Filter Pressure Drop Calculator

Filter Inputs

Filter Type Defaults are published data for one representative product of each type (see the table below).

Rated Clean ΔP (in. w.c.) Initial resistance at the rated face velocity of 500 fpm — replace with your submittal value.

Change-out (Dirty) ΔP (in. w.c.) The loaded-filter resistance at which you will replace the filters. Size the fan on this.

Face Area per Filter (ft²) Nominal face of ONE filter, e.g. 24×24 in = 4 ft², 24×12 in = 2 ft²

Airflow Through the Bank (CFM)

Number of Filters in Bank

Results

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Clean ΔP (in. w.c.)

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Change-out ΔP (in. w.c.) — fan sizing

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Mid-life ΔP (in. w.c.) — energy

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Face Velocity (FPM)

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Total Bank Face Area (ft²)

Representative Filter Data

Filter	Clean ΔP	Rated at	Change-out	Published source
MERV 8, 2" pleated	0.29"	500 fpm	1.0"	AAF PerfectPleat SC M8 0.27"; Camfil 30/30 0.31"
MERV 8, 4" pleated	0.23"	500 fpm	1.0"	AAF PerfectPleat SC M8
MERV 11, 2" pleated	0.34"	500 fpm	1.0"	AAF PREpleat M11 SC
MERV 11, 22" bag	0.24"	500 fpm	1.0"	Camfil Hi-Flo ES
MERV 13, 2" pleated	0.30"	500 fpm	1.0"	AAF PREpleat M13 (electret; mechanical-media MERV 13 pleats run higher)
MERV 13, 22" bag	0.38"	500 fpm	1.0"	Camfil Hi-Flo ES
MERV 14, 22" bag	0.45"	500 fpm	1.0"	Camfil Hi-Flo ES
MERV 14, 12" V-bank	0.29"	500 fpm	1.5"	Camfil Durafil ES2 (ASHRAE 52.2 test report)
MERV 15, 22" bag	0.62"	500 fpm	1.0"	Camfil Hi-Flo ES
HEPA 99.97%, 12" box	1.0"	250 fpm	2.0"	Typical standard-capacity rating (1,000 CFM per 24×24); change-out often 2× initial

Manufacturers’ published initial resistance and recommended final resistance. Products with the same MERV and depth differ by 30–50% or more, so use your submittal values for a final design. MERV is the ASHRAE 52.2 efficiency rating; 52.2 is a test method and sets no pressure drops. ASHRAE 62.1 requires at least MERV 8 upstream of cooling coils and other wetted surfaces.

Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This filter pressure drop calculator turns a filter type, airflow, and filter bank size into the face velocity and the pressure drop the fan has to overcome. Pick a filter type to load a representative product’s published initial resistance and recommended change-out resistance, or enter the values from your own submittal. Enter the airflow, the face area of one filter, and how many filters sit in parallel; the tool finds the face velocity and corrects the rated clean pressure drop to it.

Filters are often the largest single item in an air handler’s internal static pressure, and their resistance climbs as they load. Use the change-out pressure drop for fan selection so the system still delivers its airflow just before the filters are replaced, and the mid-life value for energy estimates. Sizing the bank for a sensible face velocity keeps the clean resistance low and stretches the time between change-outs.

Formula & Method

Face velocity	V (fpm) = Q (CFM) ÷ (A_filter × N)
Clean ΔP	ΔP_clean = ΔP_rated × (V ÷ V_rated)ⁿ
Exponent n	1.5 pleated panels · 1.4 bags and V-banks · 1.1 HEPA
Change-out ΔP	ΔP_dirty = your replacement set point (default: manufacturer’s recommended final resistance)
Mid-life ΔP	ΔP_mid = (ΔP_clean + ΔP_dirty) ÷ 2

Face velocity is the airflow divided by the total face area of the parallel filters. The default ratings are published initial resistances of representative AAF and Camfil products at 500 fpm (HEPA at 250 fpm), not values from a standard; products of the same MERV and depth vary widely, so replace them with your submittal data. The velocity exponents come from fitting manufacturers’ resistance-versus-velocity data (AAF PerfectPleat SC M8 2" and 4": about 1.5 between 300 and 625 fpm; Camfil Durafil ES2 V-bank: about 1.4); HEPA media flow is close to laminar, so 1.1 is used. The change-out pressure drop is a maintenance set point, so it is taken as entered and not scaled with velocity. Ratings are at standard air; at altitude the same actual CFM gives a slightly lower pressure drop, so the result is conservative. MERV is the ASHRAE 52.2 test rating, which specifies efficiency, not pressure drop.

Frequently Asked Questions

What face velocity should an air filter be sized for?

Most commercial pleated, bag and rigid filters are rated at 500 fpm face velocity, and designing the bank for about 300 to 500 fpm is common practice. Lower face velocity means lower pressure drop and longer filter life but needs more face area. Standard-capacity HEPA filters are rated at about 250 fpm. The calculator warns when the face velocity is above the selected filter’s rated velocity, because the clean pressure drop is then an extrapolation.

Should I use clean or dirty pressure drop to size the fan?

Size the fan and the external static pressure on the change-out (dirty) pressure drop, the loaded-filter resistance at which the filters will be replaced, because that is the worst case the fan must overcome. Manufacturers publish a recommended final resistance, typically 1.0 in. w.c. for pleated and bag filters, 1.5 in. for rigid V-bank filters and about 2 times the initial for HEPA, and the tool fills that in as the default. The mid-life value, the average of clean and change-out, is a reasonable basis for annual fan energy.

How does pressure drop change with airflow?

Filter pressure drop rises a little faster than airflow. Published resistance curves for 2 and 4 in. pleated filters fit pressure drop proportional to face velocity to about the 1.5 power, a 12 in. V-bank filter to about the 1.4 power, and HEPA media is closer to linear, so the tool uses 1.5, 1.4 and 1.1. Doubling the face velocity of a pleated filter therefore raises its clean pressure drop about 2.8 times. The change-out pressure drop is a set point you choose, so it does not scale with velocity.

Why does a higher MERV rating cost more static pressure?

Higher-MERV media captures smaller particles with finer or denser fibers, which usually raises resistance at the same face velocity, and deeper filters with more media area bring it back down. In published data a 2 in. MERV 8 pleated filter starts near 0.27 to 0.31 in. w.c. at 500 fpm, while 22 in. bags run about 0.24 in. at MERV 11 to 0.62 in. at MERV 15. MERV is the efficiency rating from the ASHRAE 52.2 test; the standard does not set pressure drops, so use the manufacturer’s data for the product you specify.

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.

Filter Pressure Drop Estimator