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Filter Pressure Drop Calculator

Work out filter pressure drop instantly with clear inputs, formula shown and shareable results.

About Filter Pressure Drop Calculator

Filter resistance is published at a 500 fpm face velocity and rises with roughly the 1.6 power of velocity for pleated media. Dust loading then multiplies that clean value, typically by two before change-out. The extra pressure is paid for in fan power for every hour the filter stays in place.

Formulas

Face velocity

V = CFM / face area ft²

Loaded drop

Δp = rated drop x (V / 500)^1.6 x loading factor

Fan penalty

kW = CFM x Δp extra / (6356 x fan efficiency) x 0.7457

Worked example

With the default values below, Filter Pressure Drop Calculator returns Pressure drop when loaded of 0.500. Change any input above and every figure here is recalculated in your browser.

Inputs used

Airflow through filterminimum 1 CFM
2,000 CFM
Filter face areaminimum 0.1 ft²
4 ft²
Clean drop at 500 fpm ratingminimum 0.01 in.wg
0.25 in.wg
Dirty-to-clean loading factorminimum 1
2

Results

Pressure drop when loaded
0.500 in.wg
Face velocity
500 fpm
Clean drop at this velocity
0.250 in.wg
Extra fan power when loaded
0.098 kW

Step by step

  1. 1Values used
  2. 2Face velocity
  3. 3Loaded drop
  4. 4Fan penalty
  5. 5Pressure drop when loaded= 0.500 in.wg
  6. 6Face velocity= 500 fpm
  7. 7Clean drop at this velocity= 0.250 in.wg
  8. 8Extra fan power when loaded= 0.098 kW

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Filter Pressure Drop figure worked out?

V = CFM / face area ft². Filter resistance is published at a 500 fpm face velocity and rises with roughly the 1.6 power of velocity for pleated media.

When should a filter be changed?

Common practice is at twice the clean pressure drop, or at the manufacturer's stated final resistance, whichever comes first. Waiting longer starves the coil of airflow.

What values do I need to enter?

This calculator takes 4 inputs: Airflow through filter, Filter face area, Clean drop at 500 fpm rating, Dirty-to-clean loading factor. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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