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Pressure Unit Converter

Convert duct static, gas manifold, refrigerant and vacuum pressures between all the units HVAC work uses — inches of water column, pascals, millibar, psi, ounces per square inch and inches of mercury.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Works for gauge readings and differences. Absolute pressures convert the same way.

Result

Inches of water column (conventional)
3.5in. w.c.
Pascals
871.8Pa
Millibar (hectopascal)
8.718mbar
Pounds per square inch
0.126psi

3.5 in. w.c. = 3.5 in. w.c. = 871.8 Pa = 0.1264 psi.

  • Gas appliance manifold and supply pressures are set by a qualified gas technician to the values on the appliance rating plate. This converter only changes units.
Millimetres of water column (conventional)
88.9mm w.c.
Kilopascals
0.872kPa
Bar
0.009bar
Ounces per square inch
2.023oz/in²
Inches of mercury (conventional)
0.257in. Hg
Millimetres of mercury / Torr
6.539mm Hg
Feet of water column (conventional)
0.292ft w.c.
Standard atmospheres
0.009atm
Show the working
  1. Convert to pascals (NIST SP 811 factor)
    3.5 in. w.c. × 249.0889 = 871.8 Pa
  2. Divide by each unit’s pascal value
    e.g. 871.8 ÷ 249.0889 = 3.5 in. w.c.

The factors

All conversions go through pascals, using NIST’s conventional values: one inch of water column is 249.0889 Pa, one inch of mercury is 3,386.389 Pa, one psi is 6,894.757 Pa and one millimetre of mercury (Torr) is 133.3224 Pa. Millimetres and feet of water and ounces per square inch are derived exactly from those.

So 1 psi is about 27.7 in. w.c., and 1 in. w.c. is about 2.49 mbar.

Gas pressures

Natural gas and propane appliance pressures in North America are usually given in inches of water column; European appliances often use millibar. The converter changes units only: the correct supply and manifold pressures are on the appliance rating plate and are set by a qualified gas technician with a manometer.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Pressure (or pressure difference)
3.5
Unit
Inches of water column (conventional)

Result

Inches of water column (conventional)
3.5 in. w.c.
Millimetres of water column (conventional)
88.9 mm w.c.
Pascals
871.8 Pa
Kilopascals
0.872 kPa
Millibar (hectopascal)
8.718 mbar
Bar
0.009 bar

3.5 in. w.c. = 3.5 in. w.c. = 871.8 Pa = 0.1264 psi.

Assumptions

  • Conventional water column (249.0889 Pa per inch) and mercury column (3,386.389 Pa per inch; 133.3224 Pa per mm) as listed by NIST.
  • Millimetres and feet of water and ounces per square inch are derived exactly from the inch and pound units.

Limitations

  • Water-column readings taken at other water temperatures differ slightly from the conventional value (by about 0.1 % at 60 °F).

Frequently asked questions

How many inches of water column are in 1 psi?

About 27.68 in. w.c.

What is 3.5 in. w.c. in millibar?

About 8.72 mbar, or 872 Pa.

Is ounces per square inch the same as in. w.c.?

No. 1 oz/in² is about 1.73 in. w.c. (one-sixteenth of a psi).

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.8 — factors for units listed alphabetically — National Institute of Standards and Technology. To pascals: inch of water, conventional 249.0889; inch of mercury, conventional 3,386.389; millimeter of mercury, conventional 133.3224; torr 133.3224; psi 6,894.757; bar 1 × 10⁵; millibar 100; standard atmosphere 101,325.

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