CFM Calculator

CFM stands for cubic feet per minute — the volume of air a fan moves every minute. It is the standard airflow rating for bathroom exhaust fans, kitchen range hoods, inline duct fans, air purifiers and HVAC equipment in North America. A fan rated at 100 CFM moves 100 cubic feet of air each minute, which is 6,000 cubic feet per hour.

Multiply the room's floor area by the ceiling height to get volume in cubic feet, multiply by the air changes per hour (ACH) the room needs, then divide by 60. For example, a 10 × 10 ft bathroom with 8 ft ceilings has a volume of 800 ft³. At 8 ACH: 800 × 8 ÷ 60 = 107 CFM. The calculator above runs this math instantly, then adds duct friction and a safety factor.

For general whole-house HVAC, the rule of thumb is 1 CFM per square foot of floor area with standard 8 ft ceilings. Room-specific needs vary a lot: a bathroom needs about 1 CFM per ft² (50 CFM minimum for a typical full bath), while a commercial kitchen needs several times more. Air changes per hour is the more precise method because it accounts for ceiling height, which the per-square-foot rule ignores.

Aim for 6–8 ACH in a residential bathroom, which is why 8 ACH is the preset in this calculator. For a standard 10 × 10 ft bathroom with 8 ft ceilings that works out to about 107 CFM — right in line with the 80–110 CFM fans most manufacturers sell for full baths. A half bath can drop to 6 ACH; a bathroom with a steam shower deserves 8–10 ACH.

A residential kitchen targets 7–8 ACH for general ventilation, but a cooking-range exhaust hood is sized differently: 100 CFM per linear foot of range is the classic rule (so a 30-inch cooktop wants at least 250 CFM), and commercial kitchens run 14–15 ACH or more per code. The Kitchen Hood preset here fills in 15 ACH for hood and commercial applications.

Grow tents typically need 20–40 air changes per hour to control heat, humidity and CO₂ — this calculator's preset uses 30 ACH. A 4 × 4 ft tent that is 6.5 ft tall holds about 104 ft³, so: 104 × 30 ÷ 60 = 52 CFM at the fan. After adding duct losses and a 20% safety factor you land near 70 CFM, which is why 4-inch inline fans rated 100–190 CFM are so common for 4 × 4 tents.

Size ducts so air velocity stays between 600 and 900 FPM for quiet residential operation. The capacity table above shows the limits: a 6-inch round duct carries roughly 120–160 CFM quietly, an 8-inch duct about 210–280 CFM, and a 10-inch duct 330–440 CFM. If your fan's CFM pushes velocity above 900 FPM, step up one duct size — the fan will move the same air with far less noise and static pressure.

Almost always air velocity. When rated CFM is squeezed through a duct that is too small, too long, or crushed by a kinked flex run, air speed climbs past about 1,000 FPM and the flow starts to whistle and rumble. The fixes are the same three things: a larger duct diameter, fewer and smoother bends, and a shorter duct run. Check the Air Velocity badge in the results panel — above 1,200 FPM the noise will be obvious.

Every foot of duct and every bend adds static pressure that chokes the fan below its free-air rating. This calculator adds 10% for every 50 feet of duct and 5% for each 90° elbow — so a 50 ft run with two elbows needs about 20% more fan than the bare-room number. Flexible corrugated duct is worse than smooth metal: expect it to eat most of a 10–20% margin on its own.

Round duct is more efficient — it has the least perimeter for a given area, so less friction. When a rectangular duct must fit inside a wall or ceiling joist bay, use the equivalent diameter the calculator computes: De = 1.3 × (w·h)^0.625 / (w+h)^0.25 (the ASHRAE formula). A 3.25 × 10 in wall stack is equivalent to a 6-inch round — but only at equal friction, so a size-for-size rectangular swap of a round duct will under-perform.

A safety factor pads the calculated CFM for real-world losses the formula cannot see: dirty filters, clogged exterior hoods, slight duct compression, and the gap between a fan's rated flow and its installed flow. 15–20% is standard practice, so pick ×1.15 or ×1.2 once you have ducting installed. Skipping it means the fan you install will almost always deliver fewer air changes than you designed for.

