PSI to Inches of Water Converter
Unit ConversionConvert psi to inches of water column, shown for water at 60°F and at 39.2°F (4°C), for low pressures such as HVAC duct static pressure.
Inches of Water (60°F)
Inches of water column, water at 60°F (NIST: 248.84 Pa per inch)
Inches of Water (39.2°F / 4°C)
27.681
Inches of water column, water at 39.2°F (NIST: 249.082 Pa per inch)
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SponsoredThis converter turns pounds per square inch into inches of water column (inH2O, also written in. w.c.). Enter a pressure in psi to see two results: inches of water at 60°F, about 27.7076 per psi, and at 39.2°F (4°C), about 27.6807 per psi.
An inch of water is the pressure at the bottom of a one-inch column of water. Water is slightly denser when cold, so the pressure of that inch depends on the water temperature, and published converters disagree because they use different temperatures without saying so. This page states both.
Inches of water measure small pressures, such as HVAC duct static pressure and chimney draft. Readings below the surrounding air, such as on the suction side of a fan, are negative, so negative psi values are accepted down to a full vacuum of about -14.7 psi.
How to Convert PSI to Inches of Water
Convert psi to pascals, then divide by the pressure of one inch of water. NIST SP 811 gives 248.84 Pa for an inch of water at 60°F and 249.082 Pa at 39.2°F, and notes that additional digits are not justified because water density is a measured value. That gives about 27.7076 inH2O per psi at 60°F and about 27.6807 at 39.2°F.
Going the other way, 1 inch of water at 60°F is about 0.0360912 psi, so multiply inches of water by 0.0361 for a quick estimate.
Why Converters Give Different Answers
Converters quote 27.68, 27.7 or 27.71 inches of water per psi because they use water at different temperatures, or a conventional inch of water based on a fixed density. The differences are about 0.1%, small for most HVAC work, but they explain why two tools disagree in the second decimal place.
PSI to Inches of Water Chart
PSI values converted to inches of water at both reference temperatures, rounded to three decimal places.
| PSI | Inches of Water (60°F) | Inches of Water (39.2°F) |
|---|---|---|
| 0.01 psi | 0.277 inH2O | 0.277 inH2O at 39.2°F |
| 0.05 psi | 1.385 inH2O | 1.384 inH2O at 39.2°F |
| 0.1 psi | 2.771 inH2O | 2.768 inH2O at 39.2°F |
| 0.25 psi | 6.927 inH2O | 6.92 inH2O at 39.2°F |
| 0.5 psi | 13.854 inH2O | 13.84 inH2O at 39.2°F |
| 1 psi | 27.708 inH2O | 27.681 inH2O at 39.2°F |
| 2 psi | 55.415 inH2O | 55.361 inH2O at 39.2°F |
| 5 psi | 138.538 inH2O | 138.403 inH2O at 39.2°F |
| 10 psi | 277.076 inH2O | 276.807 inH2O at 39.2°F |
Inches of Water to PSI
Inches of water (60°F) converted to psi, rounded to five decimal places.
| Inches of Water (60°F) | PSI |
|---|---|
| 0.1 inH2O | 0.00361 psi |
| 0.25 inH2O | 0.00902 psi |
| 0.5 inH2O | 0.01805 psi |
| 1 inH2O | 0.03609 psi |
| 2 inH2O | 0.07218 psi |
| 5 inH2O | 0.18046 psi |
| 10 inH2O | 0.36091 psi |
| 14 inH2O | 0.50528 psi |
| 28 inH2O | 1.01055 psi |
| 50 inH2O | 1.80456 psi |
| 100 inH2O | 3.60912 psi |
Inches of Water Column in HVAC
HVAC technicians measure static pressure, the resistance a duct system puts up against the fan, in inches of water column. The values are small fractions of a psi, which is why inches of water are used instead. Airflow through the same ducts is measured in cubic feet per minute, and the CFM Calculator works out the airflow a room needs.
Equipment size is a separate question from duct pressure; the HVAC Sizing Calculator estimates the heating or cooling output in BTU. Medical ventilators use a close relative, centimeters of water, as in the Mean Airway Pressure Calculator.
Frequently Asked Questions
How many inches of water is 1 psi?
1 psi is about 27.7076 inches of water at 60°F, or about 27.6807 inches at 39.2°F.
What is inches of water column?
Inches of water column, written inH2O, in. w.c. or in WC, is the pressure that holds up a column of water that many inches high. It is used for low pressures, such as duct static pressure in heating and cooling systems.
Which temperature should I use?
Use the one your instrument or specification states. Converters differ: some use 60°F and others 39.2°F (4°C), the temperature at which water is densest. If no temperature is given, the difference of about 0.1% rarely matters in HVAC work.
Can inches of water be negative?
Yes. Readings are measured from the surrounding air, so a pressure below it, such as on the return or suction side of a fan, shows as a negative number. The converter accepts negative psi down to a full vacuum, about -14.7 psi.
How many inches of water is 1 inch of mercury?
1 inch of mercury is about 13.61 inches of water at 60°F, because mercury is about 13.6 times as dense as water.
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