FreeHVACCalculators

Start here

Loading calculators…

Menu

Air Filter Pressure Drop Calculator

See how fast air moves through your filter, what its clean pressure drop is at your airflow, and — from a manometer reading — how close it is to needing a change.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Unit system

The face velocity at which the maker lists the clean (initial) pressure drop. Example values throughout — use your filter’s data.

Result

Face velocity
346fpm
Clean pressure drop at your airflow
0.303in. w.c.

346 fpm through the filter; a clean filter drops about 0.303 in. w.c. at this airflow.

Exponent used
1.357
Face area
3.47ft²
Show the working
  1. Face velocity = CFM ÷ face area
    1,200 ÷ 3.472 ft² = 346 fpm
  2. Exponent from the two rated points n = ln(ΔP₂/ΔP₁) ÷ ln(V₂/V₁)
    ln(0.5 ÷ 0.25) ÷ ln(500 ÷ 300) = 1.3569
  3. Clean drop at your velocity = ΔP₁ × (V ÷ V₁)^n
    0.25 × (346 ÷ 300)^1.357 = 0.303 in. w.c.

Face velocity and clean drop

Face velocity is airflow ÷ face area. Filter makers rate the clean (initial) pressure drop at one or more face velocities. The drop rises with velocity following a power law, ΔP ∝ V^n. Enter two rated points and the calculator fits n; or enter n yourself. Duct components follow the square law (n = 2) in DOE’s fan sourcebook; fibrous filter media usually rise less steeply.

How loaded is it?

Makers also give a recommended final pressure drop: the point to change the filter. Measure the drop across the filter with a manometer. The calculator corrects your reading to the rated velocity and shows how much of the range between clean and final has been used, and the change point at your own airflow.

Why it matters

A loaded or undersized filter raises the static pressure the blower works against, which cuts airflow and can raise blower energy. A bigger filter face lowers velocity and pressure drop together.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Face velocity and clean pressure drop
Filter face width
20 in
Filter face height
25 in
Airflow through the filter
1,200 CFM
Rated face velocity
300 fpm
Clean (initial) pressure drop at that velocity
0.25 in. w.c.
How the drop changes with airflow
From a second rated point (most accurate)
Second rated face velocity
500 fpm
Clean pressure drop at the second velocity
0.5 in. w.c.

Result

Face velocity
346 fpm
Clean pressure drop at your airflow
0.303 in. w.c.
Exponent used
1.357
Face area
3.47 ft²

346 fpm through the filter; a clean filter drops about 0.303 in. w.c. at this airflow.

Assumptions

  • Pressure drop follows a power law in face velocity, ΔP ∝ V^n, fitted to the maker’s two rated points or to the exponent you enter. The square law (n = 2) is DOE’s component equation; filter media are often nearer n = 1.
  • Face velocity uses the nominal face area (width × height).

Limitations

  • Rated and final drops are manufacturer data; the site publishes none.
  • A loaded filter does not always follow the same exponent as a clean one.

Frequently asked questions

What face velocity should a filter have?

Use the velocity at which your filter is rated as the reference; the drop rises steeply above it. For filter grilles, ACCA’s Manual D checklist lists 300 fpm as the face-velocity limit.

When should I change my filter by pressure drop?

When the drop reaches the final resistance the filter maker recommends (corrected for your airflow), or earlier if your system’s static pressure budget is tight.

My measured drop is lower than a clean filter’s. Why?

Usually air is going around the filter through gaps, the filter is torn or missing, or the probes are in the wrong place.

Sources

  1. Improving Fan System Performance: A Sourcebook for Industry — U.S. DOE (Industrial Technologies Program) with AMCA International. Component pressure loss Δp = C·ρ·(V/1,097)² (in. wg, V in fpm, ρ = 0.075 lb/ft³ standard), giving a parabolic system curve; system curves can have a static component; sheave changes alter fan speed; airflow is linear with speed and power is cubed.
  2. Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).

Spotted an error? Report a problem with this calculator — see our corrections policy.