Construction Calculator
Fire Flow Calculator
Last Updated On
Estimate the water a building needs for firefighting — ISO's needed fire flow from construction, effective area, occupancy and exposures, or the National Fire Academy's quick fireground estimate.
Fire Flow Calculator
Required fire flow in gallons per minute
Results
Required fire flow
1,750 gpm
ISO needed fire flow
Construction factor
1,750 gpm
F = 1, A = 9,000
Same Building, Each Construction Class
| Class | C (gpm) | NFF (gpm) |
|---|---|---|
| Class 1 — Frame (wood) | 2,500 | 2,500 |
| Class 2 — Joisted masonry (ordinary) ← | 1,750 | 1,750 |
| Class 3 — Non-combustible | 1,250 | 1,250 |
| Class 4 — Masonry non-combustible | 1,250 | 1,250 |
| Class 5 — Modified fire resistive | 1,000 | 1,000 |
| Class 6 — Fire resistive | 1,000 | 1,000 |
Same occupancy and exposure. Fire-resistive classes also count less of the upper floors in the effective area when entered from dimensions.
ISO’s method is NFF = C × O × [1 + (X + P)], with C = 18 × F × √A rounded to the nearest 250 gpm. The 18 × C × √A part is only the construction factor; occupancy, exposures and communication then scale it.
Effective area is not total floor area. ISO takes the largest floor plus 50% of the other floors for construction classes 1–4, and for fire-resistive classes 5–6 only 25% of up to two other floors when vertical openings are protected.
This is the ISO method. The International Fire Code (Appendix B) instead reads fire flow from a table by construction type and area, with reductions for sprinklers. The code your jurisdiction adopts governs a permit.
Results are for unsprinklered buildings. ISO does not calculate a needed fire flow for a building with an NFPA 13 system; it uses the sprinkler and hose stream demand instead.
Step-by-Step Calculation
Step 1 — Effective area
floor = 100 × 60 = 6,000 sq ft
largest floor + 50% of all other floors
effective area = 9,000 sq ft
Step 2 — Construction factor
C = 18 × 1 × √9,000 = 1,707.63 gpm
rounded to nearest 250 → 1,750 gpm
C = 1,750 gpm
Step 3 — Occupancy, exposure and communication
NFF = 1,750 × 1 × (1 + 0) = 1,750.00 gpm
Step 4 — Round
below 2,500: nearest 250 → 1,750 gpm
Result — 1,750 gpm needed fire flow (ISO)
Two Formulas for Two Questions
"Fire flow" means two different things. For planning, it is the water supply a community should be able to deliver to a building — the ISO needed fire flow, set by how the building is built, how big it is, what is in it and what stands next to it. On the fireground, it is the flow needed to put out the fire actually burning — the National Fire Academy formula, scaled by how much of the building is involved.
They are not interchangeable, and a third method is often confused with the first. The familiar 18 × F × √A is ISO's. The International Fire Code's Appendix B uses a lookup table by construction type and area instead, and that table is what governs permits wherever the IFC is adopted.
Fire Flow Formulas
1. ISO Construction Factor
2. ISO Needed Fire Flow
3. ISO Effective Area
4. NFA Fireground Estimate
Why 18 × C × √A Is Not the Answer on Its Own
The formula most people quote is only ISO's construction factor. It is rounded to the nearest 250 gpm before anything else happens, then scaled by occupancy and exposure, then rounded again. That is why 18 × 1.0 × √10,000 = 1,800 becomes 1,750 gpm, and 18 × 0.6 × √20,000 = 1,527.35 becomes 1,500.
The rounding steps matter at the boundaries. A 10,000 sq ft frame building gives a raw 2,700 gpm, rounded to 2,750 for C; with no adjustments that is above 2,500, so the final figure rounds to the nearest 500 and becomes 3,000 gpm.
This calculator checks itself against ISO's own worked examples: a 2,250 sq ft frame storage building at 1,250 gpm, a two-story furniture factory at 3,000 gpm, and a frame building with an exposure at 2,000 gpm. It reproduces all three.
