Negative Pressure Room Calculator
Exhaust offset needed to hold a room negative.
Enter your values
ⓘ Live results
Results
- Required exhaust airflow
- 418 CFM
- Offset above supply
- 18 CFM
- Estimated leakage through gaps
- 18 CFM
Quick answer
A room typically needs 50-125 CFM more exhaust than supply airflow to reliably hold negative pressure against door leakage.
Exhaust = Supply + offset CFM
How to use the Negative Pressure Room Calculator
- 1Estimate the total leakage path area — door undercuts, gaps and grilles.
- 2Set the target pressure differential, typically 0.01 in wc (2.5 Pa) for isolation rooms.
- 3Add the resulting offset airflow onto the supply airflow to size the exhaust fan.
Worked example
Using the values the calculator loads with:
Inputs
- Supply airflow400 CFM
- Door undercut gap area10 sq in
- Target pressure differential0.01 in wc
Results
- Required exhaust airflow418 CFM
- Offset above supply18 CFM
- Estimated leakage through gaps18 CFM
Frequently asked questions
What is a typical offset?
50-125 CFM is common for single-door rooms; anterooms reduce the needed offset.
How is this verified?
A manometer or smoke tube check at the door confirms real-world negative pressure once installed.
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.