CFM Calculator

The formula, and the 60 that everybody drops

Ventilation airflow starts from one line of arithmetic: CFM = volume × ACH ÷ 60. Volume in cubic feet, ACH in air changes per hour, and the 60 doing the only job it has — turning an hourly requirement into a per-minute flow rate, because CFM means cubic feet per minute.

A 20 × 15 ft room with an 8 ft ceiling holds 2,400 ft³. At six air changes an hour it needs 2,400 × 6 = 14,400 cubic feet of air every hour, which is 240 every minute. That 240 CFM is the number you take to a fan catalogue.

The trap is what happens when metric enters the room. In cubic metres the equation is simply volume × ACH, with no division whatsoever: 68 m³ at six changes an hour is 408 m³/h, because m³/h and ACH are both already hourly quantities. People who learned the imperial version reach for the 60 out of habit, apply it to a figure that is already in m³/h, and specify a fan one sixtieth the size. It is a quiet failure — the fan runs, the room does not clear, and nothing on the spreadsheet looks wrong.

This calculator runs every internal step in cubic feet and CFM and converts once at the edge, which is the only reliable way to keep the factor from creeping into the middle of a formula.

Air changes per hour is not one number

The single biggest source of wrong answers is treating ACH as a constant. It is a property of what the room does, and the spread across ordinary rooms is close to an order of magnitude.

Enclosed parking sits around 6 but is really governed by carbon monoxide sensing rather than a fixed rate. Server rooms are frequently quoted at 15, though there the real driver is heat removal, not air quality, and a proper calculation runs on kilowatts and temperature rise rather than air changes.

Every preset here is a starting point that stays editable. Local mechanical code, an industrial hygiene assessment or the equipment manufacturer's own requirement all outrank a generic table, and a calculator that hides the assumption inside its source code cannot be corrected when they do.

The second method: airflow per person

ASHRAE 62.1 does not size ventilation by air changes at all. It sizes the outdoor-air portion with two terms added together:

Vbz = Rp × people + Ra × floor area

Rp is the rate per person — 5 CFM in an office, 10 in a classroom, 7.5 on a retail floor, 20 in a weight room — and covers what the occupants themselves emit. Ra is the rate per square foot of floor — 0.06 in an office, 0.12 in a classroom, 0.18 in a restaurant dining room — and covers what the building emits: carpet, adhesives, paint, furniture.

Both terms matter, and dropping the second one is common. A large, sparsely occupied space with two people in it still has a floor, and that floor keeps off-gassing whether anyone is standing on it or not. Residential work uses ASHRAE 62.2 instead, whose whole- dwelling formula is 0.03 × floor area + 7.5 × (bedrooms + 1) CFM — a different shape of equation for a different problem.

Which method governs, and why you never add them

Two methods, two different answers, and the design rule is simple: take the larger, never the sum. They are not additive because they are not two separate contaminant loads to be handled in series — they are two lower bounds on the same airflow, and satisfying the bigger one automatically satisfies the smaller.

Which one wins is a question about density. A 3,000 ft³ storeroom with one person in it is governed by volume by a wide margin. A 1,600 ft³ meeting room with twelve people in it is governed by occupancy, and by roughly double. The calculator shows both figures side by side, marks the one that governs, and prints the ratio between them, because the ratio is what tells you whether the choice was close or decisive.

Watch what happens when you change one input. Add ten people to a room and the volume figure does not move at all — which is exactly why a volume-only calculator sails straight past the most crowded room in the building without flinching.

Bathrooms and range hoods break the pattern

Two very common jobs do not follow the room at all.

Bathroom exhaust has an absolute floor. The rule of thumb is 1 CFM per square foot of floor, but with a minimum of 50 CFM intermittent — and almost every real bathroom falls under that minimum on area alone. A 35 ft² bathroom does not get a 35 CFM fan; it gets 50. Larger or fixture-heavy bathrooms are sized per fixture instead, at roughly 50 CFM per toilet, shower or tub and 100 for a jetted tub. If the fan runs continuously rather than on a switch, ASHRAE 62.2 allows a much lower rate, around 20 CFM.

Range hoods are sized by the width of the hood, not the volume of the kitchen, because the job is capturing a thermal plume at the cooktop before it disperses. The convention is about 100 CFM per linear foot for a hood against a wall and 150 for an island, where there are no walls containing the plume on three sides. A 30 inch wall hood is therefore 250 CFM, regardless of whether the kitchen is small or cavernous.

One consequence people miss: any hood pulling more than about 400 CFM out of a reasonably tight house needs make-up air brought in deliberately. Otherwise the house goes negative, and the cheapest path for replacement air may be back down a flue.

Sizing the duct: area equals flow divided by velocity

Once you know the CFM, the duct follows from the continuity equation: area = flow ÷ velocity. In coherent units, ft³/min ÷ ft/min = ft². At 240 CFM and 600 fpm that is 0.4 ft², or 57.6 in², which is a round duct 8.56 inches across — so you install a 9 inch and move on.

Velocity is a design decision with real consequences. Low velocity is quiet and needs more sheet metal; high velocity is cheap to install and audible forever. Roughly 600 fpm suits a quiet residential branch, 700 to 900 a trunk, 500 a return, and 1,000 to 1,500 is normal in commercial work where the duct is above a hard ceiling and further from ears. Undersizing does not reduce airflow for free — it converts the missing area into noise and static pressure, and static pressure is a bill the fan pays every hour it runs.

For rectangular duct, matching the round duct's area is wrong. A rectangle has more perimeter per unit of area, therefore more friction, therefore less flow at the same pressure drop, and the flatter it gets the worse it is. The correct relation is the equivalent diameter, De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25. This tool inverts it numerically: tell it the height you have between the ceiling and the joists and it returns the width that genuinely performs like the round duct it replaces.

CFM, m³/h and L/s

These conversions are exact, not measured, because the foot is defined as exactly 0.3048 metres:

Equipment data sheets mix these freely — an American fan curve in CFM, a European diffuser in L/s, a Brazilian inline fan in m³/h — and a single sloppy conversion makes a tool useless to anyone working outside its home units. The calculator prints all three at once so nothing has to be converted by hand at the point where mistakes happen.

What this calculator does not do

It gives you the airflow requirement and a first-pass duct size. It does not compute total static pressure, so it cannot pick a fan off a curve for you: filters, grilles, elbows and flexible duct all add resistance, and flexible duct compressed into a joist bay can add a startling amount. It does not do heating or cooling load, which is a separate calculation in BTU or kW. It does not model pressure balance between supply and exhaust, contaminant-specific capture velocity, or anything smoke-control related.

Everything runs in your browser as arithmetic — nothing is uploaded, nothing is stored, and there is no account. Use it to get the number right and to sanity-check a quote; use a mechanical engineer where the code requires one.