Mastering the First Due Size Up on the Fireground

November 12, 2025 by Resgrid Team

A first due size up is the lightning-fast, systematic evaluation a fire officer makes the moment they arrive on scene. We're talking about the first 60 seconds—that’s it. This isn't just a quick look; it's the cognitive blueprint that will dictate every move made on that fireground.

Why the First 60 Seconds Dictate Everything

Think of it like a pilot's pre-flight check. Before ever pushing the throttle, a pilot runs through a meticulous checklist—aircraft systems, weather, flight plan—to guarantee a safe trip. A fire officer’s size up is no different. That initial assessment spots the immediate threats, calculates the resources you'll need, and starts to form the strategy that will lead to a safe and effective outcome.

The decisions made in that first minute have a massive ripple effect. Get the size up right, and you set a positive, decisive tone for the whole operation. Resources get where they need to go, and crews act with purpose. But a rushed or incomplete assessment? That's a recipe for tactical errors that burn through precious time, escalate property damage, and can put both civilians and firefighters in harm's way.

The Foundation of Fireground Command

Every single action that follows—from which hoseline gets pulled to where search and rescue teams are deployed—grows from this initial evaluation. It's the moment where a chaotic, dynamic scene starts to get organized into a coherent plan. A solid size up instantly answers the big questions:

What do I have? (Is it a wood-frame house or a commercial building? What are the fire conditions? Are people trapped?)
What am I doing? (Are we going offensive or defensive? Is a primary search the top priority?)
What do I need? (Do I need a second alarm? Specialized units like a truck company or heavy rescue? Mutual aid?)

Getting this right prevents costly mistakes. For instance, underestimating the fire's size and pulling a smaller hoseline only means your crew will have to back out and regroup. That's a huge setback, costing minutes you don't have and giving the fire a chance to grow exponentially. This split-second strategic thinking is absolutely vital for firefighter safety and bringing the incident under control.

The initial size-up isn't just about what you see; it's about what you understand. It's the bridge between observing a problem and implementing a solution under extreme pressure.

To really nail this down, the first-in officer is mentally checking off a list of core components. Here’s a look at what absolutely has to be evaluated.

Core Components of a First Due Size Up

Component	Key Questions to Answer
Building Construction	What type of construction? Wood-frame, ordinary, non-combustible? How will it react to fire? Where are the collapse hazards?
Occupancy	What is this building used for? Single-family home, apartment complex, commercial business? Who is likely inside?
Fire Conditions	Where is the fire? How large is it? What stage is it in (incipient, growth, fully developed)? What color is the smoke? Is it under pressure?
Life Hazard	Is anyone inside? Where are they most likely to be? What are the entry/exit points for rescue?
Arrangement	How is the building situated? Are there exposures (other buildings) at risk? What's the access like for apparatus?
Resources	What units are on scene and en route? Do I have enough personnel and equipment for the initial plan? What additional help do I need?
Water Supply	Where is the nearest hydrant? Is it a reliable source? Do I need to establish a tanker shuttle?

This table isn't just a checklist; it's a mental framework for turning chaos into order. Mastering this rapid assessment is what separates a good officer from a great one.

Saving Money by Saving Time

A sharp first due size up doesn't just save lives and property—it directly saves money. By quickly pinpointing the fire's location and size, crews get water on the fire faster. That speed limits the fire's spread, turning a potential total loss into a contained kitchen fire. It also means you call for what you actually need, avoiding the cost of dispatching unnecessary apparatus and keeping those units free for other emergencies. Explore the essential features of a comprehensive first responder platform that can give your officers the data they need for a more informed size up.

Nowhere are the stakes higher than in the middle of the night. According to the U.S. Fire Administration, a staggering 49% of all residential fire deaths happen between 11 p.m. and 8 a.m., when people are most likely asleep and vulnerable. This statistic underscores why a rapid, accurate size up in the dark is so critical—it's the first and most important step in changing the outcome from tragedy to rescue.

Breaking Down the WALLACE WAS HOTS Method

To make sense of the chaos on a fireground, experienced officers lean on mental frameworks to turn that chaos into a structured plan. One of the most effective and widely adopted is the WALLACE WAS HOTS mnemonic. Think of it as a comprehensive pre-flight checklist for an incident; it’s designed to make sure no critical detail gets missed when every second counts.

