CFM Calculator

What CFM means

CFM is cubic feet per minute: how much air a fan, a hood or a duct moves. The calculator has four cards, because the question changes with the room. The room card turns a volume and a number of air changes into airflow. The bath fan card follows the HVI rule and the Broan volume method. The range hood card uses burner heat and the width of the range. The HVAC card links airflow, heat and tons.

Every card prints its working with your numbers, so you can check it by hand. Metric mode shows litres per second, metres, kilowatts and °C beside the fan rating in CFM.

Room airflow from air changes

The room formula is volume times air changes per hour, divided by 60. An air change is one full volume of the room replaced each hour. The chips set the usual starting points: 8 for a bathroom (HVI), 15 for a kitchen under the older HVI rule, and 0.35 for whole-house ventilation (ASHRAE 62.2, quoted by the EPA). You can type any figure from 0.1 to 60.

A fan is sold in steps, so the answer shows the exact airflow and a buy size rounded up to the next 10 CFM. A fan below the exact figure moves less air than the room needs, so the buy size is never lower.

Bath fan size

HVI gives two cases. Under 100 square feet, use 1 CFM per square foot with a floor of 50 CFM. From 100 square feet, add up the fixtures: 50 for a toilet, 50 for a shower, 50 for a bathtub and 100 for a jetted tub. A bathroom of exactly 100 square feet uses the fixtures.

Broan’s help page gives a second method that adds ceiling height: volume in cubic feet times 0.13, rounded up. The card shows both and sizes the fan to the larger. A high ceiling pushes the volume figure above the HVI one.

Range hood CFM

Two rules apply, and the card takes the larger. The first is for gas ranges: add the BTU/h of every burner and divide by 100, which is the rule Proline and World Coppersmith print. The second is HVI’s guidance per linear foot of range, which depends on where the hood sits: 100 against a wall and 150 over an island as the recommended figure, 40 and 50 as the minimum.

Electric ranges use the HVI per-foot figure alone. If you add the kitchen size, the card also shows the older rule of 15 air changes an hour as a cross-check. It never sets the answer, because HVI’s current guidance is per foot of range.

These are guidance figures, not a code requirement. One thing the code does say: a hood that can move more than 400 CFM may need make-up air. The lamp turns amber above that figure as a reminder to check local code and any fuel-burning appliance in the home.

HVAC airflow

For cooling and heating, the sensible heat in air is 1.08 times the CFM times the temperature difference in °F. The 1.08 comes from the density and specific heat of air at 70 °F near sea level, and ACCA points out it is not a true constant: it falls with altitude. The card turns BTU/h into CFM and back, and shows the tons (12,000 BTU/h each).

The third mode multiplies tons by CFM per ton. The usual figure is 400, with 350 for humid climates and 450 for dry ones. Treat the answer as a range: your air handler’s install manual sets the airflow for your equipment, and a pro’s Manual J load calculation and duct design size the system. The calculator does not do either.

What it leaves out

This is a rule-of-thumb estimate for a single room, fan or hood. It does not count duct length or elbows, which cut real airflow, and it has no fan curves, no altitude correction, no downdraft sizing and no make-up air sizing. It also has no engine or carburetor CFM, which is a different question.

Fan and hood ratings on the label, your equipment’s manual and your local code decide the final choice.

Worked example

1. Inputs: 10 × 8 × 9 ft room at 8 air changes an hour.
2. Volume = 10 × 8 × 9 ft = 720 ft³
3. Airflow = 720 ft³ × 8 ÷ 60 = 96.0 CFM
4. Fan size = 96.0 CFM rounded up to the next 10 = 100 CFM

Questions people ask