Building Negative Pressure Calculator
Net pressure balance from supply and exhaust.
Enter your values
Enter the total supply air in CFM.
Enter the total exhaust air in CFM.
Enter the building volume in cu ft.
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.
- 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
At a glance
| What it does | Net pressure balance from supply and exhaust. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total supply air, Total exhaust air, Building volume |
| Main output | Net airflow |
| Formula | net = supply CFM − exhaust CFM |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Total supply air(CFM)
- Enter the total supply air in CFM.
- Total exhaust air(CFM)
- Enter the total exhaust air in CFM.
- Building volume(cu ft)
- Enter the building volume in cu ft.
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.
What do I need to enter into the Building Negative Pressure Calculator?
Just 3 values: total supply air, total exhaust air, building volume. 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 Building Negative Pressure Calculator work out the answer?
A building runs negative when exhaust exceeds supply, drawing unconditioned air in through every crack. It applies the formula net = supply CFM − exhaust CFM and shows the working so you can check each step by hand.
Is the Building Negative Pressure 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.