PSI to Inches Of Water Conversion Calculator - inH2O with Pascal Cross-Check

A psi to inches of water conversion turns a pressure reading in pounds per square inch (psi) into inches of water column (inH2O) at 4 degrees C, and back, in a single step. It uses the fixed relationship that 1 inH2O at 4 degrees C equals 249.082 pascals, 1 psi equals 6,894.757293168 pascals, and 1 inch equals 0.0254 metres, so 1 psi equals 27.6806725 inH2O and 1 inH2O equals 0.03612729 psi. This covers HVAC ductwork, differential pressure gauges, gas pipeline test points, and medical ventilator calibration without retyping a constant.

• • HVAC duct and air-handler readings: Translate a 0.5 inH2O duct static reading (typical for residential supply ducts) into the equivalent psi value for fan curves and balancing reports.
• • Differential pressure gauges and manometers: Convert a 1 inH2O or 10 inH2O manometer reading on a Magnehelic or digital differential gauge into psi for control loop setpoints and alarm thresholds.
• • Medical ventilator and CPAP calibration: Map a 10-20 cmH2O or 4-8 inH2O pressure setting on a ventilator or CPAP machine into psi for bench-test fixtures and biomedical equipment logs.
• • Natural gas pipeline and appliance test points: Convert a low-pressure gas test-point reading in inH2O (typically 5-14 inH2O for residential gas) into psi for regulator inspection records.

The standard 'inches of water' unit assumes fresh water at 4 degrees C, the conventional reference temperature for water column measurements in HVAC, gas, and medical standards. Because the inH2O unit rests on a reference water density rather than on exact SI definitions alone, the 27.6806725 inH2O/psi factor is conventionally fixed and reproducible across workflows, but it is not a defined SI constant.

For higher-pressure references, PSI to ATM Conversion uses the same 6,894.757293168 Pa/psi definition to convert psi into standard atmospheres alongside the pascal and bar rows.

The conversion is a single fixed ratio. The BIPM standard gravity, the inch, and the conventional reference density of water at 4 degrees C fix 1 inH2O at about 249.082 pascals, so 1 psi equals 6,894.757293168 / 249.082 = 27.6806725 inH2O, and 1 inH2O equals 0.03612729 psi. The same pascal value drives the kilopascal cross-check row, so every output reads from a single underlying number.

• p(psi): Pressure in pounds per square inch (psi) that you want to convert to inches of water column
• p(inH2O): Pressure in inches of water column at 4 degrees C that you want to convert to psi
• direction: Toggle that picks psi -> inH2O or inH2O -> psi so the same tool handles both directions

When the direction toggle is set to psi -> inH2O, the calculator multiplies the psi input by 27.6806725 and reports the answer in inH2O. The same psi value then drives the pascal and kilopascal cross-check rows, so the four outputs all read from one underlying number and never drift out of step.

According to NIST SP 811, 1 psi equals exactly 6,894.757293168 pascals and 1 inch equals exactly 0.0254 metres, the values the psi to inches of water conversion chain uses.

When the workflow needs to read the same psi value against atm, bar, kPa, and MPa in a single panel, PSI Conversion Calculator applies the same NIST pascal definition to the broader psi family.

Four ideas shape this conversion: the conventional reference density of water at 4 degrees C, the BIPM standard gravity, the NIST pascal and inch definitions, and the way pascals and kilopascals sit on the same physical scale.

The kilopascal cross-check row uses the same pascal anchor as Kpa to PSI Conversion Calculator, which covers the kPa -> psi leg of the same NIST-defined conversion chain.

The psi to inches of water conversion calculator has a value field and a direction toggle. Pick the direction, type the number, and read the converted pressure plus the pascal and kilopascal cross-check rows in the same result panel.

