Building Negative Pressure Calculator
Net pressure balance from supply and exhaust.
Enter your values
ⓘ Live results
Results
- Net airflow
- -150 CFM
- Pressure state
- Negative
- Equivalent air changes
- 0.56 ACH of infiltration
- Recommended correction
- Add 150 CFM of make-up air
Quick answer
A building runs negative when exhaust exceeds supply, drawing unconditioned air in through every crack.
net = supply CFM − exhaust CFM
How to use the Building Negative Pressure Calculator
- 1Add every supply fan and ventilator to get total supply.
- 2Add every exhaust fan, hood and dryer to get total exhaust.
- 3Aim for slightly positive in humid climates and neutral elsewhere.
Worked example
Using the values the calculator loads with:
Inputs
- Total supply air800 CFM
- Total exhaust air950 CFM
- Building volume16000 cu ft
Results
- Net airflow-150 CFM
- Pressure stateNegative
- Equivalent air changes0.56 ACH of infiltration
- Recommended correctionAdd 150 CFM of make-up air
Frequently asked questions
Is slight negative pressure bad?
Small imbalances are normal, but sustained negative pressure pulls in humid or polluted air and risks back-drafting.
Should hospitals be positive?
Clean spaces are kept positive; isolation and soiled rooms are deliberately negative.
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.