Needed Fire Flow by Area and Construction
ISO needed fire flow in gpm for a multi-story building with combustible (C-3) contents and no exposures:
| Effective area | Frame | Joisted masonry | Non-comb. | Fire resistive |
|---|---|---|---|---|
| 2,500 sq ft | 1,250 | 1,000 | 750 | 500 |
| 5,000 | 2,000 | 1,250 | 1,000 | 750 |
| 10,000 | 3,000 | 1,750 | 1,500 | 1,000 |
| 20,000 | 4,000 | 2,500 | 2,000 | 1,500 |
| 40,000 | 5,500 | 3,500 | 3,000 | 2,250 |
| 80,000 | 8,000 | 5,000 | 4,000 | 3,000 |
Flow grows with the square root of the area, so doubling a building raises it by about 41%, not 100%. Construction matters more: at every size, frame needs about two and a half times the construction factor of fire-resistive construction. At 80,000 sq ft the frame construction factor is 7,750 gpm, just under ISO's 8,000 gpm limit for frame — around where fire walls or sprinklers become the realistic answer rather than more water. (Its final 8,000 comes from rounding 7,750, which sits exactly between 7,500 and 8,000; ISO does not say how to break such ties, and this calculator rounds halves up.)
Effective Area Is Not Floor Area
ISO assumes a fire will not involve every floor at once, so it counts the largest floor in full and only part of the rest. For frame, joisted-masonry and non-combustible buildings that is 50% of all other floors.
Fire-resistive buildings get a much larger allowance, because their floors and shaft enclosures resist vertical spread. With protected vertical openings, only 25% of up to two other floors is added. A ten-story building of 10,000 sq ft floors then has an effective area of 15,000 sq ft and needs 1,250 gpm. Using its 100,000 sq ft total would give 3,500 gpm — nearly three times too much. If the shafts are unprotected, ISO counts 50% of up to eight other floors instead, and the same building needs 2,500 gpm.
Occupancy and Exposure Adjustments
The default building — 100 × 60 ft, two stories, joisted masonry — has a construction factor of 1,750 gpm. Its contents move the result:
| Occupancy | Factor O | Before rounding | NFF |
|---|---|---|---|
| C-1 Noncombustible | 0.75 | 1,312.5 | 1,250 |
| C-2 Limited combustibility | 0.85 | 1,487.5 | 1,500 |
| C-3 Combustible | 1.00 | 1,750.0 | 1,750 |
| C-4 Free burning | 1.15 | 2,012.5 | 2,000 |
| C-5 Rapid burning | 1.25 | 2,187.5 | 2,250 |
Exposures and communications add on top: X covers neighbouring buildings within 40 ft, P covers connecting passageways, and ISO takes the worst side and caps X + P at 0.60. The factors come from ISO's exposure tables, which depend on the facing walls, openings and distances, so this calculator takes the combined value as an input.
The NFA Formula on the Fireground
The National Fire Academy formula — one gallon per minute for every three square feet involved — is a quick estimate for an officer sizing up an interior attack. It scales with the fire, not the building: the default 100 × 60 ft building needs 4,000 gpm with both floors fully involved, but only 1,000 gpm at 25% involvement, and 500 gpm with a quarter of one floor burning.
That is why it bears little relation to ISO's 1,750 gpm for the same building. Training sources add 25% for each exposure but differ on whether to count neighbouring buildings or exposed sides, and describe the formula as intended for fires involving less than half the building; above that, operations are usually defensive.
Example Calculations
Two ISO cases and one NFA estimate:
Example 1 — 10,000 sq ft, Joisted Masonry (ISO)
Ordinary construction, combustible contents, no exposures.
C = 18 × 1.0 × √10,000 = 1,800 gpm
Rounded to the nearest 250 gpm: C = 1,750
NFF = 1,750 × 1.00 × (1 + 0) = 1,750 gpm
The often-quoted 1,800 skips ISO's rounding step
Example 2 — 50 × 40 ft, Two Floors Burning (NFA)
The fireground formula at full involvement.
Flow per floor = (50 × 40) ÷ 3 = 666.7 gpm
Two floors involved = 1,333 gpm
At 50% involvement: 667 gpm
Each exposure would add 25%: +333 gpm
Example 3 — 20,000 sq ft Effective Area, Fire Resistive (ISO)
The least combustible construction class.
