Pressure Converter

Pressure is defined as the force applied per unit area. It is a fundamental physical quantity used across meteorology, medicine, engineering, and everyday life. Different fields and regions use different pressure units, often chosen for historical or practical reasons. This guide covers the most commonly used pressure units, their origins, and how they relate to each other.

The Pascal: SI Standard

The Pascal (Pa) is the SI (International System of Units) derived unit of pressure, defined as one Newton per square meter (N/m²). Named after the French mathematician and physicist Blaise Pascal, it was adopted as the standard unit in 1971. Although Pa is scientifically rigorous, its small magnitude means that many everyday applications use kilopascals (kPa) or megapascals (MPa) for convenience.

Standard atmospheric pressure at sea level is approximately 101,325 Pa — a value known as one standard atmosphere (atm). Weather reports in many countries now use hPa (hectopascals, equivalent to millibar) for atmospheric pressure. In structural engineering, materials are typically rated in MPa or GPa.

Bar and Millibar

The bar is a metric unit of pressure defined as exactly 100,000 Pa. It is slightly less than one standard atmosphere (1 atm ≈ 1.01325 bar). Bar and its subunit millibar (mbar, equal to 100 Pa or 1 hPa) are widely used in meteorology, industrial equipment ratings, and tire pressure in many European countries.

Meteorologists often report atmospheric pressure in millibars or hectopascals (hPa). A standard sea-level pressure of 1013.25 mbar is the benchmark for many weather discussions. Industrial gas cylinders, hydraulic systems, and compressors are commonly rated in bar.

Standard Atmosphere (atm)

One standard atmosphere (atm) is defined as exactly 101,325 Pa. It was originally defined as the typical air pressure at sea level, and it remains useful as a reference baseline for high-pressure experiments, scuba diving depth calculations, and industrial process conditions.

The atmosphere is also commonly used in chemistry to describe conditions for reactions and gas behavior, often in the context of standard temperature and pressure (STP) or standard conditions. It relates closely to the bar: 1 atm ≈ 1.01325 bar.

PSI: Pounds per Square Inch

PSI (pounds per square inch) is the primary pressure unit in the United States and several other countries that use the imperial system. One psi equals approximately 6,894.76 Pa. It is widely encountered when checking tire pressure in automobiles, measuring water pressure in plumbing systems, and specifying pressure in hydraulic and pneumatic equipment.

Standard atmospheric pressure corresponds to approximately 14.696 psi, often rounded to 14.7 psi for practical use. Tire manufacturers typically recommend inflation pressures between 30 and 35 psi for passenger vehicles, though this varies widely. For reference, SCUBA tanks are typically filled to around 3,000 psi.

mmHg and Torr

Millimeters of mercury (mmHg), also called torr, is a unit historically defined by the height of a mercury column in a barometer or manometer. One mmHg equals approximately 133.322 Pa. This unit has been widely used in medicine — particularly for blood pressure measurement — and in laboratory vacuum applications.

The torr was named after Evangelista Torricelli, the Italian physicist who invented the barometer. Although SI units are officially preferred for clinical and scientific work, mmHg remains deeply embedded in medical practice globally. Blood pressure is still universally reported in mmHg: a typical healthy adult reading might be 120/80 mmHg, where the numbers represent systolic and diastolic pressure respectively. Standard atmospheric pressure corresponds to exactly 760 mmHg.

Inches of Mercury (inHg)

Inches of mercury (inHg) is another mercury-column based unit, used primarily in aviation and meteorology in the United States and a few other countries. One inHg equals approximately 3,386.39 Pa. Standard sea-level atmospheric pressure corresponds to 29.92 inHg.

Altimeters in aircraft are set using inHg as the reference pressure at the airfield. Pilots adjust this value as they fly into regions of different atmospheric pressure, ensuring accurate altitude readings. Weather broadcasts in the US often report barometric pressure in inHg alongside forecasts of high and low pressure systems.

cmH₂O: Centimeters of Water

Centimeters of water (cmH₂O) represents the pressure exerted by a one-centimeter column of water under standard gravitational acceleration. One cmH₂O equals approximately 98.07 Pa. This unit is used in medical contexts, especially for measuring low pressures such as ventilator settings, cerebrospinal fluid pressure, and airway resistance.

The unit offers a convenient scale for the small pressure differences encountered in respiratory physiology — for instance, the pressure needed to inflate the lungs typically ranges from a few cmH₂O up to about 30 cmH₂O. Medical ventilators commonly display settings in cmH₂O for this reason.