Guide

Pressure in engineering, weather and medicine

Pressure is stated in bar, pascals, psi, hPa and mmHg, in different units depending on the field. On top of that, absolute and gauge pressure are distinguished, which turns one number into two different pressures.

The SI unit is the pascal

One pascal is one newton per square metre, a very small unit. One bar is 100,000 pascals, one hectopascal 100 pascals. Air pressure at sea level is about 1013 hPa, roughly 1.013 bar. Meteorology works in hectopascals, engineering mostly in bar, Anglo-American countries in psi.

Absolute and gauge pressure

Absolute pressure refers to vacuum, gauge pressure to the surrounding air pressure. A tyre at 2.5 bar gauge holds about 3.5 bar absolute. Because both notations usually appear without a qualifier, the context must tell which is meant. For tyre pressure and working pressure it is normally the gauge value.

psi, atm and the 14.5 approximation

One psi is one pound-force per square inch and equals 6894.757293168361 pascals. One bar is therefore about 14.5038 psi. The frequently quoted 14.7 belongs to the standard atmosphere instead: 1 atm = 101,325 Pa = about 14.6959 psi. The two reference units differ by only 1.3 %, so their psi values are often mixed up.

mmHg in medicine

The mercury column is a historical pressure unit and remains common in medicine. One mmHg equals 133.322387415 pascals, about 1.333 hPa. A reading of 120 mmHg therefore equals about 160 hPa: the same numeric range as a weather figure, but a different pressure. Medical values belong with the treating professional; unit arithmetic does not replace a diagnosis.

Matching conversions

Matching tables

Unit family: Pressure

Sources