Convert between pounds per square inch (psi) and inches of water column (inH₂O) — a common conversion for HVAC duct pressure and low-pressure gas readings.
How it works
Using the conventional reference (water at 39.2°F / 4°C, standard gravity), 1 psi = 27.6799 inches of water. Enter a value in either field and the other updates instantly.
What this does not include
This uses the standard reference conditions published in NIST’s guide to the SI. Inches-of-water readings taken at significantly different water temperatures would shift very slightly from this value, an effect this calculator doesn’t model.
How to use this calculator
- Enter a value in psi or inches of water.
- The other field updates automatically.
Why use inches of water instead of psi for low pressures?
Inches of water column gives much finer resolution at the low end — HVAC duct pressures are often a fraction of a single psi, so measuring in inH₂O avoids working with awkward decimals.
What generates an inches-of-water reading?
A manometer — a simple tube of water where pressure pushes the water column up by a height proportional to the pressure — is the classic instrument for this unit.
Is inches of water the same as inches of mercury?
No — they use different reference fluids with very different densities, so the two units have a very different conversion factor to psi; don’t substitute one for the other.
