Pressure Converter

Pressure is one of the most practically important physical quantities, appearing across fields as diverse as medicine, meteorology, engineering, diving, and everyday tasks like inflating tyres. Yet pressure is also measured in a bewildering variety of units — pascals, bar, psi, atmospheres, torr, mmHg — each originating from different scientific traditions and remaining entrenched in specific industries. Understanding what each unit represents and how to convert between them is essential for anyone working across disciplines or international boundaries.

What Is Pressure?

Pressure is defined as force per unit area: the amount of force applied perpendicular to a surface, divided by the area over which that force acts. The SI unit of pressure is the pascal (Pa), defined as one newton per square metre. Because one pascal is a very small unit — atmospheric pressure at sea level is approximately 101,325 Pa — practical applications typically use kilopascals (kPa), megapascals (MPa), or entirely different unit systems altogether. The diversity of pressure units in common use reflects the fact that different fields developed their measurement conventions independently long before SI standardisation.

Common Pressure Units Explained

The bar is widely used in engineering, meteorology, and compressed gas applications. One bar equals exactly 100,000 Pa and is close to standard atmospheric pressure (1 atm = 1.01325 bar). Tyre pressures in European countries are typically quoted in bar. PSI (pounds per square inch) is the preferred unit in the United States and United Kingdom for tyre pressures, hydraulic systems, and industrial applications. Standard atmospheric pressure is approximately 14.696 psi. The atmosphere (atm) is a historical unit equal to the average air pressure at sea level, still used in chemistry and physics for expressing gas pressures. One atm equals 101,325 Pa exactly.

Millimetres of mercury (mmHg) and its equivalent torr are used primarily in medicine for blood pressure measurement and in scientific vacuum applications. Blood pressure readings such as 120/80 mmHg describe the systolic and diastolic pressures of the cardiovascular system. One mmHg equals 133.322 Pa. Inches of mercury (inHg) is used in aviation and meteorology in the United States for reporting barometric pressure and altimeter settings. Kilopascals (kPa) are the standard for tyre pressures in many countries and for medical oxygen pressure systems.

Gauge Pressure vs Absolute Pressure

An important distinction in pressure measurement is between absolute pressure and gauge pressure. Absolute pressure is measured relative to a perfect vacuum (zero pressure). Gauge pressure is measured relative to the surrounding atmospheric pressure — it tells you how much the pressure exceeds atmospheric. A tyre inflated to 32 psi gauge pressure actually has an absolute pressure of about 46.7 psi (32 + 14.7 atmospheric). When a pressure reading is not specified as gauge or absolute, context usually makes it clear: tyre pressures and blood pressure are always gauge; atmospheric and altitude-related pressures are always absolute.

In engineering, the suffix "g" (e.g., psig) denotes gauge pressure, while "a" (e.g., psia) denotes absolute. Some industries use vacuum pressure — a negative gauge pressure — to describe pressures below atmospheric. A vacuum of 500 mmHg (torr) means the pressure is 500 mmHg less than atmospheric, or approximately 260 mmHg absolute. Getting gauge vs absolute wrong can introduce serious errors in calculations involving compressed gas, fluid systems, or industrial processes.

Practical Pressure References

Knowing a few reference pressures helps anchor unit conversions in reality. Standard atmospheric pressure is 101.325 kPa, 1.01325 bar, 14.696 psi, or 760 mmHg. Car tyre pressures typically range from 28 to 35 psi (193–241 kPa). Blood pressure: normal systolic pressure is around 120 mmHg (16 kPa), and normal diastolic is around 80 mmHg (10.7 kPa). A bicycle tyre at 80 psi (552 kPa) feels rock hard. A scuba diving tank at 200 bar (2,900 psi) holds enormous compressed air energy. The deepest ocean trenches reach pressures of over 1,000 bar (approximately 15,000 psi). Keeping these reference points in mind makes it easy to sanity-check a conversion result and catch errors before they cause problems.