Door Undercut Airflow Calculator
Airflow capacity of a door undercut gap.
Enter your values
Enter the door width in in.
Enter the undercut height in in.
Enter the flow coefficient in CFM/sq in.
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.
- Gap area
- 24 sq in
- Approx airflow capacity
- 600 CFM
- Recommended undercut for 100 CFM
- 0.13 in
Quick answer
A door undercut passes roughly 25-27 CFM per square inch of gap area for the small pressure differences typical of home HVAC systems.
CFM ≈ area (sq in) x flow coefficient
At a glance
| What it does | Airflow capacity of a door undercut gap. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Door width, Undercut height, Flow coefficient |
| Main output | Gap area |
| Formula | CFM ≈ area (sq in) x flow coefficient |
| Cost | Free — no sign-up, no download |
How to use the Door Undercut Airflow Calculator
- 1Measure the door width and the gap left under the door.
- 2Multiply the two for the open gap area.
- 3Apply the flow coefficient to estimate the CFM the gap can relieve at typical residential pressures.
Inputs explained
- Door width(in)
- Enter the door width in in.
- Undercut height(in)
- Enter the undercut height in in.
- Flow coefficient(CFM/sq in)
- Enter the flow coefficient in CFM/sq in.
Worked example
Using the values the calculator loads with:
Inputs
- Door width32 in
- Undercut height0.75 in
- Flow coefficient25 CFM/sq in
Results
- Gap area24 sq in
- Approx airflow capacity600 CFM
- Recommended undercut for 100 CFM0.13 in
Frequently asked questions
How big should an undercut be?
About 3/4 to 1 inch is common, but a dedicated transfer grille or jumper duct is more reliable for higher airflow rooms.
What if the room still feels stuffy?
The undercut may be too small for the supply CFM — add a transfer grille or jumper duct instead.
What do I need to enter into the Door Undercut Airflow Calculator?
Just 3 values: door width, undercut height, flow coefficient. 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 Door Undercut Airflow Calculator work out the answer?
A door undercut passes roughly 25-27 CFM per square inch of gap area for the small pressure differences typical of home HVAC systems. It applies the formula CFM ≈ area (sq in) x flow coefficient and shows the working so you can check each step by hand.
Is the Door Undercut Airflow 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.