SettingsSettings for this calculationUS
The filter's pressure drop when new and clean, per the manufacturer's data sheet.
From the sheet, at the filter's RATED face velocity. A bank pushed faster than rated starts above its published clean figure, so a system designed to the sheet value while moving more air than the sheet assumed begins life already part of the way toward its change-out point — and the first filter change arrives sooner than the maintenance budget expects.
The pressure drop at which the manufacturer recommends replacing the filter.
A recommendation rather than a failure point: past it the filter still filters, but the fan energy needed to push through it has begun to cost more than a replacement, and on a HEPA the medium itself can be stressed. Plenty of facilities set a tighter figure than the maker's for energy reasons, and a cleanroom may set one tighter still to protect its air-change rate.
Your estimate of how far the filter has progressed from clean (0%) to final loading (100%).
The honest way to get this is to MEASURE the current pressure drop and interpolate between the two figures above — not to reason from how long the filter has been in. Loading depends on what the air actually carries, so a filter in a clean space can be barely used when its calendar age says otherwise, and one downstream of a construction area can be finished in weeks.
Estimated current pressure drop
1.5 in. w.g.
Filter pressure drop is unique to each manufacturer's media and construction — always use the actual filter's data sheet for the rated clean and final pressure drop values, not an assumed default. This calculator only linearly interpolates between the two rated values you supply; real filter loading curves are not perfectly linear.
They open the calculator with your figures already in it
HEPA Filter Bank Pressure Drop Estimator: 1.5 in. w.g. — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Filter pressure drop is inherently manufacturer- and media-specific — there is no general industry formula analogous to a duct friction chart. This calculator interpolates between the filter manufacturer's rated clean (initial) pressure drop and its rated final (replacement) pressure drop, based on a user-estimated loading percentage: current ΔP = clean ΔP + (final ΔP − clean ΔP) × (loading % ÷ 100). Typical standard HEPA filters are commonly rated around 0.5-1.5 in. w.g. clean and roughly double that at final loading, but the specific filter's data sheet governs — these are not universal constants.
Inputs used
- Manufacturer's Rated Clean (Initial) Pressure Drop (in. w.g.)
- 1
- Manufacturer's Rated Final (Replacement) Pressure Drop (in. w.g.)
- 2
- Estimated Dust Loading (%)
- 50
Confidence note: Filter pressure drop is unique to each manufacturer's media and construction — always use the actual filter's data sheet for the rated clean and final pressure drop values, not an assumed default. This calculator only linearly interpolates between the two rated values you supply; real filter loading curves are not perfectly linear.
What this calculation does not cover
- Airflow appears nowhere in this, and the rated clean and final figures are quoted at the filter's rated face velocity. Media resistance rises with flow through the pack, near enough proportionally, so the same bank running 25 percent above rated flow reads about 25 percent higher than predicted here — and a room whose fans are chasing an air-change rate is rarely sitting exactly at rated.
- This is one filter, not the resistance the fan has to overcome. Housing, frame, the prefilter ahead of it and the approach ductwork all add drop, and the prefilter's condition is what governs how quickly the HEPA loads in the first place. A fan or a drive sized on this number alone runs out of head before the HEPA ever reaches its final rating.
- Pressure drop is not the only reason a HEPA comes out. In a classified space a filter is also replaced when it fails an integrity scan, and a pinhole in the media, a damaged gasket or a poor seal at the frame will pass particles while the differential pressure reads perfectly normal. Change-out decided on this figure alone can leave a leaking filter in a room that has to be clean.
Add the equipment this sizes
This result is a specification — 1.5 in. w.g. — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Filter pressure drop is inherently manufacturer- and media-specific — there is no general industry formula analogous to a duct friction chart. This calculator interpolates between the filter manufacturer's rated clean (initial) pressure drop and its rated final (replacement) pressure drop, based on a user-estimated loading percentage: current ΔP = clean ΔP + (final ΔP − clean ΔP) × (loading % ÷ 100). Typical standard HEPA filters are commonly rated around 0.5-1.5 in. w.g. clean and roughly double that at final loading, but the specific filter's data sheet governs — these are not universal constants.
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.