Make-Up Air CFM Calculator
Replacement air for powerful exhaust fans.
Enter your values
Enter the total exhaust in CFM.
Enter the natural infiltration allowance in CFM.
Enter the duct velocity for make-up in FPM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
make-up CFM = exhaust CFM − natural infiltration
At a glance
| What it does | Replacement air for powerful exhaust fans. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total exhaust, Natural infiltration allowance, Duct velocity for make-up |
| Main output | Make-up air required |
| Formula | make-up CFM = exhaust CFM − natural infiltration |
| Cost | Free — no sign-up, no download |
How to use the Make-Up Air CFM Calculator
- 1Add up every exhaust device that can run at once — hood, bath fans, dryer.
- 2Subtract a small natural infiltration allowance for leaky homes.
- 3Size a powered or passive make-up air path for the balance.
Inputs explained
- Total exhaust(CFM)
- Enter the total exhaust in CFM.
- Natural infiltration allowance(CFM)
- Enter the natural infiltration allowance in CFM.
- Duct velocity for make-up(FPM)
- Enter the duct velocity for make-up in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Total exhaust900 CFM
- Natural infiltration allowance100 CFM
- Duct velocity for make-up500 FPM
Results
- 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
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.
What do I need to enter into the Make-Up Air CFM Calculator?
Just 3 values: total exhaust, natural infiltration allowance, duct velocity for make-up. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Make-Up Air CFM Calculator work out the answer?
Make-up air should replace roughly 80–100% of the exhausted air once natural infiltration is accounted for. It applies the formula make-up CFM = exhaust CFM − natural infiltration and shows the working so you can check each step by hand.
Is the Make-Up Air CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.