Inches of Water (inH2O) to PSI Converter

What an inch of water actually measures

An inch of water column is the pressure produced by a one-inch-tall column of water: inH2O = ρ_water × g × h, with h = 0.0254 m and g = 9.80665 m/s². Because water is light compared with mercury, the unit resolves very small pressures finely — one psi is about 27.7 inH2O, so a span that would be a fraction of a psi becomes tens of readable inches. That is exactly why duct static pressure, filter and coil differential, burner draft and low-density liquid level are graduated in inH2O (also written inWC, in. w.c. or iwg) rather than psi. Converting to psi is what you do when a gauge, a transmitter output or a spec sheet speaks psi and the rest of the system speaks water column.

The reference temperature most converters hide

Water density falls as it warms, so the inH2O-to-psi factor depends on the water temperature the unit is referenced to — and most online converters fix one value without telling you which. The three references actually used in industry differ by about 0.18%:

The selector above defaults to 60°F, the convention behind most North American DP and HVAC instrument scales. On a precision transmitter specified to ±0.1% FS, the 4°C-versus-60°F gap is roughly the same size as the accuracy budget, so it is worth matching the reference your instrument’s datasheet or governing standard uses rather than accepting whatever a hidden factor assumes.

inH2O to psi: worked examples

Worked at the 60°F reference. To convert, multiply the inches of water by the factor for your reference temperature; psi = inH2O × 0.0360918.

A single inch is the anchor worth memorising: 1 inH2O = 0.0360918 psi = 2.4884 mbar = 0.24884 kPa at 60°F. At the ISO 4°C reference the same inch is 0.0361263 psi — close, but not identical, which is the whole point of stating the reference.

DP transmitter range cheat-sheet

Low-range differential transmitters are usually catalogued in inH2O. This table converts the common spans to psi, mbar and kPa at 60°F so you can match a water-column requirement to a transmitter range without re-deriving the factor each time.

The HMK HM30 covers the bottom of this band for air and gas duct work, while the HM31 carries the higher spans and liquid service; both are graduated so the inH2O figure on the drawing maps directly onto the ordered range.

Notation traps: inH2O vs inWC vs iwg vs iwd

The same physical unit travels under several labels, and mixing them up is a common source of confusion on drawings:

The arithmetic is identical — an inch is an inch — but the reference is not: a gauge (iwg) reading is against atmosphere, while a differential (iwd) reading is the drop across two ports. Always confirm whether a stated inH2O figure is gauge or differential before you size a transmitter to it, because the same number on the wrong reference points at the wrong instrument.

Where inH2O shows up in the field

Typical bands help sanity-check a converted figure. Duct static pressure in commercial HVAC sits roughly between 0.5 and 6 inH2O; a loaded filter or coil differential is often alarmed at 1–3 inH2O; combustion air and furnace draft run from a fraction of an inch to a few inches; clean-room and containment differentials are tiny, around 0.01–0.05 inH2O; and low-density liquid level in shallow tanks can read tens of inches of water. If a converted psi value lands far outside the band you expect, the cause is usually a wrong reference temperature, a gauge-versus-differential mix-up, or a transmitter range that does not match the service.

Frequently Asked Questions

Is 1 psi 27.68 or 27.7 inches of water?

Both, depending on the water reference temperature. At 4°C (ISO) 1 psi is 27.6807 inH2O; at 60°F (the common US instrument reference) it is 27.7071 inH2O; at 68°F it is 27.7296 inH2O. The converter lets you pick the reference rather than hiding it.

What is the difference between inH2O at 4°C and 60°F?

Water is denser at 4°C, so an inch of it makes slightly more pressure: 1 inH2O = 0.0361263 psi at 4°C versus 0.0360918 psi at 60°F. The gap is about 0.18%, which matters when you calibrate a high-accuracy DP transmitter.

How many inH2O is a typical DP transmitter range?

Low-range DP transmitters are commonly built for 0–10, 0–25, 0–100, 0–250 or 0–400 inH2O. At 60°F those correspond to about 0.36, 0.90, 3.61, 9.02 and 14.4 psi respectively.

Related converters & tools

• PSI to Inches of Water — the reverse conversion
• Pressure Unit Converter — all 15 pressure units at once
• DP Level Calibration — find LRV and URV for DP level transmitters
• DP Flow Calculator — differential pressure to flow rate
• PSI to Feet of Head — the other water-column unit
• inHg to PSI — vacuum / barometric inches of mercury to psi