Door Undercut Airflow Calculator
Airflow capacity of a door undercut gap.
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ⓘ Live results
Results
- 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
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.