Pressure Converter

Water and mercury columns: which reference?

Manometric units depend on the density of the liquid. The conventional inch of water (249.08891 Pa), millimetre of water (9.80665 Pa) and centimetre of water (98.0665 Pa) assume 1000 kg/m³, which is close to water at 4 °C. The inch of water at 60 °F is about 0.1 % smaller (248.84 Pa). The conventional mmHg and inHg assume 13 595.1 kg/m³, mercury at about 0 °C. All of them use standard gravity, 9.80665 m/s².

Gauge, absolute and differential pressure

A differential pressure (ΔP), such as a filter pressure drop, converts with the same factor as any other pressure. Gauge (psig, barg) and absolute (psia, bara) readings differ by the ambient pressure, about 101.325 kPa at sea level, and that offset is not a unit conversion. Convert the number first, then add or subtract atmospheric pressure in the same unit.

Torr vs mmHg

The torr is exactly 1/760 atm (133.3223684 Pa). The conventional millimetre of mercury is 133.322387415 Pa. They differ by about 0.14 parts per million, which only matters in calibration work.

Pressure unit definitions

pascal (Pa)

The pascal (Pa) is the SI unit of pressure and stress, equal to one newton of force acting on one square metre.

kilopascal (kPa)

The kilopascal (kPa) equals 1000 pascals. It is the everyday SI unit for tyre pressure, gas supply pressure and atmospheric pressure.

megapascal (MPa)

The megapascal (MPa) equals one million pascals, which is the same as one newton per square millimetre (N/mm²).

psi (psi)

The pound-force per square inch (psi) is the US customary unit of pressure: the force of one pound-force acting on one square inch. It is exact in SI because the pound and the inch are defined exactly.

bar (bar)

The bar is a metric unit of pressure defined as exactly 100 000 pascals, close to (but slightly below) one standard atmosphere.

millibar (mbar)

The millibar (mbar) is one thousandth of a bar, exactly 100 pascals. It is numerically identical to the hectopascal (hPa) used in meteorology.

decibar (dbar)

The decibar (dbar) is one tenth of a bar, exactly 10 000 pascals. In seawater one decibar of pressure corresponds to roughly one metre of depth.

torr (Torr)

The torr is defined as exactly 1/760 of a standard atmosphere (101 325/760 Pa). It is almost, but not exactly, equal to one millimetre of mercury.

millimeter of mercury (mmHg)

The conventional millimetre of mercury (mmHg) is the pressure exerted by a 1 mm column of mercury of density 13 595.1 kg/m³ under standard gravity: exactly 133.322387415 Pa. Reference: Conventional mercury column: ρ = 13 595.1 kg/m³ and gₙ = 9.80665 m/s² (≈ mercury at 0 °C / 32 °F).

millimeter of water (mmH₂O)

The conventional millimetre of water (mmH₂O, also written mmWC or mmAq) is the pressure of a 1 mm column of water of density 1000 kg/m³ under standard gravity: exactly 9.80665 Pa. Reference: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).

centimeter of water (cmH₂O)

The conventional centimetre of water (cmH₂O) is the pressure of a 1 cm column of water of density 1000 kg/m³ under standard gravity: exactly 98.0665 Pa. Reference: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).

inch of water (inH₂O)

The conventional inch of water (inH₂O, also written in WC, "WC or in WG) is the pressure of a 1 inch column of water of density 1000 kg/m³ under standard gravity: exactly 249.08891 Pa. Reference: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).

inch of water (60 °F) (inH₂O (60 °F))

The inch of water at 60 °F is the pressure of a 1 inch column of water at 60 °F (15.6 °C) under standard gravity, about 248.84 Pa. It is roughly 0.1 % smaller than the conventional inch of water. Reference: Water at 60 °F (15.6 °C), density ≈ 999.0 kg/m³, standard gravity. NIST value, 5 significant figures.

inch of mercury (inHg)

The conventional inch of mercury (inHg) is the pressure of a 1 inch column of mercury of density 13 595.1 kg/m³ under standard gravity: 3386.388640341 Pa. Reference: Conventional mercury column: ρ = 13 595.1 kg/m³ and gₙ = 9.80665 m/s² (≈ mercury at 0 °C / 32 °F).

atmosphere (atm)

The standard atmosphere (atm) is defined as exactly 101 325 Pa, approximately the mean air pressure at sea level.

barye (Ba)

The barye (Ba) is the CGS unit of pressure, one dyne per square centimetre, equal to exactly 0.1 Pa.

All pressure conversions

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Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures.
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology.

Last reviewed: 2026-10-15.