This isn't just about memorizing a quirky acronym. It's a systematic way to conduct a thorough first due size up, breaking a complex scene down into manageable, bite-sized pieces. Each letter prompts a specific question that helps build a complete operational picture.

W A L L A C E – The Core Structure

The first half of the mnemonic is all about setting the scene. It forces you to focus on the foundational, and often static, elements of the incident.

W – Water Supply: First things first, where is your water? How reliable is it? A quick scan for hydrants is a start, but a real size up means asking more questions. Is it close enough? Is it on the same side of the street, so you don't have a hoseline stretched dangerously across traffic? Misjudging your water supply can kill an attack before it even starts, wasting precious minutes while the fire grows.
A – Apparatus and Personnel: What do you have right now, and who's on the way? Knowing you have two engines and a truck on scene versus four engines gives you a clear picture of what you can actually accomplish. This assessment directly shapes your initial assignments and helps you decide if you need to strike a second alarm right away, instead of waiting until you realize you're outmatched.
L – Life Hazard: This is often the most critical question: who is inside, and where are they? Think about the building type—a single-family home versus a multi-family apartment—and the time of day. A fire in a house at 2 AM screams high life hazard, far more than a fire in a closed commercial building at the same time. This analysis drives the urgency and focus of your search and rescue.
L – Location and Extent of Fire: Where's the fire, and how bad is it? Is it stuck in one room, or has it already pushed into the attic and other void spaces? Reading the smoke—its color, velocity, and density—is key to pinpointing the fire's heart.
A – Area of Building: How big of a monster are you fighting? A 1,200-square-foot ranch home is a completely different tactical problem than a 50,000-square-foot warehouse. The size of the building dictates everything from how much hose you need to how many crews it will take to cover the area.
C – Construction: What's this place made of? Legacy construction with beefy, solid wood joists will hold up to fire a lot longer than modern lightweight truss construction, which can fail in as little as five minutes under fire conditions. Identifying the construction type is absolutely fundamental to predicting collapse potential and keeping your firefighters safe.
E – Exposures: What else is about to catch fire? This could be the house next door, cars in the driveway, or even nearby brush. Protecting exposures is often a primary goal to keep a bad problem from getting a whole lot bigger.

W A S H O T S – Dynamic Factors

The second part of the mnemonic shifts your focus to the factors that are constantly changing and evolving on the fireground.

W – Weather: Don't underestimate Mother Nature. Wind can turn a simple room-and-contents fire into a fast-moving conflagration. Rain, snow, or extreme heat will also hammer your crews' performance and force you to adjust your tactics.
A – Auxiliary Appliances: Does the building have any built-in help? We're talking about sprinklers, standpipes, or a fire alarm system. A working sprinkler system can be your best friend, often controlling a fire before you even arrive and turning a major firefight into a simple water cleanup job.
S – Special Matters: What are the wildcards? Look for anything out of the ordinary. This could be downed power lines, solar panels on the roof, hazardous materials stored in the garage, or even an aggressive dog that complicates access.

This infographic helps visualize how the size up process is a constant balancing act between safety, resources, and rescue priorities.

As the image shows, every bit of information you gather has to be filtered through these three critical lenses. It’s how you form a coherent and effective incident action plan.

H – Height: How tall is the building? A fire on the first floor of a two-story house is one thing; a fire on the tenth floor of a high-rise is another challenge entirely. Height impacts ladder placement, the water pressure needed for standpipes, and the pure logistics of getting crews and gear into position.
O – Occupancy: What is this building used for? This is closely tied to life hazard but digs deeper into the building’s function. A school, a hospital, or a crowded nightclub each comes with its own unique set of problems related to occupant load, building layout, and potential fire spread.
T – Time: The time of day, day of the week, and even the season can drastically change the game. Time tells you about the likely number of occupants, traffic conditions that might slow your responding units, and even the structural integrity of a building (like a heavy snow load on a roof in winter).
S – Smoke: Finally, what is the smoke telling you? Smoke is the fire talking. By analyzing its Volume, Velocity, Density, and Color (VVDC), you can get incredible insight into where the fire is, what stage it's in, and its potential to flashover.

By systematically running through WALLACE WAS HOTS, an officer can move from a state of pure reaction to one of proactive command. It’s the key to making informed decisions that save lives, protect property, and ensure every firefighter on scene goes home.

