Pressure Converter

Last updated: 2026-05-13

Pressure Units: One Phenomenon, Many Scales

Pressure is force per unit area — but the units used to measure it span an extraordinary range depending on the application. Meteorologists track the atmosphere in hectopascals. Doctors record blood pressure in mmHg. Engineers rate hydraulic systems in bar or MPa. Tyre labels in the US show psi. This converter lets you move freely among all of them.

Conversion Reference

All conversions pivot on the pascal (Pa), the SI unit of pressure (1 N/m²). Every other unit is an exact or defined multiple.

Unit	In pascals (Pa)	Typical context
1 Pa	1	SI base; scientific reference
1 hPa	100	Meteorology; equals 1 mbar
1 kPa	1 000	Tyre pressure (metric), medical vacuum
1 MPa	1 000 000	Hydraulics, high-pressure engineering
1 bar	100 000	Industrial gauges, weather maps
1 mbar	100	Upper-atmosphere weather, vacuum work
1 atm	101 325	Sea-level reference; chemistry stoichiometry
1 mmHg	≈133.322	Blood pressure, manometry
1 Torr	≈133.322	Vacuum systems (near-identical to mmHg)
1 psi	≈6 894.76	Tyres and pipes in the US

When Each Unit Shows Up

Pa and kPa are the scientific defaults. Weather reports in most of the world use hPa — the numbers happen to equal the old millibar readings that meteorologists had already memorised. Standard sea-level pressure is 1013.25 hPa.

Bar is ubiquitous in European industrial gauges and tyre pressure labelling. A car tyre at 2.2 bar ≈ 220 kPa ≈ 32 psi. MPa appears whenever pressures get large: hydraulic presses, water-jetting equipment, and high-strength material testing commonly work at tens or hundreds of MPa.

mmHg is the unit the medical world settled on for blood pressure because early sphygmomanometers were literal mercury columns. Normal blood pressure is around 120/80 mmHg (systolic/diastolic). In vacuum technology, Torr covers the same range and is essentially interchangeable — the two units differ by only about 0.000 015 %.

psi remains standard for tyre pressure in the United States. Car tyres typically run 30–35 psi; bicycle tyres 80–120 psi (road) or 25–35 psi (mountain). Many modern pressure gauges show both psi and kPa or bar.

atm is less common on gauges than the others, but it is the reference pressure for tabulated thermodynamic data (standard enthalpy, boiling points at "1 atm") and appears in dive-depth calculations (roughly 1 extra atm per 10 m of seawater).

A Note on Absolute vs. Gauge Pressure

Most everyday pressure readings are gauge pressure — pressure relative to the surrounding atmosphere. A tyre gauge reads 0 when the tyre matches atmospheric pressure, not when it is a vacuum. Absolute pressure adds atmospheric pressure back in (absolute = gauge + ~101 325 Pa). Scientific equations almost always want absolute pressure; gauges almost always show gauge pressure. Keep the distinction in mind when using converted values in calculations.

Frequently Asked Questions (FAQ)

What is the difference between Pa and hPa?

1 hectopascal (hPa) equals 100 pascals (Pa). Meteorologists report atmospheric pressure in hPa because the values are conveniently close to those in millibars (1 hPa = 1 mbar). Standard sea-level pressure is 1013.25 hPa.

How is 1 atmosphere defined?

One standard atmosphere (atm) is exactly 101 325 Pa. It was originally defined as the average air pressure at sea level at 0 °C, but is now a fixed constant used as a reference pressure in chemistry and physics.

Are mmHg and Torr the same?

They are nearly identical but not exactly equal. 1 Torr is defined as 101 325/760 Pa ≈ 133.3224 Pa, while 1 mmHg is defined from the physical properties of mercury and equals approximately 133.3224 Pa. For most practical purposes they are interchangeable, but they differ by about 0.000 015 %.

Why is tire pressure measured in psi?

Psi (pounds per square inch) is used for tire pressure in the United States and some other countries because it predates metric adoption. Typical car tire pressure is 30–35 psi (207–241 kPa). Many modern gauges show both psi and kPa.