How this calculator works
Convert through pascals while keeping the same gauge, absolute or differential reference. NIST lists a conventional inch of water as 249.0889 Pa, an inch at 4 C as 249.082 Pa, and an inch at 60 F as 248.84 Pa. This page uses the published conventional factor for the main result and compares the other two columns in the tables. These are reference conversions, not a model of a particular manometer at an arbitrary temperature.
Formula
Conventional inH2O = psi x 6894.757293168 / 249.0889. psi = conventional inH2O x 249.0889 / 6894.757293168. The exact psi factor is 4.4482216152605 / 0.0254^2 Pa; the decimal used here is rounded. NIST publishes the conventional water factor to 7 significant digits, so extra digits do not imply greater physical accuracy.
Worked example
A differential pressure of 0.1 psi is about 2.76799 conventional inH2O. In reverse, 10 conventional inH2O is about 0.361273 psi. At 4 C, the same 0.1 psi is about 2.76807 inches of water.
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Frequently asked questions
Which inch of water is selected?
The conventional inch of water, using the NIST factor 249.0889 Pa. The comparison columns use the separately listed 4 C and 60 F factors.
Does converting the unit add atmospheric pressure?
No. A gauge pressure stays a gauge pressure and a differential pressure stays differential. Use the PSIG to PSIA converter to change the pressure reference.
Can an inches-of-water reading be negative?
Yes. A differential instrument can show suction as a negative reading. This converter preserves that sign; it does not interpret a negative number as an absolute pressure.
Why use inches of water for ducts?
Small air-pressure differences are easier to read in this unit. For example, a pressure difference of 1 conventional inH2O is only about 0.0361273 psi.