How to use the Pressure Converter
- Enter the pressure reading you have.
- Choose its unit and the unit you need.
- Note whether your source figure is gauge or absolute pressure before you rely on it.
- For tyres, psi and bar are the usual pair; for weather, hectopascals and inches of mercury.
- Blood pressure figures are always mmHg gauge, relative to atmospheric.
How the calculation works
The pascal — one newton per square metre — is the SI unit and the base for every conversion here. It is inconveniently small for practical work, which is why derived multiples dominate in the field: the bar is 100,000 Pa, chosen because it is close to average sea-level atmospheric pressure, and the hectopascal used in meteorology is numerically identical to the older millibar, which is why the switch caused no disruption to weather charts.
Gauge versus absolute is the distinction that causes real errors. A tyre inflated to 32 psi holds 32 psi above the surrounding air, so its absolute pressure is about 46.7 psi. Gas law calculations, compressor sizing and vacuum work all require absolute pressure; tyre gauges, hydraulic readouts and blood pressure cuffs all report gauge. Converting units correctly will not save a calculation that mixed the two references.
The mercury-column units are historical instruments preserved as units. One millimetre of mercury, also called a torr, is the pressure exerted by a 1 mm column of mercury under standard gravity — 133.322 Pa. Medicine kept mmHg because sphygmomanometers were literally mercury columns; aviation and US weather reporting kept inches of mercury for the same instrumental reason, with standard sea-level pressure being the familiar 29.92 inHg.
pressure_target = (p_source × Pa_per_source) ÷ Pa_per_target; 1 bar = 100,000 Pa; 1 psi = 6,894.757 Pa; 1 atm = 101,325 Pa; 1 mmHg = 133.322 PaSource: BIPM SI Brochure 9th edition; ISO 80000-4; NIST Special Publication 811.
Worked example
A European car's door plate specifies 2.4 bar for the front tyres and the forecourt compressor reads in psi.
- Enter 2.4 and select bar.
- Convert to pascals: 2.4 × 100,000 = 240,000 Pa.
- Convert to psi: 240,000 ÷ 6,894.757 = 34.8 psi.
- Set the compressor to 35 psi cold, and check when the tyres have not been driven on for a few hours.
2.4 bar is 34.8 psi — round to 35 psi, and remember both figures are gauge pressures measured cold.
Frequently asked questions
How many psi in a bar?+
14.5038 psi. The rough conversion of 1 bar to 14.5 psi is accurate enough for tyres.
Is a bar the same as an atmosphere?+
Very nearly but not exactly. One standard atmosphere is 101,325 Pa, while one bar is 100,000 Pa — about 1.3% lower.
What is the difference between gauge and absolute pressure?+
Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure includes it. Absolute = gauge + about 14.7 psi at sea level.
Why does weather use hectopascals?+
Because 1 hPa equals 1 millibar exactly, so the switch to SI units left every historical chart and forecast figure numerically unchanged.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.