Pressure Converter

Enter a pressure, pick the unit you are converting from and the unit you want, and the result updates as you type. The six units here — pascal, kilopascal, bar, pound per square inch, standard atmosphere and millimeter of mercury — between them cover tire placards, weather maps, dive computers and blood pressure monitors. Read the answer as the same physical push measured on a different scale: nothing about the gas or liquid changes, only the label on the ruler.

How the conversion works

Pressure is force spread over area, and every unit below is really just a fixed number of pascals — the SI unit, one newton pressing on one square meter. The converter stores each unit as an exact factor relative to the pascal and routes every conversion through it:

result = value × factor_from ÷ factor_to

The defaults make a good worked example: 32 psi into bar. One psi is 6,894.757293168 Pa, so 32 psi is 32 × 6,894.757293168 = 220,632.23 Pa. A bar is defined as exactly 100,000 Pa, and 220,632.23 ÷ 100,000 = 2.2063 bar. None of these factors are rounded guesses. The psi figure follows from the exact definitions of the pound-force and the inch, the standard atmosphere was fixed at exactly 101,325 Pa in 1954, and the conventional millimeter of mercury is 133.322387415 Pa.

From Torricelli's tube to the pascal

For a long time nobody agreed that air had weight at all. The proof came in 1643, when Evangelista Torricelli filled a glass tube about a meter long with mercury, sealed one end, and upended the open end into a dish. Torricelli had trained under Benedetto Castelli rather than under Galileo directly; he joined the elderly Galileo at Arcetri in October 1641 and assisted him for the three months before Galileo died in January 1642, then succeeded him as mathematician to the Grand Duke of Tuscany. In the tube experiment the mercury did not all run out. It settled at roughly 76 centimeters, leaving an empty space at the top — the first recorded sustained laboratory vacuum, still called the Torricellian vacuum. Torricelli reasoned that the weight of the atmosphere pressing on the dish held the column up, and because mercury is about thirteen and a half times denser than water, a manageable 76-centimeter column did the work that would otherwise have needed a water column more than ten meters tall.

Blaise Pascal pushed the idea to its test. On 19 September 1648 his brother-in-law Florin Périer, who had married Pascal's elder sister Gilberte, carried a barometer up the Puy de Dôme, a volcanic dome of about 1,465 meters in the Auvergne near Clermont-Ferrand, reading the mercury height at the base and again at the summit. The column stood measurably lower at altitude, exactly as it should if the air above weighs less the higher you climb. Pascal's name was later attached to the SI unit of pressure: the 14th General Conference on Weights and Measures adopted the pascal, one newton per square meter, in 1971.

Why one quantity carries six names

Every other unit here is a historical accident that turned out too useful to abandon. The millimeter of mercury and the closely related torr — named for Torricelli — both descend directly from his glass tube; the torr is defined as 1/760 of a standard atmosphere, which works out to 101,325 ÷ 760, or about 133.3224 Pa, within one part in seven million of the conventional mmHg. The bar and millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, a founder of modern weather forecasting, and take their name from the Greek baros, meaning weight; a bar is exactly 100,000 Pa, chosen to sit close to ordinary atmospheric pressure without matching it. The standard atmosphere itself was fixed by the 10th General Conference in 1954 at exactly 101,325 Pa, roughly the average pressure at sea level. The pound per square inch comes from a separate tradition, and its factor is built on three exact constants: the avoirdupois pound of 0.45359237 kg, fixed by the 1959 International Yard and Pound Agreement; standard gravity of 9.80665 m/s squared, which turns that mass into a pound-force of 4.4482216152605 newtons; and the inch of exactly 0.0254 meter, giving a square inch of 0.00064516 square meters. Dividing the force by the area lands on 6,894.757293168 Pa, which is why the psi factor runs to so many decimal places.

Tire pressure: psi in some countries, bar in others

Tire placards in the US, UK and Canada quote psi, most of continental Europe uses bar, and Australian placards often print kPa alongside. The same physical pressure just wears different labels:

A useful shortcut at the pump: divide psi by 14.5 to get bar, or multiply bar by 14.5 to get psi. That is accurate to within 0.03 percent, far tighter than any pump gauge. One thing the table hides is that tire and most engineering gauges read gauge pressure — the amount above the surrounding air — while the converter treats every number as an absolute value. Converting between units does not change that, but comparing a gauge reading against an absolute figure like the standard atmosphere does: a tire at 32 psi gauge is holding roughly 46.7 psi absolute once you add the atmosphere it already sits in.

Weather maps and the hectopascal

Meteorologists report sea-level pressure in hectopascals because 1 hPa equals exactly 1 millibar, the unit weather services used for most of the twentieth century. Moving from millibars to hectopascals let them adopt SI units without redrawing a single contour, since the two are numerically identical. Standard sea-level pressure is 1,013.25 hPa, which is exactly 1 atm and 101.325 kPa; strong high-pressure systems reach about 1,040 hPa and deep storm lows can fall below 950 hPa. This converter does not list hPa separately, but the conversion is trivial: divide an hPa reading by ten and enter it as kPa, so a 998 hPa low becomes 99.8 kPa.

mmHg in medicine

Blood pressure is still written in millimeters of mercury — 120/80 mmHg — because the early instruments literally pushed a mercury column up a glass tube. The Austrian physician Samuel Siegfried Karl Ritter von Basch built a workable sphygmomanometer in 1881, the Italian internist Scipione Riva-Rocci produced the familiar arm-cuff version in 1896, and in 1905 the Russian physician Nikolai Korotkoff added the listening technique, using the sounds in the artery to mark the systolic and diastolic points. The mercury has long since gone from the instruments, but the unit outlived it: modern monitors are electronic and still display mmHg, even in countries that are otherwise strictly metric. Only a few health systems chart blood pressure in kilopascals. A systolic reading of 120 mmHg is 15.9987 kPa, which is why 16 kPa turns up as the metric shorthand.

Frequently asked questions

How do I convert psi to bar for tires?

Multiply psi by 0.0689476, or simply divide by 14.5 if you are standing at an air pump. A placard value of 32 psi works out to 2.2063 bar, which European pumps usually round to 2.2. Going the other way, 2.5 bar is 36.3 psi.

Is 1 bar the same as 1 atmosphere?

Close but not equal. One standard atmosphere is 101,325 Pa while a bar is exactly 100,000 Pa, so 1 atm equals 1.01325 bar — a difference of about 1.3 percent. Scuba divers usually ignore the gap and treat each bar as one atmosphere of depth-equivalent pressure, which is fine at recreational precision.

What does mmHg mean on a blood pressure monitor?

It stands for millimeters of mercury, the height a column of mercury would reach under that pressure. A typical healthy reading of 120/80 mmHg corresponds to 16.0 and 10.7 kPa. Monitors display mmHg in almost every country, so in practice there is rarely anything to convert.

Why do weather forecasts use hPa?

Because one hectopascal equals exactly one millibar, weather services could adopt SI units without changing a single number on their charts. Standard sea-level pressure is 1,013.25 hPa, strong highs reach about 1,040 hPa, and deep storm lows can drop below 950 hPa. To use hPa values here, divide by ten and enter them as kPa.

Is a torr the same as a mmHg?

For any practical purpose, yes. The torr is defined as 1/760 of a standard atmosphere, which is 133.3224 Pa, while the conventional millimeter of mercury is 133.322387415 Pa — a difference of less than one part in seven million. Vacuum engineers say torr, doctors say mmHg, and the mmHg option here serves both.