Pressure

What pressure units mean

Pressure is force spread over an area. In SI that means pascals (N/m²). Everyday gauges rarely show raw pascals, so this page focuses on three common display units: psi (pounds-force per square inch), bar (exactly 100,000 Pa), and kilopascals (kPa, exactly 1,000 Pa).

Tyre stickers in the US often use psi. Many European cars and workshop gauges use bar. Weather charts, some HVAC specs, and blood-pressure contexts more often use kilopascals or millimetres of mercury — kPa is the SI-friendly middle ground included here.

Conversion factors

This converter uses exact SI links. One bar is defined as 100,000 Pa and one kilopascal as 1,000 Pa. Psi is built from the exact pound-force and inch definitions in SI (so 1 psi ≈ 6,894.757 Pa). Editing any field recalculates the others from a common pascal value so all three stay consistent.

• 1 bar = 100 kPa
• 1 psi ≈ 6.895 kPa
• 1 bar ≈ 14.5038 psi

Worked example

A cold car tyre listed at 32 psi is about 2.206 bar or 220.6 kPa. Absolute atmospheric pressure at sea level is roughly 14.7 psi / 1.013 bar / 101.3 kPa — gauge readings on tyres are usually relative to ambient air, not absolute vacuum.

When to use which unit

Use psi when matching a US tyre placard or compressor gauge. Use bar for many European tyre and hydraulic markings. Use kPa when you want a straightforward SI multiple (1 bar = 100 kPa) without dealing with tiny pascal numbers. Do not mix gauge and absolute pressure without knowing which the source document means.

Common mistakes

• Treating “bar” and “atmosphere” as identical — standard atmosphere is ≈ 1.01325 bar, not exactly 1.
• Confusing gauge pressure (relative to ambient) with absolute pressure (relative to vacuum).
• Reading blood pressure in mmHg and assuming it is already in kPa without converting.

FAQs

Is this gauge or absolute pressure?

The maths convert magnitude between units only. Enter the same kind of reading you started with (gauge or absolute).

Why not include mmHg or atm?

Those appear elsewhere in specialised contexts; this page keeps the three units most people need for tyres, compressors, and technical datasheets.

Full guide: Pressure units: psi, bar, and kPa

Related: Power, Temperature, Liquids.