The CFM formula, and the 60 everybody drops
Ventilation airflow comes from one line of arithmetic: CFM = volume × ACH ÷ 60. Volume in cubic feet, ACH in air changes per hour, and the 60 does exactly one thing. It turns 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, or 240 every minute. Take that 240 CFM to the fan catalog.
The trouble starts when metric shows up. In cubic meters the equation is just volume × ACH, with no division: 68 m³ at six changes an hour is 408 m³/h, because m³/h and ACH are both already hourly. People who learned the imperial version reach for the 60 out of habit, apply it to a figure already in m³/h, and spec a fan one sixtieth the size. Nothing warns you. The fan runs, the room doesn't clear, and the spreadsheet looks fine.
This calculator does every internal step in cubic feet and CFM and converts once, at the end. That is the only reliable way to keep the factor from sneaking into the middle of a formula.
Air changes per hour depends on the room
Most wrong answers come from treating ACH as a constant. It depends on what the room does, and across ordinary rooms the spread is close to an order of magnitude.
| Space | Typical ACH | What is being diluted |
|---|---|---|
| Bedroom | 5 | CO₂, moisture from breathing |
| Living room | 6 | General occupancy, furnishings |
| Classroom | 6 | Dense occupancy, check the per-person rule too |
| Bathroom | 8 | Moisture, odor |
| Residential kitchen | 15 | Grease, combustion products, steam |
| Welding or paint shop | 20+ | Fume, solvent vapor |
| Commercial kitchen | 30 | Continuous heat and grease load |
Enclosed parking sits around 6, though carbon monoxide sensing governs it more than any fixed rate. Server rooms often get quoted at 15, but what drives them is heat removal, not air quality, and a proper calculation for one runs on kilowatts and temperature rise instead of air changes.
Every preset here is a starting point you can edit. Local mechanical code, an industrial hygiene assessment or the equipment maker's own requirement all outrank a generic table. When they do, you need to be able to change the number, and a calculator that hides the assumption in its source code won't let you.
The second method: CFM per person
ASHRAE 62.1 doesn't size ventilation by air changes at all. It sizes the outdoor-air portion as 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 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.
You need both terms, and people often drop the second. A big, sparsely occupied space with two people in it still has a floor, and that floor keeps off-gassing whether anyone stands on it or not. Residential work uses ASHRAE 62.2 instead. Its whole-dwelling formula is 0.03 × floor area + 7.5 × (bedrooms + 1) CFM, a different equation for a different problem.
Which ventilation method governs, and why you never add them
Two methods give two answers, and the design rule is short: take the larger, never the sum. They don't add because they aren't two separate contaminant loads handled one after the other. They are two lower limits on the same airflow, and meeting the bigger one meets the smaller one automatically.
Which one wins comes down to 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 is governed by occupancy, by roughly double. The calculator shows both figures side by side, marks the one that governs and prints the ratio between them. The ratio tells you whether the call was close or clear-cut.
Try changing one input. Add ten people to a room and the volume figure doesn't move at all. That's how a volume-only calculator walks right past the most crowded room in the building and never notices.
Bathroom fans and range hoods break the pattern
Two very common jobs don't follow the room at all.
Bathroom exhaust has a hard minimum. The rule of thumb is 1 CFM per square foot of floor, with a floor of 50 CFM intermittent, and almost every real bathroom falls under that minimum on area alone. A 35 ft² bathroom doesn't get a 35 CFM fan. It gets 50. Larger or fixture-heavy bathrooms are sized per fixture instead: roughly 50 CFM per toilet, shower or tub, and 100 for a jetted tub. If the fan runs continuously instead of 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 catching a thermal plume at the cooktop before it spreads. The convention is about 100 CFM per linear foot for a hood against a wall and 150 for an island, where no walls hold the plume in on three sides. So a 30 inch wall hood is 250 CFM, whether the kitchen is small or cavernous.
One thing people miss: any hood pulling more than about 400 CFM out of a reasonably tight house needs make-up air brought in on purpose. Without it the house goes negative, and the cheapest path for replacement air may be back down a flue.
Sizing the duct: area equals CFM divided by velocity
Once you have the CFM, the duct follows from the continuity equation: area = flow ÷ velocity. In consistent units, ft³/min ÷ ft/min = ft². At 240 CFM and 600 fpm that's 0.4 ft², or 57.6 in², a round duct 8.56 inches across. Install a 9 inch and move on.
