Water Column Converter (mmH₂O, inH₂O)

Understanding the water column pressure converter — concept and principle

Small pressures in heating, ventilation and air-conditioning (HVAC), gas appliances, cleanrooms and filter testing are often given as a height of water: inches of water column (inH₂O, "in. w.c.") or millimeters of water (mmH₂O). This converter turns those values into pascals, hectopascals (millibar), kilopascals and psi, and back, so you can compare a fan curve, a manometer reading and a spec sheet that use different units.

How it is calculated

The calculator uses "conventional" water-column units, defined with a water density of 1,000 kg/m³ and standard gravity g = 9.80665 m/s²:

• 1 mmH₂O = 1,000 × 9.80665 × 0.001 = 9.80665 Pa
• 1 inH₂O = 25.4 mmH₂O = 249.08891 Pa
• 1 hPa = 1 mbar = 100 Pa; 1 kPa = 1,000 Pa; 1 psi = 6,894.757293 Pa

Your value is converted to a common base and then into the target unit.

The principle

The pressure at the bottom of a column of liquid is p = ρ × g × h: density times gravity times height. A manometer works exactly this way. Push air into one side of a U-tube of water and the level difference h tells you the pressure difference. Water is light, so small pressures produce easy-to-read heights, which is why HVAC technicians like it. Because real water density changes with temperature, older tables also list inH₂O at 39.2 °F (about 249.082 Pa) or 60 °F (about 248.84 Pa). The conventional unit avoids that ambiguity by fixing ρ and g.

Worked example

Default: 1 inH₂O (conventional) to pascals.

1. Convert to mmH₂O: 1 × 25.4 = 25.4 mmH₂O.
2. Convert to pascals: 25.4 × 9.80665 = 249.08891 Pa.
3. Result: 1 inH₂O = 249.08891 Pa, which the list also shows as 2.4908891 hPa, 0.24908891 kPa and 0.036127292 psi.

How to read the result

These are tiny pressures. Atmospheric pressure is about 101,325 Pa, roughly 407 inH₂O, so 1 inH₂O is about a quarter of one percent of the air pressure around you. Many residential air handlers are rated for roughly 0.5 inH₂O of total external static pressure; readings far above the rating usually point to restrictive ducts or a clogged filter. Gas appliance pressures are also commonly specified in inH₂O, so always compare with the manufacturer's rating plate.

Common mistakes

• Mixing up inH₂O with inHg (inches of mercury), which is about 13.6 times larger.
• Confusing gauge pressure (relative to the room) with absolute pressure.
• Reading pascals as hectopascals: 249 Pa is 2.49 hPa, not 249 hPa.

Frequently asked questions

How many pascals is 1 inch of water?

About 249.09 Pa using the conventional definition.

How many mmH₂O are in 1 kPa?

About 101.97 mmH₂O.

Is "in. w.c." the same as inH₂O?

Yes. Both mean inches of water column.

Sources: NIST Special Publication 811 (2008), Appendix B · BIPM, SI Brochure, 9th edition, 2019 (standard gravity and the pascal)