Pressure Converter
Pa, kPa, bar, psi, atm, mmHg, inH₂O, and Torr — one value, every dialect.
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The engineering
Every industry kept its own gauge: US process plants and tire shops run psi, European P&IDs run bar (conveniently ≈1 atm), HVAC balancing reports inches of water, vacuum and medical people still speak mmHg/Torr, and the SI paperwork wants pascals — which are so small that real numbers arrive in kPa and MPa. The bar is not an SI unit and not an atmosphere: it's 100 kPa exactly, 1.3% below atm, a gap that matters in calibration labs and nowhere near a tire.
The classic mistakes are gauge-versus-absolute (psig vs psia differ by one whole atmosphere — sizing a relief valve on the wrong one is a bad day) and the near-miss pairs: mmHg vs Torr differ by parts in 10⁷ (fine), but kgf/cm² vs bar vs atm all sit within 3% of each other, close enough to pass a sanity check and still be wrong. When a datasheet just says 'bar', ask whether it means gauge.
Where this math comes from
Evangelista Torricelli built the mercury barometer in 1643 and gave pressure its first unit — the height of the column itself, which survives as mmHg on every blood-pressure cuff. Blaise Pascal then had his brother-in-law Périer carry a barometer up the Puy de Dôme in 1648, proving air pressure falls with altitude and that the 'sea of air' is real and weighable.
Industry added its own scales as steam arrived: Eugène Bourdon's 1849 curved-tube gauge put psi (or kgf/cm², depending on the country) on every boiler, and the dial-gauge dialects hardened for a century. The SI only got its own pressure unit in 1971, when the 14th CGPM named the newton-per-square-meter after Pascal — arriving so late that the older units had already colonized their industries, which is why this card exists.
- 1643Evangelista TorricelliMercury barometer — pressure measured as a column height.
- 1648Blaise Pascal & Florin PérierPuy de Dôme experiment: pressure falls with altitude.
- 1849Eugène BourdonBourdon-tube gauge puts a pressure dial on every boiler.
- 197114th CGPMThe pascal adopted as the SI unit of pressure.
See the full timeline of the math behind every calculator →
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