Pressure Converter

Force spread over area

Pressure is the quiet workhorse of physics: force divided by the area it acts on. The SI unit, the pascal, is small enough that a sheet of A4 paper resting on a desk exerts only a few pascals. Real-world pressures live in kPa, bar, atm, PSI, and the stubbornly persistent millimetre of mercury, each suited to a different trade.

Mercury, mountains, and the birth of the unit

The story begins in 1643 with Evangelista Torricelli, who inverted a glass tube of mercury into a dish and watched the column settle at roughly 760 mm. He had built the first barometer and demonstrated, against centuries of doctrine, that air has weight. The unit torr bears his name, and one torr is defined as exactly 1/760 of a standard atmosphere.

Blaise Pascal pushed the idea further by carrying a barometer up the Puy de Dôme volcano in 1648, showing the column shortened with altitude. Centuries later, the SI unit was named in his honour.

Industries and their preferred scales

Meteorologists in Europe report air pressure in hectopascals or millibars (the two are numerically identical). American forecasters use inches of mercury. Hospitals worldwide measure blood pressure in mmHg despite metrication efforts. Engineers tightening bolts and inflating tyres work in PSI, while structural and materials specialists climb into MPa and GPa. Vacuum technicians, almost uniquely, still talk in torr.

The mistakes that bite

Three errors recur across labs and workshops:

1. Bar versus atmosphere. They are close but not equal. One bar is exactly 100,000 Pa; one atm is exactly 101,325 Pa. The 1.3 percent gap is invisible in a tyre but ruinous in calibration work.
2. Gauge versus absolute. A tyre gauge reading 32 PSI means 32 PSI above atmospheric. The absolute pressure is closer to 47 PSIA. Engineers write PSIG and PSIA precisely to avoid this confusion.
3. mmHg versus inHg. Both measure mercury columns, but in different units; 760 mmHg equals about 29.92 inHg.