Pressure Converter

About the Pressure Converter

Convert pressure and stress between pascals, kilopascals, megapascals, N/mm², kN/m², bar, millibar, hectopascals, psi, ksi, psf, kgf/cm² (ksc), kgf/m², t/m², standard atmospheres, mmHg, inHg, torr and water columns in mm, cm, m, inches and feet. Type in either box and the other updates; the table underneath shows the value in every unit, and the arithmetic uses exact fractions, so 1 kgf/cm² is exactly 98,066.5 Pa and a psi is derived from the exact pound and inch.

Gauges usually read the pressure above the surrounding air. Gauge and absolute pressure under the result adds or removes the atmospheric pressure — 2 barg is 3.01325 bara at sea level — using the standard atmosphere of 101.325 kPa or a local value that you type or estimate from your altitude.

Common uses

• Tyre, compressor and cylinder gauges in psi, bar and kgf/cm² — including psig, psia, barg and bara.
• Engineering stresses and strengths in MPa, N/mm², ksi and t/m², and design loads in kN/m² and psf.
• Weather and altitude in hPa, mbar and inHg; vacuum in torr and mmHg.
• HVAC, gas and pump work in inches, millimetres and metres of water column.
• Physics and chemistry homework with exact factors.

Gauge and absolute pressure

A gauge measures the difference from the air around it, so an open gauge reads 0 even though the air presses at about 101 kPa. Absolute pressure is measured from a perfect vacuum:

• absolute = gauge + atmospheric, for example 2 barg + 1.01325 bar = 3.01325 bara
• gauge = absolute − atmospheric; a vacuum gives a negative gauge reading, down to −1.01325 bar at most

The letters g and a only say which reference is used: psig and psia are both psi. The standard atmosphere is exactly 101,325 Pa (101.325 kPa, 1.01325 bar, about 14.696 psi). Real air pressure is lower at altitude and changes with the weather; weather reports usually give it corrected to sea level, so for an accurate absolute value use a barometer reading from where the gauge is. The altitude estimate uses the pressure-altitude formula of the US National Weather Service (weather.gov) for the lowest layer of the standard atmosphere, up to 11,000 m.

Where the conversion factors come from

• bar = 100,000 Pa and millibar = 100 Pa exactly (BIPM SI Brochure, 9th edition, Table 8).
• standard atmosphere = 101,325 Pa, technical atmosphere = 1 kgf/cm² = 98,066.5 Pa and kgf/m² = 9.80665 Pa, all exact (NIST SP 811, Appendix B).
• psi = one pound-force per square inch: 0.45359237 kg × 9.80665 m/s² ÷ (0.0254 m)² ≈ 6,894.757 Pa; ksi = 1,000 psi and psf = psi ÷ 144.
• torr = 1/760 atm ≈ 133.322368 Pa, while the conventional millimetre of mercury is 133.322387415 Pa — the two differ by about 0.000014 %.
• water columns are conventional: water of 1,000 kg/m³ under standard gravity, so 1 mmH₂O = 9.80665 Pa and 1 inH₂O = 249.08891 Pa.

NIST Handbook 44 tabulates the inch of water at 4 °C instead (249.082 Pa, about 0.003 % less than the conventional value).

Pressure and stress units in engineering

Stress has the same unit as pressure, so material strengths and soil pressures convert here too: 1 N/mm² = 1 MPa, 1 kN/m² = 1 kPa, and 1 t/m² (tonne-force per square metre, used for soil bearing capacity) = 9.80665 kPa. In US practice, steel strengths are often in ksi (1 ksi ≈ 6.895 MPa) and floor loads in psf (1 psf ≈ 47.88 Pa). Gauges marked “kg/cm²” mean kilogram-force per square centimetre, also written ksc or at.

Limitations

• Mercury and water columns use the conventional densities; a real manometer at room temperature reads slightly differently.
• The altitude estimate is the standard atmosphere, not your actual air pressure, which also depends on the weather.
• This is a unit converter: it does not check whether a pressure is safe for a tyre, a cylinder or a structure.
• Results are exact fractions shown to 10 significant digits (marked ≈ when rounded); very large or small values use scientific notation.

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