Pressure Units Explained, From Pascal to PSI

Pressure is force divided by the area it acts on. The same force pressing on a smaller area gives a higher pressure. That is why a pressure unit is always a force unit over an area unit, such as newtons per square meter or pounds-force per square inch.

Many pressure units exist because different fields grew up with different tools: mercury columns in medicine and vacuum work, atmospheres in chemistry, bar in industry and psi in the United States.

The pascal (Pa) is the SI unit of pressure: one newton per square meter. It is a very small pressure, so prefixes are common.

• Kilopascal (kPa) = 1000 Pa. Used for tire pressure in many countries, building and HVAC work, and weather in some places.
• Megapascal (MPa) = 1,000,000 Pa, which equals one newton per square millimeter. Used for material strength, concrete and hydraulics.
• Hectopascal (hPa) = 100 Pa. Used in weather reports and on weather maps.

The bar is exactly 100,000 Pa, so 1 bar = 100 kPa. It is not an SI unit, but it is defined from the pascal and is common in Europe for tires, compressors, diving and industry.

A millibar (mbar) is one thousandth of a bar, which is exactly 100 Pa. That makes the millibar and the hectopascal the same size: 1 mbar = 1 hPa. Meteorologists used millibars for decades and now mostly use hectopascals, so the numbers on weather charts did not change.

The standard atmosphere (atm) is exactly 101,325 Pa, which is 1013.25 hPa or 14.6959 psi. It is a reference value close to average air pressure at sea level and is used in chemistry and physics.

The torr is defined as exactly 1/760 of a standard atmosphere, about 133.3224 Pa. It is used mostly for vacuum measurements.

The millimeter of mercury (mmHg) is the pressure of a 1 mm column of mercury under standard gravity, about 133.3223874 Pa. It is used for blood pressure. The torr and the mmHg differ by less than one part in a million (1 Torr ≈ 0.999999858 mmHg), so for everyday purposes they are interchangeable.

A blood pressure reading such as 120/80 mmHg means a peak of 120 mmHg and a low of 80 mmHg above atmospheric pressure, which is about 16 kPa and 10.7 kPa.

The psi is one pound-force per square inch, about 6.89476 kPa. It is the standard pressure unit in the United States for tires, compressors, plumbing and hydraulics. For high stresses, US engineers use ksi (1000 psi), about 6.89476 MPa.

When two documents use different units, convert once at the end of the calculation and keep the full factor until then.