Negative Pressure Room Calculator
Exhaust offset needed to hold a room negative.
Enter your values
Enter the supply airflow in CFM.
Enter the door undercut gap area in sq in.
Enter the target pressure differential in in wc.
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.
- 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
At a glance
| What it does | Exhaust offset needed to hold a room negative. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Supply airflow, Door undercut gap area, Target pressure differential |
| Main output | Required exhaust airflow |
| Formula | Exhaust = Supply + offset CFM |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Supply airflow(CFM)
- Enter the supply airflow in CFM.
- Door undercut gap area(sq in)
- Enter the door undercut gap area in sq in.
- Target pressure differential(in wc)
- Enter the target pressure differential in in wc.
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.
What do I need to enter into the Negative Pressure Room Calculator?
Just 3 values: supply airflow, door undercut gap area, target pressure differential. 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 Negative Pressure Room Calculator work out the answer?
A room typically needs 50-125 CFM more exhaust than supply airflow to reliably hold negative pressure against door leakage. It applies the formula Exhaust = Supply + offset CFM and shows the working so you can check each step by hand.
Is the Negative Pressure Room 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.