How this instrument works
The bar is a metric unit the SI never adopted. Vilhelm Bjerknes, the Norwegian meteorologist who put weather forecasting on a physical footing, pushed it around 1909 — from the Greek baros, weight — because he wanted a round figure near sea-level air pressure. One bar is exactly 100 kPa, or 100,000 pascals. The standard atmosphere is 1.01325 bar, so he overshot by 1.3 percent and nobody minded. Millibars ruled synoptic charts for seventy years until the hectopascal quietly replaced them, digit for digit.
The pound per square inch descends from two definitions pinned down a lifetime apart. A pound-force is the international avoirdupois pound — 0.45359237 kg since the 1959 yard and pound agreement — multiplied by standard gravity, 9.80665 m/s² since the third CGPM fixed it in 1901. That product is 4.4482216152605 newtons exactly. A square inch is 0.0254 m squared, which is 0.00064516 m² exactly. Divide force by area and one psi is 6894.757293168… pascals, with no measurement anywhere in the chain.
Dividing 100,000 pascals by that figure gives 14.5037737730209215…, which is the reduced fraction 129032000000000⁄8896443230521. It repeats instead of terminating because the square inch carries 127² down into the denominator, and 127 divides neither two nor five. This sheet applies 14.503773773, the ratio truncated at nine decimals, which sits low by about one part in seven hundred billion — far past what any gauge, transducer or dead-weight tester can resolve.
- Type your reading into Bar (bar). The field opens at 2, a typical cold tyre pressure on a European door placard.
- Read the answer from PSI (psi) directly beneath it, carried to six decimals and recomputed on every keystroke.
- Working backwards from psi? Divide your figure by 14.503773773, or nudge Bar (bar) until PSI (psi) lands on it.
- Check whether your instrument shows gauge or absolute pressure before matching the result to a spec sheet: barg pairs with psig, bara with psia.
- Coming from megapascals, remember 1 MPa is exactly 10 bar — enter ten times your MPa figure.
Worked example — a 2 bar placard on a forecourt gauge
A car built for the German market carries a door-jamb sticker calling for 2.0 bar cold in the front tyres. Enter 2 into Bar (bar) and PSI (psi) returns 29.007547546. On a forecourt hose graduated in whole pounds you would dial 29 and stop, and the thousandths you throw away are trivial beside the real variable: one degree Celsius of overnight cooling moves that same tyre by roughly half a pound, which is why placards always specify a cold reading.
Scale up and the decimals start earning their keep. An espresso group head held at 9 bar is 130.53 psi. A 200 bar diving cylinder is 2900.75 psi, which is why the same steel bottle sells in the United States as a 3000 psi tank — that rating is 206.84 bar, a genuinely different number rather than a translation of the first.
Questions
Is one bar exactly 14.503773773 psi?
Not quite — that value is truncated. Both units are defined exactly, so their ratio is an exact rational number, 129032000000000⁄8896443230521, whose decimal form opens 14.5037737730209215 and runs on without repeating for a long while. Nine decimals is where this sheet stops, leaving the result low by roughly one part in seven hundred billion. No pressure instrument ever built resolves anything close to that.
What is the difference between barg and psig?
Gauge pressure is measured against the surrounding air; absolute pressure is measured against vacuum. A tyre reading 2 barg actually holds 3.01325 bar absolute, because the atmosphere is already pressing back at 1.01325 bar. Convert first, then add or subtract — zero barg is 14.696 psia. Feeding a gauge figure into an absolute specification is the most common mistake in pressure work, and it costs 14.7 psi every time.
Is a bar the same as an atmosphere?
No, though they sit within one and a half percent of each other. The standard atmosphere is fixed at 1.01325 bar, or 14.695949 psi. A third unit muddies older equipment: the technical atmosphere, one kilogram-force per square centimetre, equals 0.980665 bar and 14.223343 psi. Japanese and Soviet-era gauges are frequently marked kgf/cm², and reading those graduations as metric leaves you two percent low.
Why does this conversion never come out even?
Because the inch drags a 127 into the arithmetic. An inch is 0.0254 m, which is 127⁄5000, so a square inch is 127²⁄5000² of a square metre. Any reduced fraction whose denominator holds a prime other than two or five produces a repeating decimal, and 127 is prime. The bar, by contrast, is a clean power of ten above the pascal, so every bit of untidiness here arrives from the imperial side of the equation.
Should technical documents quote bar or pascals?
Pascals, strictly. The BIPM does not encourage the bar, and NIST Special Publication 811 asks writers to prefer kPa or MPa. In practice, hydraulics, pneumatics, diving and European process plants still run on it, and dislodging that habit mid-industry costs more than tolerating it. If you need a bridge between the two habits, 1 MPa is exactly 10 bar and 145.037737730 psi.
Does psi mean the same thing in the UK and the United States?
Yes. The inch and the avoirdupois pound have been identical on both sides of the Atlantic since 1959, so the psi carries none of the imperial-versus-US-customary split that divides the gallon or the fluid ounce. Watch its neighbours instead: psf is pounds per square foot, one one-hundred-and-forty-fourth of a psi, while ksi means a thousand psi and is standard in structural steel work.