Pressure Converter

Enter a pressure in psi, bar, kPa, atmospheres or millimeters of mercury and this converter shows it in every other unit at once, with the pascal (the SI unit) as the base. It covers everyday jobs such as reading a European tire placard in psi, checking a pump or compressor rated in bar, or making sense of blood pressure and vacuum readings in mmHg.

The converter changes units only — it does not switch between gauge and absolute pressure. A tire gauge reading of 32 psi means 32 psi above the surrounding air (psig); the absolute pressure inside the tire is about 46.7 psi (psia). Add or subtract one atmosphere yourself when a formula needs absolute values, as the ideal gas law and combined gas law do.

The bar and the standard atmosphere are exact definitions in the SI system as published by NIST and the BIPM; the psi follows from the international pound (0.45359237 kg) and inch (25.4 mm). One torr is 1/760 of an atmosphere, which differs from 1 mmHg by less than one part in a million, so you can read torr as mmHg. For inches of mercury (inHg, used on US barometers and in aviation), 1 inHg ≈ 3,386.39 Pa ≈ 33.86 hPa — convert to kPa here first, then divide by 3.386.

Values are approximate and show gauge pressure unless marked absolute. They are useful for sanity-checking a conversion — if your answer for a car tire comes out at 30 bar, a factor has gone wrong.

• Gauge (psig, barg): pressure above the local atmosphere. Tire gauges, compressors, water pressure and blood pressure all read this way, so a flat tire shows 0.
• Absolute (psia, bara): pressure above a perfect vacuum. Weather reports, gas-law calculations and altitude work use absolute values.
• Converting between them: absolute = gauge + atmospheric pressure. At sea level that is +14.7 psi, +1.013 bar or +101.3 kPa; at altitude the atmosphere is lower, so use the local barometric reading.
• Vacuum: often given as a negative gauge value or in mmHg/inHg below atmosphere. A perfect vacuum is −14.7 psig, or 0 psia.

The psi dominates in the US for tires, air tools, plumbing and hydraulics. The bar is standard on European tire placards, pumps, coffee machines and dive gear, and the kPa appears on Canadian, Australian and Asian tire placards and on many HVAC instruments. Meteorologists use hectopascals (1 hPa = 0.1 kPa = 1 millibar), so a standard 1013.25 hPa reading is 101.325 kPa. mmHg survives in medicine for blood pressure, and duct static pressure in HVAC is usually in inches of water — see the static pressure calculator for that work.