Range Hood CFM Calculator

Range hood CFM calculator: two methods, one answer

A range hood CFM calculator gives the airflow your hood should move. For a gas range, add the burner BTUs and divide by 100. For any range, allow 100 CFM per linear foot of width under a wall hood and 150 CFM per foot under an island hood. A 36 in gas range with 48,000 BTU needs 480 CFM, because the BTU number is larger than the 300 CFM width number.

The formulas in plain words

Think of the burners as a kettle boiling over. A bigger flame sends up more steam, so the fan needs to pull harder.

1. BTU method (gas): CFM = total burner BTU per hour ÷ 100.
2. Width method: CFM = range width in feet × 100 for a wall hood, or × 150 for an island hood.
3. Recommended CFM = the larger of the two for a gas range. For electric or induction, only the width method applies.
4. Makeup air check: if the result is over 400 CFM, look at your code for a makeup air rule.

Worked examples

These match the calculator exactly.

Example 1: a 36 in gas range, 48,000 BTU, wall hood.

• BTU method: 48,000 ÷ 100 = 480 CFM
• Width method: 36 ÷ 12 = 3 ft × 100 = 300 CFM
• Recommended: 480 CFM
• Makeup air: over 400 CFM, so check your code

Example 2: a 30 in electric cooktop, island hood.

• Width method: 30 ÷ 12 = 2.5 ft × 150 = 375 CFM
• Recommended: 375 CFM
• Makeup air: under 400 CFM, so usually no trigger

Reference table

Range width	Wall hood (100 CFM per ft)	Island hood (150 CFM per ft)
30 in	250 CFM	375 CFM
36 in	300 CFM	450 CFM
48 in	400 CFM	600 CFM

Gas ranges often exceed these numbers on the BTU method. A 36 in gas range with 60,000 BTU needs 600 CFM.

Makeup air and the 400 CFM line

A hood pulls air out of the house, and that air has to come back in. The 2009 to 2015 editions of the IRC required makeup air for kitchen exhaust systems capable of moving more than 400 CFM. The 2018 edition (M1503.6) ties the rule to homes with a fuel-burning appliance that is neither direct-vent nor mechanically drafted inside the air barrier. A hood over 400 CFM then needs makeup air at about the exhaust rate. Local codes differ. Proline Range Hoods calls the idea that every hood over 400 CFM needs makeup air a myth and says it depends on your code, so ask your building department.

How to measure

• Add up the BTU rating for each burner, from the range manual or the burner label.
• Measure the width of the cooktop in inches.
• Note whether the hood goes on a wall or over an island.
• Measure the duct run and count the elbows, since both reduce airflow.

Common mistakes

• Reading the hood’s maximum CFM at its loudest setting as a target. Pick a hood you will actually run.
• Using the wall rule for an island.
• Ignoring duct losses. A long run with elbows cuts the airflow.
• Skipping makeup air on a powerful hood in a tight house.
• Choosing too much. Large hoods are loud and pull conditioned air out of the house.

Sources

• Proline Range Hoods, CFM guide: gas BTU ÷ 100, electric width × 10, and duct allowances of 1 CFM per foot, 25 CFM per turn, 40 CFM for a roof cap (prolinerangehoods.com/blogs/blog/cfm-need-range-hood).
• Continental Fan, residential range hood guidance based on HVI: 100 CFM per foot for wall hoods, 150 CFM per foot for island hoods (continentalfan.com).
• Fine Homebuilding, The Basics of Makeup Air: 400 CFM threshold across the 2009 to 2018 IRC editions (finehomebuilding.com/project-guides/kitchens/the-basics-of-makeup-air).
• Fantech catalog quoting IRC M1503.6 (2018 text).

We could not read the HVI document directly or the full IRC text, so confirm the code section and edition with your local jurisdiction.

Related calculators

Size the heating load with the BTU calculator. Choose a duct with the duct size calculator. A tight home also loses heat through the hood, so see the heat loss calculator.