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Calculation Machine

Filter Pressure Drop Calculator

Face velocity and pressure loss across an air filter.

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

Your values

ⓘ Live results

Results
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.

The formula

face velocity = CFM / (w x h / 144)

Worked example

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

Result

  • Face area3.47 sq ft
  • Face velocity346 FPM
  • Estimated clean drop0.185 in wc
  • Dirty drop (2x)0.371 in wc
  • VerdictGood sizing

How to use the Filter Pressure Drop Calculator

  1. 1Enter system airflow and the nominal filter dimensions.
  2. 2Aim for a face velocity around 300 FPM or lower.
  3. 3If velocity is high, add a second filter grille or go to a deeper media filter.

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.

About this calculator