Pressure Converter

One converter for ten pressure units

Pressure shows up on tire sidewalls, weather broadcasts, scuba tanks and refrigerant gauges, and each of those worlds picked a different unit. This page carries all ten common ones: pascal, kilopascal, megapascal, bar, millibar, psi, atmosphere, millimeter of mercury, inch of mercury and kilogram per square centimeter.

Type a value, pick the unit you have and the unit you want, and the result appears with the same pressure restated in every other unit. The default is the 32 psi printed on a typical passenger car tire, which is 220,632 pascals, 220.63 kilopascals, 2.2063 bar, 2,206.3 millibar, 2.17747 atmospheres, 1,654.88 mmHg, 65.153 inHg and 2.2498 kilogram per square centimeter.

Two extra panels handle the most common questions: tire pressure between psi and bar, and weather pressure between inches of mercury and hectopascals.

The exact factors and where they come from

The pascal is the SI unit of pressure, one newton per square meter, and every other unit here is defined against it. The SI Brochure from the BIPM lists the bar among the units accepted for use with the SI, and the appendix tables of NIST Special Publication 811 give the factors this page uses.

The standard atmosphere was fixed at exactly 101,325 pascals in 1954, which is the 1 atm of every engineering table, and the torr is 1/760 of that atmosphere, so 760 torr is 101,325 pascals. The conventional inch of mercury is 25.4 mm of mercury and lands on 3,386.389 pascals.

The psi row ties the imperial system to the metric one. The pound-force is the force that holds one pound mass against standard gravity of 9.80665 m/s2, which is 4.448222 newtons, and one square inch is 0.00064516 square meters. One divided by the other gives 4.448222 / 0.00064516 = 6,894.757293 pascals.

Gauge pressure and absolute pressure

This is the most common source of wrong pressure numbers, and it is not a conversion error. It is a question about the zero point. An absolute reading counts all the pressure above a perfect vacuum, so the atmosphere at sea level reads 1.01325 bar absolute and nothing outdoors reads zero. A gauge reading counts only the pressure above the surrounding air, so the same atmosphere reads 0 bar(g) on a bench gauge and 0 psig on a tire gauge.

Tire gauges, hand pumps and most service manifold gauges read gauge pressure. Barometers, absolute sensors and dive computers read absolute. The two differ by one atmosphere: 32 psig + 14.696 = 46.696 psia, so a tire at 32 psi holds about 46.7 psi of real pressure. In bar, 2.2063 bar(g) + 1.01325 = 3.21957 bar(a).

Where a label writes bar(a) or bar(g), the letter tells you which scale it is on, and equipment sold in Europe often carries it for that reason. A bare 32 psi on a tire means gauge, because that is what the gauge on the garage wall reads.

Tire pressure, psi to bar and back

Car tires are quoted in psi in the United States and in bar almost everywhere else, so a European placard reads 2.2 bar where an American one reads 32 psi. Those are the same pressure, linked by 14.5038 psi per bar. A 32 psi tire is 2.2063 bar, and a 2.2 bar tire is 31.908 psi.

The usual passenger car range is 28 to 36 psi, or 1.93 to 2.48 bar. Below about 26 psi a tire flexes more than it was designed to, runs hot and wears its shoulders; above about 40 psi the center of the tread carries the load. The placard figure is a cold inflation pressure, measured after the car has sat for three hours, which is why a tire reads a few psi higher right after a drive.

The tire panel takes either unit, so the number from the door sticker and the number from an airline gauge are one keystroke apart.

Weather pressure, inHg and hectopascals

United States broadcasts quote barometric pressure in inches of mercury and most of the rest of the world quotes the same atmosphere in hectopascals. The NOAA JetStream explanation of air pressure notes that meteorology used the millibar since 1929, and that the hectopascal was adopted so the numbers would not change: 1 hPa is 100 pascals and 1 mbar is 100 pascals, so a 1970 weather map and a modern one carry the same digits.

Standard sea level pressure is 1013.25 hPa, which is 29.9213 inHg, and that pair is what every barometer on a wall is compared against. Forecasts round the inch value to two decimals and the metric value to the nearest hPa, so 30.12 inHg and 1,020.0 hPa describe the same high.

The inch of mercury is a real column, not a label: one inch of mercury at 32 degrees Fahrenheit exerts 3,386.389 pascals, the same pressure as 25.4 mm of mercury. At 5,280 feet of elevation the standard pressure falls to about 834 hPa, or 24.6 inHg, which is why an altimeter needs a local setting.

Where each unit comes from

The bar entered French engineering in 1904 and stayed, which is why the BIPM still lists it as accepted for use with the SI. The millibar was the daily unit of the weather map, and the hectopascal exists so the same number keeps working after the switch to SI. The standard atmosphere is a laboratory reference fixed at 101,325 pascals, and the torr carries the name of Evangelista Torricelli, who built the first mercury barometer in 1643.

The kilogram per square centimeter is the leftover from the metric gravitational system, where pressure was quoted as kilogram force per square centimeter rather than newtons per square meter. It survives on hydraulic machinery and pressure vessels, and it is a force over an area, not a mass over an area, which is why it is 98,066.5 pascals rather than 10,000.

Working ranges you will meet

Pressure numbers get easier to judge once you know the band they sit in. Home water supply runs 30 to 60 psi, a car tire 28 to 36 psi, and a road bicycle tire 80 to 130 psi, which is 5.52 to 8.96 bar because the volume of air is so much smaller. A scuba cylinder holds 200 to 232 bar, or 2,901 to 3,365 psi, and a first stage regulator drops that to about 140 psi above ambient before the second stage delivers it at breathing pressure.

Refrigeration is the widest band a homeowner meets. A home air conditioner runs 60 to 120 psi on the low side and 250 to 400 psi on the high side on a warm day. Industrial work leaves that range behind: a 30 MPa concrete specification is 4,351 psi, and a hydraulic press can run at 3,000 psi or more.

Two rows need a note. The diving row is absolute: at 30 meters of seawater the pressure is about 4 atmospheres, or 4.05 bar, so a dive computer shows roughly 3 atmospheres of water pressure on top of the surface reading. The air conditioner row is gauge pressure on a sealed system, which is what the manifold gauge in a technician's hand displays.

Mistakes this converter catches

The first mistake is the gauge and absolute mix: a tire at 32 psig is 46.696 psia, and a cylinder at 200 bar(g) is 201.01325 bar(a). Add the atmosphere when the spec says absolute, and leave it off when the spec says gauge. The tell is the instrument: pumps and tire gauges read gauge, barometers read absolute.

The second is a decimal slip between bar and millibar, or pascal and kilopascal. 220 kPa is 2.2 bar, and reading 220 kPa as 220 bar gives a pressure 100 times too high. The third is kgf/cm2, where the same 32 psi is 2.2498 kgf/cm2, and reading that figure as 22,498 would put it 100 times too low. The fourth is treating bar and atmosphere as the same thing, when 100,000 / 101,325 = 0.98692 atm per bar, a 1.3 percent gap that matters on a depth rating.

The fifth is rounding too early. Keep full precision through the conversion and round once at the end, since a gauge rated to 1 percent cannot resolve a difference that was rounded away three steps before it.