How many CFM do you need?

What size bathroom exhaust fan do I need?

The Home Ventilating Institute recommends 1 CFM per square foot of floor for bathrooms up to 100 square feet, and at least 50 CFM even for the smallest one. An 8 by 10 ft bathroom is 80 square feet, so an 80 CFM fan.

How do I size a fan for a large bathroom?

Above 100 square feet, add up the fixtures: 50 CFM each for a toilet, a shower and a bathtub, and 100 CFM for a jetted tub. A bathroom with a toilet, a shower and a jetted tub needs 50 + 50 + 100 = 200 CFM.

How many CFM does a range hood need?

HVI recommends 100 CFM per linear foot of cooktop width for a hood against a wall and 150 CFM per foot for an island hood. A 30 inch (2.5 ft) range under a wall hood: 250 CFM; a 36 inch island cooktop: 450 CFM. The International Residential Code requires makeup air for hoods that can move more than 400 CFM.

How do I calculate CFM from air changes per hour?

Multiply the room volume in cubic feet by the air changes per hour and divide by 60. A 12 × 10 ft room with an 8 ft ceiling is 960 cubic feet; 8 air changes an hour is 960 × 8 ÷ 60 = 128 CFM. HVI suggests at least 10 air changes an hour for a powered attic ventilator.

What does CFM mean?

Cubic feet per minute — how much air a fan moves. One CFM is 0.47 liters per second or 1.7 cubic meters per hour. Long or narrow ducts cut a fan's real airflow, so pick one rated at your duct's static pressure, and duct it outside, not into the attic.

What are sones?

A sone is a unit of loudness used on fan labels. Lower is quieter: a fan rated at 1 sone is about as loud as a quiet refrigerator, while higher numbers are noisier. The fan's CFM and its sones are usually printed on the same label.

How many CFM does ASHRAE require for a bathroom?

ASHRAE Standard 62.2 for residential ventilation asks for 50 CFM of intermittent exhaust or 20 CFM of continuous exhaust for a bathroom. The Home Ventilating Institute guideline on this page is a separate rule of thumb and can give a different number.

Does duct length change the CFM?

Yes. A fan's rating is measured under test conditions with limited duct resistance. Long runs, elbows and a restrictive wall cap increase the resistance and reduce the air that actually moves. Use the shortest, straightest duct you can, and choose a fan rated for the static pressure of your duct.

How the calculation works

There are three methods. For a bathroom up to 100 square feet, the airflow is 1 CFM per square foot of floor, with a minimum of 50 CFM. For a larger bathroom, the guideline adds the fixtures: 50 CFM for a toilet, 50 for a shower, 50 for a bathtub and 100 for a jetted tub. A range hood is sized by the cooktop width in feet times 100 CFM against a wall or 150 CFM over an island. For any room, the volume in cubic feet times the air changes per hour divided by 60 gives the CFM, because there are 60 minutes in an hour.

Worked examples

A 9 × 11 ft bathroom is 99 square feet, so it needs 99 CFM. A 10 × 14 ft bathroom (140 square feet) is above the 100 square foot limit, so the fixtures are counted: toilet, shower and tub give 3 × 50 = 150 CFM. A 36-inch (3 ft) cooktop needs 300 CFM under a wall hood and 450 CFM for an island hood. A 14 × 12 ft room with an 8 ft ceiling holds 1344 cubic feet, and for 6 air changes per hour the airflow is 1344 × 6 ÷ 60 = 134.4 CFM.

Common mistakes

• Choosing by the label CFM alone. The rated airflow is under test conditions. Real ducts reduce it, so add margin for long runs.
• Using the room area for a large bathroom. Above 100 square feet, the area rule gives a number that is too high for the room, and the guideline switches to counting fixtures.
• Forgetting makeup air for a big range hood. A strong hood pulls air from the house, and the code requires makeup air above a threshold. See the answer above.

Sources and related calculators

Bathroom and hood guidelines: Home Ventilating Institute (HVI) recommendations; makeup air: International Residential Code; residential ventilation: ASHRAE Standard 62.2. Air changes: standard volume-flow arithmetic. Related: BTU calculator, attic ventilation calculator and ventilation rate calculator.