Pa vs inH₂O for Filter Pressure Drop
By Scientific Converters · Updated · Factors from NIST SP 811 and the BIPM SI Brochure
Short answer: One conventional inch of water is exactly 249.08891 Pa, so filter pressure drops convert at about 250 Pa per inH₂O: 125 Pa ≈ 0.50 inH₂O and 1.0 inH₂O ≈ 249 Pa.
The exact factor
The conventional inch of water is the pressure of a 1-inch column of water with a density of 1000 kg/m³ under standard gravity: 1000 × 9.80665 × 0.0254 = 249.08891 Pa, exactly. So 1 Pa = 0.0040146308 inH₂O. Filter datasheets from Europe and Asia list pressure drop in pascals; US datasheets, magnehelic gauges and building controls use inches of water.
Which inch of water?
NIST SP 811 lists three variants: the conventional inch of water (249.08891 Pa), water at 39.2 °F / 4 °C (249.082 Pa) and water at 60 °F (248.84 Pa). They differ by at most 0.1 %, which is far below the accuracy of a filter gauge. Use the conventional value unless a specification names a temperature. Millimetres of water (mmH₂O, mmAq) use the same density, so 1 inH₂O = 25.4 mmH₂O exactly.
Typical filter pressure drops
- Clean pleated HVAC filter: about 50–100 Pa (0.2 inH₂O to 0.4 inH₂O) at rated airflow.
- Change-out point: often about 250 Pa (1 inH₂O), or as the manufacturer specifies; final values of 450 Pa or more are used for fine filters.
- HEPA filter, clean: about 250 Pa (1 inH₂O) at rated face velocity.
- Cleanroom room-to-room differential: commonly 5–20 Pa (0.02 inH₂O to 0.08 inH₂O).
Values are typical design figures; always use the manufacturer's data and the standard that applies to your installation.
Quick mental conversions
- Pa → inH₂O: divide by 250 (error 0.4 %). 125 Pa ≈ 0.5 inH₂O.
- inH₂O → Pa: multiply by 250. 0.3 inH₂O ≈ 75 Pa.
- mmH₂O → Pa: multiply by 10 (error 2 %). The exact factor is 9.80665.
- mbar → Pa: multiply by 100 (exact). 1 mbar ≈ 0.4 inH₂O.
Pa to inH₂O table
| Pa | inH₂O | mmH₂O | mbar |
|---|---|---|---|
| 5 Pa | 0.02007 inH₂O | 0.5099 mmH₂O | 0.05 mbar |
| 10 Pa | 0.04015 inH₂O | 1.02 mmH₂O | 0.1 mbar |
| 12.5 Pa | 0.05018 inH₂O | 1.275 mmH₂O | 0.125 mbar |
| 25 Pa | 0.1004 inH₂O | 2.549 mmH₂O | 0.25 mbar |
| 50 Pa | 0.2007 inH₂O | 5.099 mmH₂O | 0.5 mbar |
| 75 Pa | 0.3011 inH₂O | 7.648 mmH₂O | 0.75 mbar |
| 100 Pa | 0.4015 inH₂O | 10.2 mmH₂O | 1 mbar |
| 125 Pa | 0.5018 inH₂O | 12.75 mmH₂O | 1.25 mbar |
| 150 Pa | 0.6022 inH₂O | 15.3 mmH₂O | 1.5 mbar |
| 200 Pa | 0.8029 inH₂O | 20.39 mmH₂O | 2 mbar |
| 250 Pa | 1.004 inH₂O | 25.49 mmH₂O | 2.5 mbar |
| 300 Pa | 1.204 inH₂O | 30.59 mmH₂O | 3 mbar |
| 350 Pa | 1.405 inH₂O | 35.69 mmH₂O | 3.5 mbar |
| 450 Pa | 1.807 inH₂O | 45.89 mmH₂O | 4.5 mbar |
| 500 Pa | 2.007 inH₂O | 50.99 mmH₂O | 5 mbar |
| 750 Pa | 3.011 inH₂O | 76.48 mmH₂O | 7.5 mbar |
| 1000 Pa | 4.015 inH₂O | 102 mmH₂O | 10 mbar |