psi to Inches of Water Conversion

Low-pressure work often needs a unit more sensitive than psi. A pressure drop across an air filter, a gas valve outlet, or an HVAC duct section may be only a few tenths of a psi, but it can be several inches of water column. This calculator converts pressure between pounds per square inch and inches of water, commonly written as inH2O or in. w.c.

The page is a unit conversion, not a flow calculation. It does not say how much air or gas moves through a duct, and it does not size a fan. It only changes the pressure unit. For other pressure scales, use the pressure converter. For volume-flow problems, compare with the flow rate calculator. If the pressure comes from water service or fixtures, the water demand calculator may fit the broader design question.

Why inches of water are used

An inch of water is the pressure produced by a one-inch-tall column of water under standard gravity at a stated water density. Because a visible water column can respond to very small pressure differences, the unit became common in manometers and low-pressure instrumentation. In the field, a technician may read a furnace manifold pressure, fan static pressure, room pressure differential, or filter pressure drop in inches of water instead of psi.

One psi is a relatively large low-pressure number. At the calculator’s reference factor, 1 psi equals 27.6799 inches of water. That means a pressure such as 0.25 psi becomes 6.91998 inches of water, a value that is much easier to compare with appliance labels and manometer graduations.

Formula used by the calculator

For psi to inches of water, the calculation uses the two pascal factors:

$\text{inH2O} = \text{psi} \cdot \frac{6894.757293168361}{249.082}$

For inches of water back to psi, it divides by the same factor:

$\text{psi} = \text{inH2O}\cdot\frac{249.082}{6894.757293168361}$

The calculator accepts nonnegative values and identifies the conventional 4 °C reference. Actual liquid-column calibration can depend on density and local gravity, so use an instrument’s specified basis when it differs.

Example

Use the default psi-to-inH2O mode with 1 psi. The calculator multiplies:

$1 \cdot 27.680672602469713 = 27.680672602469713\ \text{inH2O}$

It displays the primary result as 27.6807 inH2O and shows the conventional reference basis.

For a more typical HVAC-style value, enter 0.5 psi:

$0.5 \cdot 27.6799 = 13.83995\ \text{inH2O}$

In reverse mode, the default inches-of-water value is 27.6799. The calculator divides:

$\frac{27.6799}{27.6799} = 1\ \text{psi}$

That symmetry helps when a gas appliance manual gives a water-column pressure but another gauge reads in psi.

Reference table

The 3.5 and 7 inH2O rows are included because gas appliance and manifold pressure examples often live in that range. Always use the manufacturer’s specified pressure and test procedure, not a generic table, when adjusting equipment.

Domains and common pitfalls

Inches of water belongs in low-pressure HVAC, combustion, filtration, laboratory, and building-pressure work. It is not the same as inches of rainfall, inches of pipe diameter, or inches of water depth in a tank unless the problem is explicitly about hydrostatic pressure. A column-height pressure unit can describe air or gas pressure because it names the equivalent water-column height, not the fluid being measured.

The most common mistake is rounding too aggressively. Rounding 27.6799 to 27.7 is fine for quick estimates, but calibration forms and commissioning records should keep enough digits to match the instrument. Another mistake is confusing gauge and absolute pressure. Most inches-of-water manometer readings in HVAC are gauge or differential readings. If an application needs absolute pressure, state that separately. Finally, check whether the source says inH2O, cmH2O, mmH2O, Pa, kPa, or psi before entering a value; those units are easy to mix up in handwritten notes.

For connected electrical or mechanical calculations that combine pressure with power or work, use the power calculator or energy calculator only after the underlying physical relationship is clear.

Accuracy and limits

The calculator keeps the defined or cited relationship through the calculation and rounds only the displayed result. A converted number does not become more precise than the source measurement. Keep additional digits for chained calculations, then round to the precision justified by the original value; also preserve any reference basis or notation convention named with the input.

Sources

• NIST, SI Units — SI pressure context and derived-unit relationships.
• BIPM, The International System of Units — official SI unit definitions used when relating pressure to base units.
• Engineering ToolBox, Water Density, Specific Weight and Thermal Expansion — reference information for water density and specific weight.