How to Read Smoke and Building Construction

When you roll up on a scene, you're hit with a flood of information. But out of all that chaos, two things are telling you the real story: the building itself and the smoke pouring out of it. These aren't just background details. They're actively showing you the fire's history, its current mood, and where it's planning to go next. Learning their language is what separates a good tactical decision from a bad one.

A building's construction is its DNA. It tells you exactly how a fire will travel, where it can hide, and—most importantly—when that structure is going to give up and fail. Ignoring these clues is just asking for trouble, because every type of building plays by a different set of rules.

Decoding the Building's Story

Figuring out construction types isn't just for the academy; it's a survival skill. A fire in an old brick-and-joist building is a completely different animal than one ripping through a modern home built with lightweight trusses.

Legacy Construction (Ordinary/Type III): Think of your classic, older buildings with masonry walls and solid wood framing inside. They tend to be a bit more forgiving. The beefy, dimensional lumber can take a beating from the fire before failing, which might buy your crews more time for an interior attack. The catch? Decades of renovations often create a maze of concealed spaces where fire can run wild without you knowing it.
Modern Lightweight Construction (Wood-Frame/Type V): Today's homes are a different story. They're often built with engineered wood like I-joists and trusses, all held together by little metal gusset plates. They're efficient to build, but under fire, these components can fail spectacularly and without much warning—sometimes in as little as five to ten minutes of direct flame contact. Spotting this kind of construction immediately cranks up the risk and shrinks your operational clock.

Get this wrong, and the consequences are severe. Mistaking lightweight for legacy could lead an officer to push crews inside just moments before a floor or roof gives way. A solid size up is what keeps that from happening.

Letting the Smoke Talk to You

Smoke is your most dynamic, real-time report from the fireground. It’s telling you everything you need to know about the fire's location, intensity, and what stage it's in. To read it right, you need to break down four key attributes: Volume, Velocity, Density, and Color (VVDC).

Smoke is the fire talking. A slow, lazy wisp of white smoke tells a very different story than a turbulent, thick, black column of smoke pushing violently from every crevice. One says "minor," the other screams "impending flashover."

Let's dig into what each attribute is really telling you:

Smoke Attribute	What It Tells You	Practical Example
Volume	This shows you how much fuel is being burned. More smoke usually means a bigger, more developed fire.	Light smoke from a single window suggests a contained fire; smoke pouring from the entire second floor means you've got a well-established problem.
Velocity (Pressure)	This is all about heat. Fast, turbulent smoke means high heat and a fire that's getting ready to flash.	Smoke lazily drifting from an eave points to low heat. Smoke jetting out like it's shot from a cannon signals intense, dangerous conditions.
Density	This reveals how much unburned fuel is in the smoke. The thicker it is, the higher the risk of a smoke explosion.	If you can see through it, the fire is burning pretty cleanly. If it's so thick you can't see your hand in front of your face, that smoke is loaded with fuel.
Color	This gives you clues about what's burning and how much oxygen the fire has. Black smoke often means plastics, while lighter smoke can mean wood.	Dark, churning black smoke often points to a ventilation-limited fire that's starving for air. Open it up the wrong way, and you'll trigger rapid fire growth.

Beyond the immediate tactics, a good size-up helps you anticipate the extent of the damage, which is crucial for the smoke damage restoration that follows. Mastering how to read both the building and the smoke is the difference between guessing and making an educated, tactical call that protects property, saves money, and most importantly, makes sure everyone goes home.

Using Technology for a Faster, More Accurate Size Up

Modern tech gives first responders a critical edge, letting the first due size up kick off the second those dispatch tones drop. This is a huge shift. Instead of a purely on-scene assessment, it’s now a pre-arrival analysis that can completely change an officer's opening moves, turning pure reaction into proactive strategy. With the right software and mobile apps, vital intel is pushed directly to responding units, cutting down the time and the cognitive load on an incident commander.

This pre-arrival intelligence is a genuine game-changer. Picture a crew rolling up on a commercial structure fire. Instead of arriving blind, the officer pulls up a pre-incident plan on their tablet while they're still en route. Right there, they can see hydrant locations, utility shutoffs, and known hazards like solar panels or specific chemicals stored inside. That information fuels smarter, faster decisions before the truck even comes to a stop.

Pre-Planning for Peak Performance

Pre-incident planning is really the foundation of this technological advantage. Think of it as the digital version of knowing your district like the back of your hand. By documenting building layouts, access points, and potential hazards in one central system, departments build a powerful, actionable library of data. When a call drops for a location with a pre-plan, that data gets pushed right out to the crews heading that way.