Velocity is a design decision, and you live with it. Low velocity is quiet and takes more sheet metal. High velocity is cheap to install and you hear it 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 sits above a hard ceiling, farther from people's ears. An undersized duct doesn't save you anything on airflow. It turns 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, so more friction, so 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 really performs like the round duct it replaces.
Converting CFM, m³/h and L/s
These conversions are exact, not measured, because the foot is defined as exactly 0.3048 meters:
| From | To | Multiply by |
|---|---|---|
| CFM | m³/h | 1.699 |
| CFM | L/s | 0.4719 |
| m³/h | CFM | 0.5886 |
| L/s | CFM | 2.119 |
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. One sloppy conversion makes a tool useless to anyone working outside its home units. The calculator prints all three at once, so you never convert by hand at the moment mistakes happen.
What this CFM 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 can't pick a fan off a curve for you. Filters, grilles, elbows and flexible duct all add resistance, and flex duct squashed into a joist bay can add a startling amount. It doesn't do heating or cooling load, which is a separate calculation in BTU or kW. It doesn't model pressure balance between supply and exhaust, contaminant-specific capture velocity, or anything related to smoke control.
Everything runs in your browser as arithmetic. Nothing is uploaded, nothing is stored, and there's no account. Use it to get the number right and to sanity-check a quote. Where the code requires a mechanical engineer, hire one.
Frequently asked questions
What is the formula for CFM?
CFM equals room volume in cubic feet times air changes per hour, divided by 60. A 2,400 ft³ room at 6 air changes per hour needs 2,400 × 6 ÷ 60 = 240 CFM. The 60 has one job: CFM is per minute and air changes per hour is per hour, so the hours have to turn into minutes somewhere. In cubic meters the same calculation is volume × ACH with no division at all, because m³/h and ACH are both hourly. Dividing that result by 60 anyway is the most common mistake in the whole subject, and it gets you a sixtieth of the fan you need.
How many air changes per hour does a room need?
That depends on what happens in the room, and the spread is much wider than most people expect. A bedroom is fine at 5, a general living space at 6, a bathroom wants 8, a residential kitchen 15, a welding or paint shop above 20, and a commercial kitchen 30 or more. A calculator that assumes 6 for everything is off by a factor of five at the top of that range. The presets here fill in the field and leave it editable, because the code that applies to your project outranks any table on a website.
Should I size ventilation by volume or by the number of people?
Run both and use the larger. The volume method (air changes per hour) dilutes what the space itself produces: cooking, solvents, moisture, materials off-gassing. The per-person method under ASHRAE 62.1 dilutes what the occupants produce, mainly carbon dioxide and odor. A big empty warehouse is governed by volume. A packed 12-person meeting room is governed by occupancy, often by a factor of two or more. Meet one requirement, ignore the other, and you get a stuffy room with the fan running.
How do I convert CFM to m³/h and liters per second?
One CFM is exactly 1.69901079552 m³/h and 0.4719474432 L/s, because a foot is defined as exactly 0.3048 meters. In practice, 1 CFM ≈ 1.699 m³/h ≈ 0.472 L/s. The other way, 1 m³/h ≈ 0.5886 CFM and 1 L/s ≈ 2.119 CFM. Rounding 1.699 to 1.7 adds about 0.06% of error. Once, that's harmless. After three conversions you can't trace it.
What size duct do I need for a given CFM?
Duct cross-section equals airflow divided by air velocity. 240 CFM at 600 feet per minute needs 0.4 ft², a round duct of 8.56 inches, so you fit a 9 inch. Velocity is a design choice you make, and it changes with the job: around 600 fpm for a quiet residential branch, 800 to 900 for a trunk, 1,000 to 1,500 in commercial work. An undersized duct doesn't just move a little less air. It makes noise you'll hear every day and static pressure the fan pays for in energy for as long as it runs.
Can a rectangular duct just have the same area as the round one?
No, and this one costs real money. A rectangle has more wetted perimeter than a circle of the same area, so it has more friction and carries less air at the same pressure drop. The right conversion is the equivalent-diameter formula, De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25, and this calculator inverts it for you: give it the height you have above the ceiling and it returns the width that performs like the round duct it replaces. Matching area alone undersizes the rectangle every time, and the flatter the duct, the bigger the error.