C = 18 × 0.6 × √20,000 = 1,527.35 gpm
Rounded to the nearest 250 gpm: C = 1,500
NFF = 1,500 gpm with C-3 contents and no exposures
The same area in wood frame needs 4,000 gpm
What This Figure Is For
Use the ISO figure for planning: checking whether a site's water supply is likely to be adequate, or comparing how construction and layout change the demand. It is not a permit requirement. Where the International Fire Code is adopted, the fire code official sets the required fire flow from Appendix B's table, which differs from ISO's formula and reduces the flow for sprinklered buildings. Results here are for unsprinklered buildings, and ISO itself does not calculate a needed fire flow for a building with an NFPA 13 system. Get the available flow and pressure from the water purveyor early — demands above a few thousand gallons per minute are often more than a single location can deliver. Use the NFA figure only as a fireground estimate of the flow to control a fire that is burning.
Frequently Asked Questions
- How do you calculate needed fire flow?
- By the ISO method, NFF = C × O × [1 + (X + P)]. C is the construction factor, 18 × F × √A, where F is the construction coefficient and A the effective area; it is rounded to the nearest 250 gpm. O is the occupancy factor, and X + P covers exposures and communications. For a 10,000 sq ft joisted-masonry building with combustible contents and no exposures, C = 1,800 rounded to 1,750, and the NFF is 1,750 gpm.
- Is the ISO formula the same as the IFC fire flow?
- No. The 18 × F × √A formula is ISO's. The International Fire Code's Appendix B sets fire flow from a table by construction type and fire-flow calculation area, with reductions for sprinklers. Both are used, but for different purposes — ISO for water-supply planning and insurance grading, the IFC table for permits where it is adopted — and they do not give identical answers.
- What are the ISO construction coefficients?
- F = 1.5 for frame (class 1), 1.0 for joisted masonry (class 2), 0.8 for non-combustible and masonry non-combustible (classes 3 and 4), and 0.6 for modified fire resistive and fire resistive (classes 5 and 6). Because the flow scales with F, the same building in frame needs two and a half times the construction factor of fire-resistive construction.
- What is the effective area for fire flow?
- Not the total floor area. ISO takes the largest floor plus 50% of all other floors for classes 1–4. For fire-resistive classes 5 and 6, it adds only 25% of up to two other floors if vertical openings are protected, or 50% of up to eight other floors if not. A ten-story fire-resistive building of 10,000 sq ft floors with protected shafts has an effective area of 15,000 sq ft, not 100,000.
- How is needed fire flow rounded?
- Twice. ISO rounds the construction factor C to the nearest 250 gpm, then rounds the final NFF to the nearest 250 gpm below 2,500 and to the nearest 500 gpm above. The construction factor is limited to 8,000 gpm for frame and joisted masonry and 6,000 for other classes or any one-story building, with a 500 minimum; the NFF itself runs from 500 to 12,000 gpm.
- What occupancy factors does ISO use?
- 0.75 for noncombustible contents (C-1), 0.85 for limited combustibility (C-2), 1.00 for combustible (C-3), 1.15 for free-burning (C-4) and 1.25 for rapid-burning (C-5). On a building with a 1,750 gpm construction factor, that moves the needed fire flow from 1,250 gpm to 2,250 gpm.
- What is the NFA fire flow formula?
- Required flow = (length × width) ÷ 3 × the percentage involved, for each floor burning, plus 25% for each exposure. A 50 × 40 ft floor fully involved needs about 667 gpm, so two floors need 1,333 gpm. It is a quick fireground estimate for an offensive interior attack — not a water-supply design figure.
- Why do the ISO and NFA methods give such different answers?
- They answer different questions. ISO estimates the water supply a community should have available for a building. The NFA formula estimates the flow to put out the fire actually burning, scaled by how much is involved. For a 100 × 60 ft two-story joisted-masonry building, ISO gives 1,750 gpm while the NFA formula gives 4,000 gpm with both floors fully involved — and 1,000 gpm at 25% involvement.
- What fire flow does a house need?
- ISO does not use the formula for one- and two-family dwellings of two stories or fewer. It sets 500 gpm where the nearest building is more than 30 ft away, 750 at 21–30 ft, 1,000 at 11–20 ft and 1,500 at 10 ft or less, each for one hour.
- Does this apply to sprinklered buildings?
- No. ISO does not calculate a needed fire flow for a building with an NFPA 13 sprinkler system; the requirement is the sprinkler demand plus the hose stream. The IFC table also reduces fire flow for sprinklered buildings. Results here are for unsprinklered buildings, and the fire code official sets the actual requirement.