Filter Pressure Drop Calculator
Face velocity and pressure loss across an air filter.
Enter your values
Enter the airflow in CFM.
Enter the filter width in in.
Enter the filter height in in.
Enter the clean drop at 300 fpm 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.
- Face area
- 3.47 sq ft
- Face velocity
- 346 FPM
- Estimated clean drop
- 0.185 in wc
- Dirty drop (2x)
- 0.371 in wc
- Verdict
- Good sizing
Quick answer
Filter face velocity is airflow divided by filter face area; staying near 300 FPM keeps pressure drop low.
face velocity = CFM / (w x h / 144)
At a glance
| What it does | Face velocity and pressure loss across an air filter. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Filter width, Filter height, Clean drop at 300 FPM |
| Main output | Face area |
| Formula | face velocity = CFM / (w x h / 144) |
| Cost | Free — no sign-up, no download |
How to use the Filter Pressure Drop Calculator
- 1Enter system airflow and the nominal filter dimensions.
- 2Aim for a face velocity around 300 FPM or lower.
- 3If velocity is high, add a second filter grille or go to a deeper media filter.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Filter width(in)
- Enter the filter width in in.
- Filter height(in)
- Enter the filter height in in.
- Clean drop at 300 FPM(in wc)
- Enter the clean drop at 300 fpm in in wc.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow1200 CFM
- Filter width20 in
- Filter height25 in
- Clean drop at 300 FPM0.15 in wc
Results
- Face area3.47 sq ft
- Face velocity346 FPM
- Estimated clean drop0.185 in wc
- Dirty drop (2x)0.371 in wc
- VerdictGood sizing
Frequently asked questions
Why do high-MERV filters cause trouble?
They restrict more, so an undersized filter grille pushes static pressure past the blower's limit.
Rule of thumb for size?
Allow about 2 sq ft of filter face per 400 CFM, or 4 sq ft per ton.
What do I need to enter into the Filter Pressure Drop Calculator?
Just 4 values: airflow, filter width, filter height, clean drop at 300 fpm. 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 Filter Pressure Drop Calculator work out the answer?
Filter face velocity is airflow divided by filter face area; staying near 300 FPM keeps pressure drop low. It applies the formula face velocity = CFM / (w x h / 144) and shows the working so you can check each step by hand.
Is the Filter Pressure Drop 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.