Multiply CFM by 1.699 to get cubic meters per hour, or by 0.4719 to get liters per second. Going the other way, divide m³/h by 1.699 or L/s by 0.4719. For example, a 100 CFM bathroom fan equals 170 m³/h or 47 L/s. Switch the calculator to metric and every result is shown in m³/h and L/s automatically.

It depends on ceiling height and the target ACH. A 2,000 sq ft home with standard 8 ft ceilings holds about 16,000 ft³ of air. At 6 ACH that is 16,000 × 6 ÷ 60 = 1,600 CFM for the whole house, and at 8 ACH it rises to about 2,133 CFM. Whole-house ventilation is normally delivered room by room, so the whole-house table above shows the continuous baseline ventilation recommended for each home size.

ASHRAE 62.2, the residential ventilation standard, calls for roughly 15 CFM of outdoor air per person plus a small rate per square foot — which works out to at least 0.35 air changes per hour of outdoor air for a typical home. For a family of four, plan on around 60 CFM of continuous fresh-air supply before you count any spot ventilation from bathroom or kitchen exhaust fans.

The standard is about 400 CFM per ton of cooling, with a working range of 350–450. Humid climates run closer to 350 CFM per ton so the coil dehumidifies more, while dry climates run closer to 450. To convert the other way, divide the system airflow by 400: a 1,200 CFM blower is roughly a 3-ton system. Note this sizes cooling delivery — it is separate from the ventilation CFM the calculator above computes.

Yes. Oversizing causes drafty registers, whistling grilles, higher energy bills, and poor humidity control — in air conditioners, airflow that is too fast can actually leave moisture in the air because air passes over the cold coil before it can condense and drain. Excess CFM can also short-circuit ventilation, pulling fresh air straight to the exhaust before it has mixed with room air. Match the CFM to the room's purpose instead of assuming bigger is better.

CFM is the volume of air moved at the actual conditions where it is measured, while SCFM (standard cubic feet per minute) is normalized to standard conditions — typically sea-level pressure and 68–70 °F. Compressor output is usually quoted in SCFM so different units can be compared fairly; a pump-up test result in SCFM may read slightly lower than the compressor's nominal CFM rating at altitude or high temperature.

A carbon filter inline on the duct run adds meaningful resistance, and scrubbing odor also depends on keeping air speed inside the filter's rated range. The common grow-room practice is to multiply the base CFM by 1.25 for duct resistance, then by another 1.25 if you run a carbon filter or unusually long or bendy ducting — a combined 56% bump. See the grow room section above for a full worked example.

Engine airflow uses a different formula: carb CFM = (engine size in cubic inches × max RPM × volumetric efficiency) ÷ 3456. A 350 cubic-inch V8 spinning to 6,000 RPM with 0.85 volumetric efficiency needs about (350 × 6,000 × 0.85) ÷ 3456 ≈ 517 CFM. This has nothing to do with room ventilation — it is a separate automotive use of the CFM unit covered in the section above.

Multiply the unit's cooling capacity by 400 CFM per ton: AHU CFM = tons × 400, adjusted within the 350–450 CFM-per-ton range for humidity. A 5-ton AHU therefore delivers about 2,000 CFM, and a 10-ton unit about 4,000 CFM. If the spec sheet lists capacity in TR (tons of refrigeration), the math is identical because 1 TR equals one standard ton — so a 15 TR air handler moves roughly 6,000 CFM. Compare the nameplate airflow against the design value: a large gap usually means clogged filters, a fouled coil or duct restrictions.

About 400 CFM. One TR (ton of refrigeration) equals 12,000 BTU/h of cooling — the same as one standard ton — and blowers are sized at roughly 400 CFM per ton. So a 1 TR split or window AC circulates about 400 CFM through the room, a 1.5 TR unit about 600 CFM and a 2 TR unit about 800 CFM. Remember this is recirculated air that cools and dehumidifies; the fresh-air ventilation target for the room still comes from its ACH requirement.