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Calculation Machine

Negative Pressure Room Calculator

Exhaust offset needed to hold a room negative.

Reviewed by the Calculation Machine team · How we check our maths

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.

The formula

Exhaust = Supply + offset CFM

Worked example

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

Result

  • Required exhaust airflow418 CFM
  • Offset above supply18 CFM
  • Estimated leakage through gaps18 CFM

How to use the Negative Pressure Room Calculator

  1. 1Estimate the total leakage path area — door undercuts, gaps and grilles.
  2. 2Set the target pressure differential, typically 0.01 in wc (2.5 Pa) for isolation rooms.
  3. 3Add the resulting offset airflow onto the supply airflow to size the exhaust fan.

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