PSI to Inches of Water (inH2O) Converter

Why scale psi back into inches of water

Plenty of instruments speak psi — a gauge, a transducer output, a pump or line spec — while the system they feed is built around inches of water. Air-handling drawings, filter and coil ΔP alarms, draft setpoints and low-pressure transmitter scales are all graduated in inH2O because the unit gives fine resolution at low pressure: one psi is about 27.7 inH2O, so a span of a fraction of a psi spreads across tens of readable inches. Converting psi into inH2O is what lets you set, check or compare a psi figure against a water-column scale without losing that resolution.

The factor, and the reference temperature behind it

To go from psi to inches of water you divide by the same temperature-dependent factor used in the reverse direction: inH2O = psi ÷ factor(T), which is the same as psi × (inH2O per psi). The three industry references differ by about 0.18%:

Because it shares the factor table with the inches of water to psi converter, the two pages reconcile: convert psi to inH2O here, push the result back through the reverse tool at the same reference, and you should land on the psi you started with. That round-trip is a quick self-check that you used a consistent reference temperature.

psi to inH2O worked examples

Worked at the 60°F reference, where 1 psi = 27.7071 inH2O. Multiply the pressure in psi by that factor to get inches of water.

One atmosphere is a useful intuition anchor: 14.696 psi works out to about 407 inH2O — more than 33 feet of water — which is why water column is reserved for the low-pressure end and psi takes over once the numbers grow.

psi to inH2O to mbar to kPa cross-table

DP work often crosses unit systems in one conversation — inH2O on the drawing, mbar on the European transmitter, kPa in the control system. This cross-table (60°F reference) lets you read all four at once:

Matching a psi transmitter to an inH2O-graduated system

When a system is scaled in inches of water but the available transmitter is specified in psi, the question is whether the psi range actually covers the inH2O span you need. A 0–5 psi transmitter spans about 0–139 inH2O at 60°F, which comfortably contains a typical 0–100 inH2O duct requirement; a 0–1 psi unit reaches only about 28 inH2O and would be over-ranged by a 0–250 inH2O job. Convert the target inH2O span to psi first, then choose a range that puts the working point in the upper two-thirds of span for the best turndown and accuracy. For native low-DP work, a transmitter graduated directly in inH2O — like the HM30 or HE30 — avoids the conversion entirely.

Notation: inH2O, inWC, iwg and iwd

Once you produce an inH2O figure from psi, label it for the reference it will be read against. inWC and in. w.c. are plain synonyms for the unit; iwg (inches of water, gauge) is HVAC static pressure referenced to atmosphere; iwd (inches of water, differential) is the drop measured across two taps. A psi gauge reading naturally maps to iwg, while a psi differential between two ports maps to iwd. Carrying the right suffix prevents a correct number from being applied to the wrong reference downstream.

Frequently Asked Questions

How many inches of water are in 1 psi?

At the 60°F reference, 1 psi = 27.7071 inH2O. At the ISO 4°C reference it is 27.6807 inH2O, and at 68°F it is 27.7296 inH2O. The converter lets you choose.

What is 5 psi in inches of water?

About 138.5 inH2O at 60°F (5 × 27.7071). That is roughly 34.5 kPa or 345 mbar.

How do I convert psi to inWC for HVAC static pressure?

Multiply psi by the inH2O-per-psi factor for your reference temperature (27.7071 at 60°F) and label the result iwg, since HVAC static pressure is gauge referenced to atmosphere.

Related converters & tools

• Inches of Water to PSI — the reverse conversion
• Pressure Unit Converter — all 15 pressure units at once
• DP Level Calibration — LRV and URV for DP level transmitters
• Feet of Head to PSI — feet of head, the other water column
• PSI to inHg — psi to inches of mercury for vacuum setpoints