Why use a square-law estimate?

Many airflow resistances approximately scale with flow squared over a limited range.

What is rated final resistance?

It is the manufacturer-recommended pressure loss at which a disposable filter should be replaced.

Can this compare unrelated filters?

No. Use tested curves for each exact model and size.

Why calculate face velocity?

Pressure drop and performance depend partly on airflow per face area.

Air Filter Pressure Drop Calculation

Estimate filter pressure drop at a new airflow from a rated test point, face dimensions, and loading factor. Review assumptions, limitations and current source information before using the result.

Reference Airflow

Pressure Drop at Reference Airflow

Target Airflow

Filter Face Width

Filter Face Height

Loading or Design Multiplier

Estimated Loaded Design Drop

0.36 in. w.g.

Estimated Clean Pressure Drop: 0.288
Estimated Clean Pressure Drop in Pascals: 71.7
Target Face Velocity: 345.6
Target-to-Reference Flow Ratio: 1.2
Clean Pressure Drop Increase: 44

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This is an approximation between operating points. A manufacturer pressure-drop curve is preferred.

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How to Use This Calculator

Use a manufacturer or measured reference point for the exact filter, then enter target airflow, face size, and a documented loading factor.

Formula & Methodology

The calculator uses square-law scaling: new drop equals reference drop multiplied by the airflow ratio squared.

Worked example

A filter at 0.20 in. w.g. and 1,000 CFM is estimated near 0.288 in. w.g. at 1,200 CFM before applying the loading multiplier.

Real behavior can be nonlinear and changes with media, pleats, dust loading, humidity, bypass, and velocity distribution.

HVAC screening estimate only. Verify clean and final resistance and confirm blower airflow at total external static pressure.

Related Guides

How to Estimate Air Filter Pressure Drop at Different Airflows
Estimate static pressure drop across air filters at different airflow rates using square-law scaling from a known reference point. Calculate how filter loading increases resistance in HVAC and industrial filtration systems.

Frequently Asked Questions

Why use a square-law estimate?: Many airflow resistances approximately scale with flow squared over a limited range.
What is rated final resistance?: It is the manufacturer-recommended pressure loss at which a disposable filter should be replaced.
Can this compare unrelated filters?: No. Use tested curves for each exact model and size.
Why calculate face velocity?: Pressure drop and performance depend partly on airflow per face area.

Published 8/18/2026