1. 1 Pick the conversion direction: Use the dropdown to switch between Pound per square inch (psi) to Inches of water (inH2O) and Inches of water (inH2O) to Pound per square inch (psi). The default is psi -> inH2O for the most common HVAC duct, gauge, and ventilator workflow.
2. 2 Type the pressure value: Enter the value in the unit set by the direction toggle. Try 1 for the 27.6807 inH2O/psi sanity check, 0.5 for a typical residential duct static, 4 for a low CPAP setting, or 10 for a higher-pressure medical gas line.
3. 3 Read the converted pressure: The black box at the top of the result panel shows the answer in the target unit. For 1 psi, the answer is 27.6806725 inH2O; for 1 inH2O, the answer is 0.03612729 psi. The unit label switches with the direction toggle.
4. 4 Cross-check the pascal and kilopascal rows: The result panel also shows the same pressure in pascals and kilopascals. Compare against a published chart or a separate calculator to confirm the factor is being applied correctly.
5. 5 Switch direction for the reverse: Toggle the direction dropdown to flip between psi -> inH2O and inH2O -> psi. The same value and cross-check rows are kept, so a round trip takes two clicks and never loses precision.

A residential HVAC supply duct reads 0.5 inH2O on a Magnehelic gauge. Toggle the direction to inH2O -> psi and type 0.5. The calculator returns 0.01806365 psi alongside 124.541 Pa and 0.1245 kPa, and toggling back to psi -> inH2O rounds the same value to 0.5 inH2O.

When the source gauge reads bar rather than inH2O, Bar to PSI Conversion reads the bar input against the same psi row so the inH2O and pascal cross-checks can be chained in the same workflow.

The advantage of a factor-driven psi to inches of water conversion is that the result is reproducible to the precision of the input, and the cross-check rows make it self-validating, removing the rounding and misplaced-decimal mistakes that creep in when each conversion is done by hand.

• • Conventionally fixed factor: The 1 psi = 27.6806725 inH2O ratio is set by the exact NIST pascal and inch definitions, the BIPM standard gravity, and the conventional reference density of water at 4 degrees C, so the calculator returns the same value once the reference conditions are accepted.
• • Bidirectional in one tool: Switch the direction toggle to flip between psi -> inH2O and inH2O -> psi, with the same cross-check rows in place, so a round-trip never loses precision.
• • Two cross-check rows: Every result includes the equivalent pressure in pascals and kilopascals, catching transcription errors early when a psi value was rounded to 1 or 2 significant figures on a gauge.
• • Worked duct and ventilator examples: The 1 psi reference, the 0.5 inH2O residential duct, and the 4-10 inH2O ventilator setting appear in the explanation, covering air balancing, gas distribution, and medical calibration.

The psi to inches of water conversion is conventionally fixed, but the usefulness of any specific number depends on what the input represents, on the precision of the reading, and on whether the rest of the workflow is using the same temperature and gravity reference.

• • The conversion factor is conventionally fixed at the 4 degrees C reference, but it assumes pure fresh water. Salt water, glycol mixtures, and other manometer fluids use a different density, so the same column reading gives a different psi value.
• • Inches of water column is a gauge-style low-pressure unit. Datasheets that quote bar, kPa, or MPa will not match the inH2O result without an extra conversion step through the pascal anchor.

Inches of water column is most useful below about 5 psi (about 138 inH2O). Above that, datasheets usually move to psi, kPa, or bar, where the same pressure reads as a large inH2O value.

According to BIPM SI Brochure, the standard value of gravity is exactly 9.80665 m/s^2, which sets the pascals-per-metre-of-water step but not the inH2O unit itself.

According to the NIST Chemistry WebBook, the reference density of pure liquid water at 4 degrees C is about 999.972 kg/m^3, giving 1 inH2O = 249.082 Pa and 1 psi = 27.6806725 inH2O under the NIST pascal.

For datasheets and engineering reports that prefer atm, kPa, mbar, or torr instead of inH2O, ATM Conversion Calculator applies the same 101,325 Pa/atm standard to the broader pressure ladder.