This straight-up eliminates guesswork and saves precious seconds on the fireground. An officer isn't wasting that first critical minute hunting for a hydrant or trying to figure out the building's layout. They already know. That lets them focus on the dynamic stuff—like smoke conditions and immediate life hazards.

By using pre-planning software, departments can turn unknown variables into known quantities. This preparation leads directly to more efficient operations, which saves money by reducing property damage and preventing firefighter injuries.

Real-Time Data and On-Scene Awareness

The benefits don't stop with pre-plans. Modern dispatch and personnel management systems give you real-time situational awareness. The incident commander can see exactly which units are responding, their ETAs, and the qualifications of the people on board. It paints a clear picture of the resources you have to throw at the problem.

We're seeing a major shift toward data-driven decisions in the fire service. In 2023 alone, one platform documented over 500,000 fire incidents, with more than 300,000 personnel using the software for reporting and analytics. This shows just how much the industry is leaning into this approach.

This screenshot gives you a sense of how a dashboard can provide an immediate overview of personnel status and unit availability.

This level of insight allows a commander to make resource allocation decisions with confidence, making sure the right people and gear get assigned to the most critical tasks from the get-go.

Practical Examples and Actionable Cost Savings

When you integrate technology into your size up, you see tangible benefits that hit your budget and improve how you operate.

Reduced Property Damage: A crew uses a pre-plan and immediately spots a sprinkler system connection on Side Charlie. By getting water to it fast, they contain the fire to the room of origin. That simple action prevents a multi-million dollar loss and saves a local business.
Enhanced Firefighter Safety: Pre-arrival info flags a building with lightweight truss roof construction. The officer immediately adjusts tactics for a defensive operation, keeping crews out of a structure with a high risk of early collapse and avoiding a potential line-of-duty injury or worse.
Optimized Resource Allocation: An IC checks their management app and sees a specialized hazmat team is just five minutes out. Instead of sending a standard engine company into a potentially nasty situation, they can stage them and wait for the right unit. This prevents costly decon procedures and potential exposures.

By bringing these tools into the fold, departments can make smarter, safer, and more cost-effective decisions on every call. If you're looking to get started, you can explore some of the powerful first responder apps from Resgrid that deliver these kinds of advantages.

Common Size Up Mistakes and How to Avoid Them

Look, even the sharpest officers can get tripped up in the chaos of those first few minutes on scene. That initial first due size up is a pressure cooker, and tiny missteps can snowball into huge problems. Getting a handle on these common pitfalls is the first real step to building a solid command presence.

These aren't rookie mistakes; they're slips in the process that happen to everyone when stress and sensory overload kick in. By knowing what to look for, you can build your own personal SOPs to fight these tendencies. That leads to safer crews, smarter resource allocation, and, at the end of the day, better outcomes for everyone.

Getting Tunnel Vision on the Fire

It's the oldest trap in the book: staring at the flames. It’s only human. The fire is the big, loud, obvious problem right in front of you. But this kind of tunnel vision makes you blind to the critical clues hiding in the parts of the building that aren't on fire.

While an officer is zeroed in on the fire blowing out the front windows, they can completely miss a family waving for help from a second-story window around the side. That same tunnel vision might also make you miss a secondary fire taking off on an exposure or the subtle signs of a wall getting ready to come down.

You have to see the fire scene as a whole system, not just one problem. The fire is just the symptom; the entire building and everyone in it is the patient. Focusing only on the flames is like a doctor looking at a rash but ignoring the patient's vitals.

Failing to Complete a 360-Degree Survey

Skipping a full 360-degree walk-around is probably the most dangerous shortcut anyone can take on a fireground. The urge to get crews inside and get to work is strong, but what you can't see will absolutely hurt you and your team. The back of the building almost always tells a different, more honest story than the front.

Here’s a classic example: The first-in officer sees moderate smoke pushing from the front of a two-story house and immediately orders an interior attack. What they don't see—because they didn't do a lap—is that the entire back of the house is fully involved. The fire has already chewed through the floor joists, and a hidden basement fire is raging underneath. That missed intel sends the interior crew into a situation with extreme, and totally unknown, risk.

The fix is to make it a non-negotiable part of your personal process: always walk the building before committing crews inside, whenever you can. If the building's too big, your next move should be assigning another officer or unit to cover the sides you can't see and report back immediately. This simple discipline is one of the most effective safety tactics you have.

