How bar, psi, kPa and atm relate
All four units measure the same thing, force spread over an area, so converting between them is pure multiplication. The kilopascal is the metric anchor: a pascal is one newton pressing on one square metre, and the other three units are defined against it exactly.
Bar is the round metric unit, deliberately set at 100 kPa because that is close to the weight of the atmosphere. Psi, pounds per square inch, is the imperial unit still printed on British tyre gauges and bike pumps. The atmosphere is the odd one out: it is the average air pressure at sea level, 101.325 kPa, so 1 atm is 1.3 percent more than 1 bar. The two are close enough that people treat them as interchangeable in conversation, but not on a calibration certificate. The handiest rule of thumb links the two units you actually meet: 1 bar is 14.5 psi, so divide psi by 14.5 to get bar and multiply to go back.
Tyre pressures in psi and bar
Tyres are where most people meet this conversion. British handbooks and forecourt gauges lean on psi, while European cars and many garage inflators read in bar, and some list kPa for good measure. The right pressure for your car is on a sticker inside the driver's door frame or in the handbook, usually with different figures for front and rear and a higher set for a fully loaded car, so check yours rather than borrowing a number from a similar model. Check when the tyres are cold, since a few miles of driving warms the air and raises the reading. The common range converts like this:
- 30 psi = 2.07 bar = 206.8 kPa
- 32 psi = 2.21 bar = 220.6 kPa
- 34 psi = 2.34 bar = 234.4 kPa
- 36 psi = 2.48 bar = 248.2 kPa
Underinflated tyres wear faster and drag more fuel through the engine, so a monthly check pays for itself; our fuel cost calculator will show you what a tank actually costs.
Worked examples
Where else these units turn up
Road bike tyres run far harder than car tyres, typically 80 to 120 psi, which is 5.5 to 8.3 bar, and a decent track pump shows both scales. An espresso machine brews at around 9 bar, about 130 psi, which is why the gauge on the front sweeps so far. Weather forecasts use millibars or hectopascals, which are the same size unit: 1 millibar is 0.1 kPa, and the sea-level average of 1013 millibars is exactly 1 atm. Divers think in atmospheres because every 10 metres of depth adds roughly 1 bar of water pressure on top of the air above. Whatever the context, the conversion factors never change, so the tool above covers the lot: type a value, pick the unit it is in, and read off the other three. For the rest of the garage maths, our MPG converter and speed converter work the same way.
Frequently asked questions
What is 32 psi in bar?
32 psi is 2.21 bar, or 220.6 kPa. To convert any tyre pressure, multiply psi by 0.0689 to get bar, or multiply bar by 14.5 to get psi. Most UK tyre pressures sit between 30 and 36 psi, which is roughly 2.1 to 2.5 bar.
What is the difference between bar and atm?
They are close but not identical. A bar is a round metric unit of exactly 100 kPa, while an atmosphere is the average air pressure at sea level, defined as 101.325 kPa. So 1 atm is 1.01325 bar, about 1.3 percent higher than 1 bar.
Why do UK tyre pressures use psi?
Habit, mostly. Psi (pounds per square inch) is an imperial unit that stuck around on British forecourts and pressure gauges, while car handbooks now usually print both psi and bar. European equipment tends to read in bar, which is why many garage inflators let you switch between the two.
How many kPa are in a bar?
Exactly 100. The bar was deliberately defined as 100,000 pascals, so kPa to bar is just a matter of moving the decimal point two places. That makes kPa handy as a bridge unit: convert anything to kPa first and bar follows instantly.
Are these conversions exact?
Yes, apart from display rounding. Bar, kPa and atm are all defined exactly in pascals, and psi comes from the exactly defined pound and inch, giving 1 psi = 6.894757 kPa to the precision shown. The tool uses the full factors internally and rounds only for display.