Pressure Converter

Using the Pressure Converter

1. Pick the source unit - pascal, kilopascal, hectopascal (millibar), bar, standard atmosphere, pound-per-square-inch (psi), millimeter of mercury or torr. Between them, they cover meteorology, tire gauges, vacuum rigs and medical blood-pressure cuffs.
2. Type the value into the input. Output is recomputed on every keystroke via the Preact onInput handler.
3. Pick the target unit. The conversion uses pascal as its internal pivot so no matter which pair you pick, the round trip is one multiply and one divide.
4. Swap sides with the swap button to go the other way without retyping.
5. Copy the result to the clipboard through the Clipboard API.

How the Conversion Works Internally

Every unit is registered as a factor relative to the pascal, the SI coherent derived unit for pressure (N/m^2, SI Brochure 9th ed., Table 4). The chain of definitions is: 1 bar = 100,000 Pa exactly (IUPAC 1982 choice of standard pressure); 1 atm = 101,325 Pa exactly (CGPM 1954); 1 torr = 101325 / 760 Pa exactly (IUPAC); 1 mmHg historically differed from torr by a few parts per million and has been treated as 133.322387415 Pa since the adoption of standard gravity (g_n = 9.80665 m/s^2, CGPM 1901). The psi uses the exact pound-force 4.4482216152605 N and the exact square inch 0.00064516 m^2, yielding 6894.757293168361 Pa per psi. Because every factor is rational or exact-rational, the only rounding you see comes from the 10-significant-digit output formatter.

Where You Actually Use This

• Reading a tire placard in psi when your local gauge shows bar or kPa.
• Cross-referencing a barometric pressure report (1013 hPa) against a US weather site that prints inches of mercury.
• Tuning espresso pump pressure - nine bars, the classic brew target, converts to about 130.5 psi or 900 kPa.
• Vacuum physics, where chamber pressures are quoted in torr, mbar or Pa interchangeably depending on the vendor.
• Medical records using mmHg for blood pressure versus European references in kPa.
• Industrial pneumatic specs mixing bar, psi and kPa in the same datasheet.

Pitfalls

The biggest trap is gauge vs absolute pressure. A tire gauge reading 32 psi means 32 psi above atmospheric - the absolute pressure inside the tire is 32 + 14.7 = 46.7 psi. Engineers write "psig" for gauge and "psia" for absolute to disambiguate, but labels slip. This converter treats every input as a number; if you have a gauge reading and want absolute, add standard atmosphere (101,325 Pa) after conversion. A second pitfall is "inches of mercury": inHg at 0 C is the meteorological standard, inHg at 60 F is the American industry convention for fuel and oil work, and the two differ by about 0.3%. This page does not yet expose inHg - ask via feedback if you need it. A third: pounds per square foot, megapascals (MPa, common for concrete strength) and technical atmosphere (at, 98,066.5 Pa) are also absent. For now, convert via kPa or psi as an intermediate.

Where the Units Came From

The pascal is named for Blaise Pascal, whose 1648 Puy-de-Dome experiment proved barometric pressure varies with altitude. The bar was coined by Vilhelm Bjerknes around 1906 for convenience in meteorology - exactly 10^5 pascal. The atmosphere was standardized in 1954 by the 10th CGPM at 101,325 Pa precisely to end disagreements between physicists and engineers about which "atmosphere" to use. The torr honors Torricelli, who built the first mercury barometer in 1643. The psi dates to the late 19th century and survives mostly in US engineering tradition - ASME piping codes, aviation, tire industry. NIST SP 811 table lists all the factors used by this converter; see Appendix B.8 for the canonical values.

Alternatives

Google and Spotlight do simple pressure conversions but round aggressively and do not distinguish gauge from absolute. For engineering work involving steam, refrigerants or real gases, the IAPWS and NIST REFPROP packages are the correct tools because they couple pressure to temperature and composition. Python's pint library propagates pressure units through a formula chain, catching unit mismatches at runtime - helpful when pressure combines with area, volume or velocity into something else. For meteorological data specifically, a barometric altitude calculator is better because pressure drops exponentially with altitude and a simple unit conversion loses that physics.