Pressure Converter

Tyre pressure is in psi in some countries and bar in others, weather forecasts use millibar or inches of mercury, and engineers like pascals. This converter moves between pascals, kilopascals, megapascals, bar, millibar, atmospheres, psi, millimetres of mercury and inches of mercury, with every result shown at once.

How to use it

1. Enter the pressure you have.
2. Choose the unit it's in and the unit you want.
3. Read the headline result and the other equivalents.
4. Swap units to flip it.

When it helps

• Checking a tyre pressure given in psi against a bar gauge.
• Reading weather pressure in millibar or inches of mercury.
• Comparing equipment ratings from manufacturers in different countries.
• Engineering and diving calculations.

Good to know

• One standard atmosphere is 101,325 pascals, about 14.7 psi, and very close to 1 bar.
• The tool converts numbers only. Whether a reading is gauge or absolute pressure depends on the instrument, and the two differ by about one atmosphere.
• A torr is almost exactly one millimetre of mercury.

How the conversion works

Pressure is force per area, and the SI unit, the pascal, is one newton per square metre. It's a tiny amount, which is why kilopascals and megapascals are common. Everything here converts through pascals: a bar is 100,000 Pa, an atmosphere 101,325, a psi 6,894.76, and a millimetre of mercury about 133.32.

The mercury units come from the barometer, where air pressure supports a column of mercury about 760 mm high at sea level. That's why a standard atmosphere is 760 mmHg, and why weather reports in some countries still quote inches of mercury, around 29.92 for a standard day.

Worked example: car tyres

A car manual lists tyre pressure as 2.3 bar and your pump reads in psi. Enter 2.3 with bar selected and read the psi line: about 33.4. Another handy case is a tyre label in kilopascals, such as 240 kPa, which is 34.8 psi.

Remember that tyre gauges show gauge pressure, the pressure above the surrounding air. Absolute pressure is about one atmosphere higher. The converter handles the numbers only, so make sure both figures use the same reference.

Where each unit is used

Meteorologists use hectopascals or millibars, which are identical, with a typical value of 1013. Divers use bar, with each 10 metres of water adding about one bar. Hydraulic systems and engineering use megapascals, and vacuum work uses torr or pascals. Blood pressure still uses mmHg, such as 120 over 80.

Because units differ so widely between industries, specifications from overseas suppliers often need converting. A pressure vessel rated 150 psi is about 10.3 bar, a useful check before you connect equipment.

Safety and accuracy

For anything where safety matters, such as gas cylinders, pressure cookers or compressed air equipment, always follow the manufacturer's stated limits and units. Use conversions to understand them, not to override them.

Rounding can add up, so keep several digits during a calculation and round only at the end. The table here shows enough digits for most practical needs.

Questions