Gauge or absolute?
This is the question that causes real pressure errors, and it is not a unit question at all. The same physical pressure has two different numbers depending on what it was measured against.
Gauge pressure — written psig, or barg — is measured relative to the surrounding air. A tyre gauge reading zero on a flat tyre is telling you the tyre is at atmospheric pressure, not that it contains nothing. Absolute pressure — psia, or bara — is measured relative to a vacuum, so the same flat tyre reads about 14.7 psia.
The gap between them is one atmosphere: about 14.696 psi, 1.013 bar, or 101.325 kPa. And because it is a fixed offset rather than a scale factor, no conversion can bridge it. Multiplying a psig figure by any number will never turn it into psia; you have to add local atmospheric pressure, and that is a physical fact about the measurement rather than an arithmetic step.
So this converter converts between units and deliberately does not offer a gauge-to-absolute switch. A tool that did would have to invent a value for local atmospheric pressure — which changes with altitude and weather — and present it as part of the answer. What it does instead is say plainly which question is which, so that you check the reference on your own source before trusting the number.
A rough guide to which you have: anything read off a dial attached to the thing being measured is almost certainly gauge. Anything in a thermodynamics calculation, a gas law, or a vacuum specification is almost certainly absolute — gas laws require it, in the same way they require kelvin rather than Celsius.
Tyre pressure, in detail
Tyres are where most people meet pressure units, and they meet three at once: psi in the US and UK, bar across much of Europe, and kPa on many placards and on Japanese and Korean vehicles. 32 psi is 2.21 bar is 221 kPa — and a placard showing 220 is kPa, not something else.
Two things matter more than the conversion. First, the number to use is the vehicle’s, not the tyre’s. The figure moulded into the sidewall is the tyre’s maximum, not a recommendation; the correct pressure is on the placard in the driver’s door jamb, in the fuel filler flap, or in the handbook, and it often differs front to rear and rises for a loaded vehicle.
Second, the placard figure is a cold pressure. Driving heats the air and raises the reading by roughly 3 to 5 psi, so checking a warm tyre and setting it to the placard figure leaves it under-inflated once it cools. Cold weather does the reverse: pressure falls about 1 psi for every 5.5 °C drop, which is why tyre warning lights appear on the first cold morning of autumn without anything being wrong.
All tyre pressures are gauge pressures, which is why a tyre at “zero” still has air in it.
The torr and mmHg near-miss
The torr and the millimetre of mercury are two units that are almost, but not exactly, the same — and the difference is a good illustration of why this site stores exact definitions rather than decimals.
A torr is defined as exactly 1/760 of a standard atmosphere, which makes it 101325/760 pascals — a repeating decimal, about 133.3224 Pa. A millimetre of mercury is defined instead from the density of mercury and standard gravity, giving exactly 133.322387415 Pa.
They agree to about one part in seven million, which is far below the resolution of any instrument that reports in either unit, so in practice they are interchangeable and most references treat them as identical. They are still not the same number, and a converter that quietly maps one onto the other is making a decision rather than performing a conversion. Both are listed here with their own factors.
Where you meet them: mmHg is the standard unit for blood pressure worldwide and for barometric pressure in aviation and older meteorology; the torr belongs to vacuum work, where a rough vacuum is measured in torr and a high vacuum in millitorr.
Which unit belongs where
Pressure has more units in active use than any other dimension on this site, and each belongs to a field rather than to a country.
psi — US and UK tyres, compressed air, hydraulics, plumbing. ksi — a thousand psi, used for material strengths in US engineering. bar— European tyres, diving, industrial and process equipment; it is near enough one atmosphere to be intuitive, being 100 kPa against the atmosphere’s 101.325.
kPa and MPa — the SI units, standard in engineering worldwide and on most modern tyre placards; concrete and steel strengths are quoted in MPa. atm — chemistry and physics, where a round reference pressure is wanted. mmHg and torr — medicine, meteorology and vacuum work, as above.
kgf/cm² deserves a note: also written “at”, the technical atmosphere, it is close to a bar without being one, and it still appears on older European and Indian equipment and on Japanese machinery. Reading it as a bar overstates by about 2%, which is usually harmless and occasionally is not.
Doing it in a spreadsheet
Write the factors out rather than relying on unit codes: =A1*0.0689475729 for psi to bar, =A1*14.5037738 for bar to psi, =A1*6.89475729 for psi to kPa, and =A1*101.325 for atmospheres to kPa. All are exact to the digits shown.
For the gauge-and-absolute step, keep it visible as its own cell rather than folding it into a conversion: =A1+14.696 takes psig to psia at sea level, and =A1-14.696 comes back. Putting local atmospheric pressure in its own named cell is better still, because at 1,000 m of altitude it is nearer 13.0 psi than 14.7 — and a sheet that hides that behind a constant will be wrong by 12% for anyone working at height, with nothing on screen to show why.
Sources and methodology
Nothing here is fetched and there is no data feed: every figure is computed from what you type. The pascal, bar and atmosphere are defined exactly, and psi is derived exactly from the 1959 pound and standard gravity rather than typed in as a decimal. The source that matters most is NIST’s guide, which is explicit that gauge pressure has to be identified as such — the distinction this page is built around.