mmH₂O (mmAq) to Pa
Convert mmH₂O (mmAq), used for duct static and filter pressure drop, to Pa, kPa, inWG, mmHg and psi. Typical duct and air handler pressures are listed below in both units.
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What mmAq means
mmAq (mmH₂O, also written mmWC) is the pressure of a 1 mm water column. The conventional definition uses 1,000 kg/m³ and standard gravity 9.80665 m/s², so 1 mmH₂O = 1,000 × 9.80665 × 0.001 = 9.80665 Pa.
inWG (inH₂O) is a 25.4 mm column, 249.089 Pa. 1 mmHg is 133.322 Pa and 1 psi is 6,894.76 Pa. The 1 kgf/cm² used for pump head is a 10 m water column (10,000 mmH₂O) = 98.0665 kPa.
Static, velocity and total pressure
Inside a duct, total pressure = static + velocity pressure. Velocity pressure is Pv = ρV²/2: air at 1.2 kg/m³ and 10 m/s gives 1.2 × 10² ÷ 2 = 60 Pa = 6.1 mmH₂O. Fan static pressure is chosen from the sum of duct, filter, coil and outlet losses.
Typical duct and air handler pressures, reference only (mmH₂O and Pa)
Typical ranges for early estimates. Manufacturer data comes first for filters, coils and equipment.
| Item | mmAq mmH₂O | Pa | inWG inH₂O |
|---|---|---|---|
| Duct friction rate (low velocity, equal friction) | 0.08 ~ 0.15 /m | 0.785 ~ 1.47 /m | 0.0031 ~ 0.0059 /m |
| Room pressurization (room to corridor) | 0.5 ~ 2.5 | 4.9 ~ 24.5 | 0.02 ~ 0.098 |
| Diffuser or grille loss | 1.5 ~ 5 | 14.7 ~ 49 | 0.059 ~ 0.2 |
| Medium filter, clean | 5 ~ 15 | 49 ~ 147 | 0.2 ~ 0.59 |
| Medium filter, change-out | 20 ~ 30 | 196 ~ 294 | 0.79 ~ 1.2 |
| Cooling coil (wet) | 10 ~ 25 | 98.1 ~ 245 | 0.39 ~ 0.98 |
| Fan coil external static | 3 ~ 10 | 29.4 ~ 98.1 | 0.12 ~ 0.39 |
| Air handler fan static (low-velocity ducts) | 50 ~ 100 | 490 ~ 981 | 2 ~ 3.9 |
Pa to mmH₂O, inWG, mmHg and psi table
Pa values from SI data in other units (four significant digits). Same factors as the converter above.
| Pa | mmAq mmH₂O | inWG inH₂O | mmHg | psi | kPa |
|---|---|---|---|---|---|
| 1 | 0.102 | 0.004015 | 0.007501 | 0.000145 | 0.001 |
| 5 | 0.5099 | 0.02007 | 0.0375 | 0.0007252 | 0.005 |
| 10 | 1.02 | 0.04015 | 0.07501 | 0.00145 | 0.01 |
| 25 | 2.549 | 0.1004 | 0.1875 | 0.003626 | 0.025 |
| 50 | 5.099 | 0.2007 | 0.375 | 0.007252 | 0.05 |
| 100 | 10.2 | 0.4015 | 0.7501 | 0.0145 | 0.1 |
| 250 | 25.49 | 1.004 | 1.875 | 0.03626 | 0.25 |
| 500 | 50.99 | 2.007 | 3.75 | 0.07252 | 0.5 |
| 1,000 | 102 | 4.015 | 7.501 | 0.145 | 1 |
| 2,500 | 254.9 | 10.04 | 18.75 | 0.3626 | 2.5 |
| 5,000 | 509.9 | 20.07 | 37.5 | 0.7252 | 5 |
| 10,000 | 1,020 | 40.15 | 75.01 | 1.45 | 10 |
Sources
- NIST Special Publication 811 (2008), Appendix B: conventional factors for mmH₂O, inH₂O, mmHg and psi
- CGPM 1901, standard acceleration of gravity g = 9.80665 m/s²
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 21 Duct Design (static, velocity pressure and friction)
Frequently asked questions
What is a 50 mmH₂O fan static pressure in Pa and inWG?
50 × 9.80665 = 490.3 Pa and 50 ÷ 25.4 = 1.97 inWG. A US schedule value of 2 inWG is 50.8 mmH₂O (498.2 Pa).
Are mmAq, mmH₂O and mmWC the same unit?
Yes. All three are the pressure of a 1 mm water column, 9.80665 Pa. Aq is from the Latin aqua and WC stands for water column.
How do I convert a duct friction rate from Pa/m to mmH₂O/m?
Divide Pa/m by 9.80665: 1 Pa/m = 0.102 mmH₂O/m, and the common 0.1 mmH₂O/m is 0.98 Pa/m. The duct size calculator takes Pa/m.
Last updated 2026-10-12.