How this instrument works
One pascal is a newton spread over a square metre, so small that practical work counts in thousands of them. Sea-level atmosphere runs at 101.325 kPa, garden taps at roughly 400 kPa, filled scuba cylinders near 20 000 kPa. The 14th General Conference on Weights and Measures adopted this name in 1971, honouring Blaise Pascal, whose brother-in-law carried a barometer up the Puy de Dôme in 1648 and proved that air has weight.
A pound per square inch descends from two frozen definitions rather than any experiment. Standard gravity was pinned at 9.80665 m/s² in 1901 and the avoirdupois pound at 0.45359237 kg in 1959, which makes one pound-force 4.4482216152605 newtons precisely. Divide by one square inch — 0.00064516 m², since an inch is 25.4 mm exactly — and 6894.757293168362 Pa drops out. No measurement was involved; it is arithmetic on treaty text.
Turn that identity upside down and a kilopascal becomes 0.14503773773020922… psi, a repeating rational that this sheet truncates at eleven decimals. Discarding its tail shifts an answer by roughly one part in 700 billion, whereas a good forecourt dial resolves perhaps half a psi. Which unit you meet is a matter of geography and trade. Vehicle placards across Australia, New Zealand and much of Europe are printed in kPa, and so are water-main and irrigation specifications; American hydraulic fittings, compressor gauges, well-head data and ASME vessel stamps stay stubbornly in psi. Diving straddles both worlds, since cylinders are rated near 20 000 kPa in Europe and 3000 psi in North America.
- Type your reading into the Kilopascals (kPa) field. It opens at 200 kPa, a common cold-tyre placard figure.
- Read the PSI (psi) line below it — the figure recomputes on every keystroke, with no button to press.
- Working from psi instead? Multiply your figure by 6.894757293 to get kilopascals, then enter that to confirm the round trip.
- Keep both numbers on one datum. A gauge reads pressure above the surrounding air, so pair gauge with gauge and absolute with absolute.
- For workshop use, one decimal place is finer than any tyre gauge can resolve; the extra digits are there for calibration paperwork.
Worked example — a placard in kPa, a dial in psi
A hatchback sold in Melbourne carries a door-pillar placard reading 200 kPa front and rear, measured cold. Your compressor came from an American supplier and its dial is marked only in psi. Enter 200 into the Kilopascals (kPa) field and the PSI (psi) line answers 29.007547546.
You inflate to 29 psi and stop. What you threw away, 0.0075 psi, sits well inside one needle-width on that dial. Push it back in reverse as a check: 29.007547546 × 6.894757293168362 returns 200.000000 kPa to six decimals, so nothing has quietly leaked out of this arithmetic.
Questions
Is 0.14503773773 an exact factor?
It is an exact value, rounded. Because 1 psi is defined as 6894.757293168362 Pa, a kilopascal is exactly the fraction 1290320000000⁄8896443230521 psi, whose decimal expansion opens 0.14503773773020922 and then repeats forever. Cutting it at eleven places, as this sheet does, moves an answer by about 1.4 parts per trillion — on a 200 kPa tyre that is four hundredths of one nanopsi, millions of times finer than any dial, transducer or dead-weight tester will ever register.
Does this handle gauge pressure as well as absolute?
Yes. Multiplying rescales the number without touching its reference point, so 200 kPa gauge becomes 29.01 psig and 200 kPa absolute becomes 29.01 psia. Confusing the two is the classic blunder in this territory: at sea level they sit 101.325 kPa apart, which is 14.696 psi — roughly half a car tyre. Cylinder and compressor readings are almost always gauge; thermodynamic tables and gas-law calculations want absolute.
Why do Australian placards specify kPa when the air hose reads psi?
Australia and New Zealand metricated vehicle documentation during their 1970s changeover, so manufacturers stamp placards in kPa, while service-station gauges and drivers reading them never fully let go of psi. Daily arithmetic at an air hose follows: 200 kPa is 29.0 psi, 220 kPa is 31.9 psi, 250 kPa is 36.3 psi. Many modern forecourt units now show both scales, which is why those pairings look oddly unrounded.
Is a psi in Britain the same as a psi in the United States?
Identical. Since 1959, when six standards bodies signed their yard and pound agreement, imperial and US customary have shared one avoirdupois pound of 0.45359237 kg and one inch of 25.4 mm, and pressure needs nothing more. Gallons and tons still differ across an ocean; psi does not. One genuine trap sits nearby — pound-force against pound-mass — but pressure always means force per unit area, so a psi counts force.
How do bar and atmospheres line up with these numbers?
A bar is defined as exactly 100 kPa, so 1 bar is 14.503773773 psi — that is the same digit string as this factor, shifted two places, which is why European hydraulic and espresso specifications quoted in bar map so cleanly. A standard atmosphere is 101.325 kPa, or 14.695948776 psi, the familiar 14.7. Weather bulletins use hectopascals, where 1013.25 hPa is that identical standard atmosphere.
Does temperature change the conversion?
Never the factor, but often the pressure you feed it. At fixed volume, absolute pressure tracks absolute temperature. A tyre sitting at 200 kPa gauge on a 20 °C morning holds 301.3 kPa absolute; warm it to 30 °C and that climbs near 312 kPa absolute, or 210 kPa gauge — an extra 1.5 psi that appeared without a single molecule entering. Placard figures assume cold tyres for exactly this reason.