Water Column to PSI Converter (inWC, ftH₂O)

1 inch of water column (inWC) equals 0.0361 psi, and 1 psi equals 27.68 inWC. To convert, multiply inches of water by 0.036127 or divide by 27.68. Feet of water convert at 0.4335 psi per foot, or 2.307 ft per psi.

It is for gas fitters, HVAC technicians and anyone reading a manometer, pump curve or tank gauge. Enter a reading in any unit — inches or feet of water, mmH₂O, cmH₂O, kPa, mbar, Pa, inHg or bar — and read it in every other unit.

How to use this converter

1. Type the reading into the value box and pick its unit (inH2O for a gas manometer, ftH2O for pump head, mbar or kPa for European gauges).
2. Choose the unit you want in "Convert to".
3. Read the headline result, and scan the list below it for the same pressure in every other unit.
4. Use the share link to send the exact conversion to a colleague or customer.

How it's calculated

Every unit is converted to pascals and back, using the factors in NIST SP 811 Appendix B.8:

• 1 inH₂O = 249.0889 Pa (conventional: water density 1,000 kg/m³, standard gravity 9.80665 m/s²)
• 1 ftH₂O = 2,989.067 Pa, 1 mmH₂O = 9.80665 Pa
• 1 psi = 6,894.757 Pa, 1 inHg = 3,386.389 Pa

So psi = inWC × 249.0889 ÷ 6894.757, which simplifies to psi = inWC × 0.036127 and psi = ft of water × 0.4335. Temperature-specific units differ slightly: NIST lists 249.082 Pa at 39.2 °F (4 °C) and 248.84 Pa at 60 °F. The difference is about 0.1%.

Worked examples

1. Checking a natural gas supply

A manometer on the gas line reads 7 inWC. 7 × 249.0889 ÷ 6894.757 = 0.253 psi, or 17.4 mbar. That matches PG&E's standard delivery pressure and sits inside the 7–10 inWC range DOE test rules specify for natural gas appliances.

2. Pump head to psi

A pump lifts water 60 ft from a well. 60 × 0.4335 = 26.0 psi of static head, before you add pipe friction and the pressure you want at the tap.

3. A tank level from a pressure gauge

A gauge at the bottom of an open water tank reads 3 psi. 3 × 2.307 = 6.92 ft (83.0 in) of water above the gauge. This holds for fresh water; salt water, fuel or other liquids need their own density.

Inches of water column to psi

Multiply inches of water column by 0.036127. The table covers the fractions of an inch used for duct static pressure, the 3.5–14 inWC range of gas appliances, and 138.4 and 277 inWC (5 and 10 psi):

Psi to inches of water column

Multiply psi by 27.680 to get inches of water column: 0.25 psi is 6.92 inWC and 0.5 psi is 13.84 inWC.

Feet of head to psi

To convert feet of head to psi, multiply by 0.4335. A 100 ft column of fresh water exerts 43.4 psi at the bottom, whatever the width of the pipe or tank.

Psi to feet of head

To convert psi to feet of head, multiply by 2.307 (USGS rounds this to 2.307 ft per psi for fresh water). A pump that must deliver 40 psi needs about 92.3 ft of head before friction losses.

Gas appliance pressures in inWC and psi

Gas pressures at the appliance are low enough that installers measure them in inches of water column rather than psi. The figures below come from utility tariffs, federal test procedures and the fuel gas sizing appendix of the International Residential Code (as adopted by Seattle).

Your utility, appliance data plate and local code set the actual pressures for your installation. Gas work carries fire and carbon monoxide risk; have a licensed gas fitter test and adjust pressures. This page only converts units.

Where you'll see water column units

• Gas lines and regulators: supply and manifold pressures in inWC, or mbar on metric gauges.
• HVAC: duct static pressure is measured in fractions of an inch of water. DOE's furnace fan test procedure (10 CFR 430 Appendix AA, Table 1) tests at 0.50–0.55 inWC external static pressure for units with a factory-installed evaporator coil, 0.65–0.70 inWC for units designed to be paired with a coil, and 0.30–0.35 inWC for mobile home units. 0.5 inWC is only 125 Pa, or 0.0181 psi.
• Pumps and wells: head is quoted in feet or metres of water. Convert to psi to compare with a pressure switch or gauge.
• Tank level sensors: a pressure sensor at the bottom of a tank reads liquid height directly when calibrated in water column.

When not to use a simple conversion

The converter gives the static pressure of a column of fresh water at NIST's conventional density. That is the standard definition of these units, so it is right for converting gauge readings. It is less accurate for moving water: flowing pipes lose pressure to friction, so the pressure at a tap is lower than the static head alone predicts. For hot water, sea water or other liquids the density differs, and a foot of liquid produces a different pressure. In those cases use the conversion as a starting point and correct for density or friction separately.

Common mistakes

• Mixing inWC and inHg. An inch of mercury is about 13.6 times an inch of water. Some vacuum and barometric gauges read inHg, so check the gauge face.
• Using the water factor for other liquids. Head in feet converts to psi at 0.4335 only for fresh water. For heavier or lighter liquids, multiply by the specific gravity. The weight per gallon calculator gives densities for common liquids.
• Confusing psig and psia. Water column gauges read pressure relative to the air around them (gauge pressure), not absolute pressure.
• Rounding 27.68 to 28. Fine for a quick estimate, but it adds about 1% error.

Frequently asked questions

Sources & method

• NIST SP 811 Appendix B.8 — inch of water (conventional) = 249.0889 Pa; psi = 6894.757 Pa
• USGS GWPD 12 — 2.307 ft of water per psi
• PG&E Gas Rule 2 — standard delivery pressure 7 in. water column
• 10 CFR 430 Subpart B App. D2 §2.3.2 — natural gas 7–10 in. w.c., propane 11–13 in. w.c.
• 10 CFR 430 Subpart B App. AA, Table 1 — furnace fan test external static pressure (in. wc)
• Seattle Residential Code 2012, Appendix A (IRC fuel gas sizing) — 3.5 in. w.c. manifold pressure

Results are estimates for general information. Found an error? It helps everyone — see our methodology.