PSI to Inches of Water Conversion Calculator — psi to inH₂O
Convert psi (pounds per square inch) to inches of water column (inH₂O) — the unit used in HVAC duct pressure, low-pressure gas systems and medical ventilators. 1 psi = 27.6807 inH₂O at 4 °C (standard NIST).
psi
1 psi = 27.6807 inH₂O at 4 °C — standard NIST conversion
- 1
Convert to pascals
1 × 6,894.757 = 6,894.761 psi = 6,894.757 Pa (from pound-force and international inch definitions). - 2
Convert to inches of water
6,894.76 ÷ 249.089 = 27.67991 inH₂O = 249.089 Pa (conventional NIST value at 4 °C).
How does this calculator work?
1 psi = 27.6807 inH₂O = 702.89 mmH₂O = 6 894.757 Pa = 2.036 inHg. Reverse: 1 inH₂O ≈ 0.03613 psi = 249.089 Pa. HVAC duct pressures use inH₂O because psi values are impractically small (0.1 inH₂O ≈ 0.0036 psi). Multiply psi by 27.68 to get inH₂O.
Formula
How this is calculated
An inch of water column (inH₂O or in WC) is the pressure exerted by a one-inch-tall vertical column of pure water. At 4 °C, water reaches its maximum density of approximately 999.84 kg/m³, so 1 inH₂O = 0.0254 m × 999.84 kg/m³ × 9.806 65 m/s² ≈ 249.089 Pa. This is the conventional NIST/ASTM value; at 68 °F (20 °C) the water density is slightly lower, giving ≈ 248.84 Pa — a difference of less than 0.1 %, negligible for most applications.
Converting from psi: 1 psi = 6 894.757 Pa (from pound-force and international inch definitions), divided by 249.089 Pa/inH₂O gives 1 psi = 27.6807 inH₂O. HVAC engineers frequently specify duct static pressures in tenths or hundredths of an inch WC — for example, 0.25 inH₂O ≈ 0.009 psi, a pressure difference too small to read on a standard psi gauge but easily measured with a manometer.
The millimetre of water column (mmH₂O) is simply inH₂O × 25.4; it is common in European medical and laboratory contexts. Inches of mercury (inHg) is used in aviation altimetry (standard sea-level = 29.92 inHg). All values round to six significant figures.
Frequently asked questions
HVAC duct static pressures are very small — typically 0.1 to 2 inH₂O (≈ 0.004 to 0.07 psi). Using inH₂O gives readable whole or decimal numbers, whereas psi values would need four decimal places. It also maps directly to manometer readings with water as the indicator fluid.
Yes, slightly. The conventional NIST value (249.089 Pa) uses water at 4 °C (maximum density). At 20 °C the factor is ≈ 248.84 Pa, about 0.1 % lower. For most engineering purposes the 4 °C factor is used as the standard, regardless of actual water temperature.
In HVAC, "inches water gauge" (inWG or inWC) always means gauge pressure (above atmospheric). When converting from psig, first convert to absolute psi by adding 14.696 if you need absolute pressure — or work in gauge throughout and simply multiply inWG × 27.6807 to get psi gauge.
Also known as
TG we-Calculate Editorial Team. (2026). PSI to Inches of Water Conversion Calculator — psi to inH₂O [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/psi-to-inches-of-water-conversion-calculator
TG we-Calculate Editorial Team. "PSI to Inches of Water Conversion Calculator — psi to inH₂O." TG we-Calculate. 2026. https://we-calculate.com/calculator/psi-to-inches-of-water-conversion-calculator.
TG we-Calculate Editorial Team, "PSI to Inches of Water Conversion Calculator — psi to inH₂O," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/psi-to-inches-of-water-conversion-calculator
@misc{wecalculate_psi_to_inches_of_water_conversion_calculator, title = {PSI to Inches of Water Conversion Calculator — psi to inH₂O}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/psi-to-inches-of-water-conversion-calculator}}, year = {2026}, note = {TG we-Calculate} }
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