Underestimating Resource Needs

This is another big one—failing to call for help early enough. It’s that classic "I think we can handle it" mindset. Then, 10 minutes later, you realize the fire is running away from your crews. Now you're playing a desperate game of catch-up while the help you desperately need is still several minutes out.

Remember how long it takes just for the first truck to get there. A study by the City of Gresham, Oregon, showed their average response time for the first-arriving unit was about 6.5 minutes, which includes dispatch time, turnout, and travel. That should tell you any extra help you call for is going to take time, too. You can check out more of their fire department response time analysis on GreshamOregon.gov.

The solution is dead simple: if you think you might need more help, call for it right then. It’s always better to turn units around than to wish you had them. It costs you nothing and could save a building or a life. If you need a hand managing and tracking your people and apparatus, you can always check out the tools available in our Resgrid support documentation.

Frequently Asked Questions About First Due Size Up

Getting the first due size up right is a skill you never stop honing. It's a constant process of learning and getting better. To help clear things up, we've put together answers to some of the questions we hear most often from officers and firefighters out on the front lines.

Think of these as straight-to-the-point insights you can take with you on your next call.

How Long Should a First Due Size Up Take?

You should aim to get your initial size up done and reported within the first 60 seconds on scene. That quick, initial look is what your first radio report and strategic calls are built on.

But here's the critical part: the size up isn't a one-and-done deal. It's a cycle. It keeps going for the entire incident. Every time you get a new piece of information—a crew report, a change in the smoke, a groan from the structure—you're doing another size up and tweaking the plan.

What Is the Single Most Important Part of a Size Up?

Every piece of the puzzle matters, but if you have to pick one, it’s identifying the life hazard. Figuring out if people are inside and what their chances are—the survivability profile—drives every single risk-benefit decision you make from that moment on.

This is what sets the pace and urgency of the whole operation. It’s the difference between sending crews in for a high-risk primary search and pulling back to a defensive attack to protect the buildings next door. Every other decision flows from how you answer the human question.

The question "Is anyone inside?" and the follow-up "Can we save them?" must be at the forefront of every incident commander's mind. The answer shapes the entire moral and tactical fabric of the response.

How Do You Perform a Size Up on a Large Commercial Building?

Doing a 360 walk-around on a massive warehouse or a long strip mall just isn't practical for one person. On these calls, the size up has to be a team effort. Splitting it up saves precious time and can prevent tactical mistakes that lead to huge property loss.

Here's how it usually breaks down among the first arriving units:

First Due Officer: The first officer on scene grabs a view of the front (Side Alpha) and one of the adjacent sides (Bravo or Delta). This gives them eyes on at least two sides of the building right away.
Assign Other Units: That officer then tasks another incoming unit—maybe the second-due engine or the first truck—to check the remaining sides.
Radio Reports: The crews doing the reconnaissance need to get on the radio with a quick, clear report of what they're seeing. They should hit the key points: fire conditions, entry/exit points, and any hazards on their side.

This teamwork approach pieces together the full 360 view. This is where pre-incident plans are worth their weight in gold, giving you a cheat sheet on the building's layout and hazards before you even step off the rig.

Can You Do a Size Up Before Arriving on Scene?

Absolutely. The size up starts the second the tones drop, not when you set the air brake. This "pre-arrival size up" is all about building a mental picture of the incident before you even get there.

You have to use every bit of information you can get your hands on:

Dispatch Information: The address, call type, and what the caller reported are your first clues.
Knowledge of the Area: Are you heading to an industrial park full of tilt-slabs, a neighborhood with old balloon-frame houses, or a new development with lightweight construction?
Time of Day: A fire in a school at 2 PM is a completely different animal than a fire there at 2 AM.
Weather Conditions: Wind is a huge factor. So is snow, rain, or extreme heat. All of it changes your tactical playbook.

Modern tools like CAD notes and pre-planning software can give you a massive head start here. Getting eyes on building info, known hazards, and hydrant locations before you see the smoke column lets you arrive with a draft of your game plan already in your head.

A faster, more accurate size up leads to better outcomes, reduced property loss, and enhanced firefighter safety. Resgrid provides the dispatch, management, and reporting tools needed to give your first responders the critical information they need before and during an incident. See how you can streamline your operations by visiting https://resgrid.com.