Estimate clean and dirty filter pressure drop from face velocity and MERV rating. Plan for replacement schedule and fan oversizing.
| MERV | Clean ΔP | Dirty ΔP | Application |
|---|---|---|---|
| MERV 1–4 | 0.05" | 0.12" | Residential, pre-filter only |
| MERV 8 | 0.10" | 0.25" | Commercial pre-filter, pleated |
| MERV 11 | 0.18" | 0.40" | Allergy, schools, light commercial |
| MERV 13 | 0.25" | 0.55" | ASHRAE 170 healthcare, offices |
| MERV 14 | 0.30" | 0.70" | High-end commercial, hospitals |
| MERV 16 | 0.50" | 1.10" | Labs, cleanrooms, near-HEPA |
| HEPA (MERV 17+) | 0.80" | 2.00" | OR, cleanroom, pharmaceutical |
This filter pressure drop estimator turns a MERV rating, airflow, and filter bank area into the clean, dirty, and design pressure drop across an air filter, plus the face velocity. Enter the airflow in CFM, the face area of one filter in square feet, and how many filters sit in parallel; the tool spreads the airflow over the total area to find face velocity, then scales the filter's rated pressure drop to that velocity.
Filter pressure drop is a real chunk of the fan's external static pressure, and it grows as the media loads with dust. Sizing the bank for a sensible face velocity keeps that penalty low, extends time between change-outs, and avoids oversizing the fan. Use the dirty pressure drop for fan selection so the system still delivers airflow at the end of the filter's life.
| Face velocity | V (fpm) = Q (CFM) ÷ (Afilter × N) |
| Velocity scaling | r = (V ÷ 500)1.8 |
| Clean ΔP | ΔPclean = ΔPclean,rated · r |
| Dirty ΔP | ΔPdirty = ΔPdirty,rated · r |
| Design ΔP | ΔPdesign = (ΔPclean + ΔPdirty) ÷ 2 |
The face area A times the filter count N gives the total free area the air passes through, and airflow divided by that area is the face velocity. The rated clean and dirty pressure drops are tabulated at a 500 fpm nominal face velocity; the tool corrects them to the actual velocity using the exponent 1.8, because filter pressure drop rises roughly as velocity raised to the 1.8 power rather than linearly. The design value is the simple average of clean and dirty. Rated ΔP values and MERV bands follow standard ASHRAE / ASHRAE 52.2 MERV practice; confirm against the manufacturer's submittal for a final design.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.
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.