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Calculation Machine

Make-Up Air CFM Calculator

Replacement air for powerful exhaust fans.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Make-up air required
800 CFM
Duct free area
230 sq in
Round duct diameter
17.1 in
Heating load of that air at 50 °F rise
43,200 BTU/h

Quick answer

Make-up air should replace roughly 80–100% of the exhausted air once natural infiltration is accounted for.

The formula

make-up CFM = exhaust CFM − natural infiltration

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Total exhaust900 CFM
  • Natural infiltration allowance100 CFM
  • Duct velocity for make-up500 FPM

Result

  • Make-up air required800 CFM
  • Duct free area230 sq in
  • Round duct diameter17.1 in
  • Heating load of that air at 50 °F rise43,200 BTU/h

How to use the Make-Up Air CFM Calculator

  1. 1Add up every exhaust device that can run at once — hood, bath fans, dryer.
  2. 2Subtract a small natural infiltration allowance for leaky homes.
  3. 3Size a powered or passive make-up air path for the balance.

Frequently asked questions

What happens without make-up air?

The house goes negative, doors get hard to open and combustion appliances can back-draft carbon monoxide.

Does make-up air need heating?

In cold climates yes, otherwise you dump freezing air into the kitchen every time the hood runs.

About this calculator