Pressure units: psi, bar, and kPa

Pressure is force spread over an area. Scientists measure it in pascals (newtons per square metre), but everyday gauges almost never show raw Pa — the numbers would be huge. Tyre stickers, compressor dials, and engineering sheets therefore use friendlier units such as psi, bar, and kilopascals (kPa).

What each unit is

• Pascal (Pa) — the SI unit. 1 Pa = 1 N/m².
• Kilopascal (kPa) — 1,000 Pa. Convenient for weather charts and many metric technical specs.
• Bar — defined as exactly 100,000 Pa (100 kPa). Close to one standard atmosphere, which is why it appears on European tyre and hydraulic gear.
• psi — pounds-force per square inch. Still dominant on US and many older Commonwealth tyre placards.

Exact conversion factors

Our Pressure converter links the three display units through pascals using SI-exact definitions for the pound-force and inch:

• 1 bar = 100 kPa = 100,000 Pa
• 1 psi ≈ 6,894.757 Pa ≈ 6.895 kPa
• 1 bar ≈ 14.5038 psi

A standard atmosphere is about 101.325 kPa or 14.696 psi — not exactly 1 bar, though 1 bar is a tidy SI round number nearby.

Gauge pressure vs absolute pressure

Most tyre and workshop gauges show gauge pressure: the difference from surrounding air. Absolute pressure counts from vacuum instead. Converting units does not change which reference you started with — 32 psi gauge is still “32 psi above ambient” after you write it as 2.2 bar. Mixing gauge and absolute values without saying so is a classic engineering error.

Worked examples

A cold car tyre listed at 32 psi is about 2.21 bar or 221 kPa. A bicycle tyre at 6 bar is about 87 psi. Sea-level atmospheric pressure is roughly 101 kPa absolute — tyre stickers remain gauge readings on top of that.

When to use which unit

• psi — US tyre placards, many North American compressors.
• bar — European tyre markings, lots of industrial hydraulics.
• kPa — SI-friendly documentation, some HVAC and meteorology contexts.

Common mistakes

• Treating “bar” and “atmosphere” as identical.
• Reading blood pressure in mmHg and assuming it is already kPa.
• Confusing pressure (force/area) with power or energy.

FAQs

Is higher psi always safer for tyres?

No — follow the vehicle placard for load and size. Overinflation changes handling and wear.

Why not show mmHg here?

Clinical blood-pressure units are a specialised domain; this guide focuses on mechanical gauge units.

Does altitude change gauge readings?

Ambient pressure changes with altitude; absolute pressure of a sealed tank does not, but gauge reading relative to new ambient will.

Tyres, workshops, and datasheets

Passenger-car tyre placards usually quote cold inflation pressures. “Cold” means before a long drive warms the air inside the tyre and raises gauge readings. Workshop compressors may show bar even when a vehicle handbook shows psi — convert once, then label your notepad so you do not reinflate to the wrong scale. Industrial datasheets sometimes list both; when only one appears, convert with the SI chain through pascals rather than chaining approximate mental factors twice.

Related mechanical quantities

Pressure is not flow, torque, or power. A hydraulic system can show high pressure while moving little fluid per second. If you are comparing motors or heaters, switch to the power or energy guides instead of forcing everything into bar.

Sources

SI brochure (BIPM) for the pascal; NIST / CODATA relationships for the international pound-force and inch used to define engineering psi.