Pressure Converter

This converter changes a pressure reading from one unit into any of the other eighteen it supports — pascals, kilopascals, bar, psi, atmospheres, torr, millimetres and centimetres of mercury, millibar, and the various force-per-area units used in engineering. Type the value, pick what it is now and what you want it to be, and the result appears without a page reload.

Pressure is force spread over an area, which is why every unit on the list is really a force unit divided by an area unit. A pascal is one newton per square metre. A psi is one pound-force per square inch. Once you see that, the long unit list stops looking arbitrary.

Why are there so many pressure units?

Because different trades settled on different references before anyone agreed on a standard. Weather services grew up measuring the height of a mercury column, so they still talk in millimetres of mercury and millibar. Tyre and hydraulic work in the United States uses psi. Continental European engineering uses bar because it is almost exactly atmospheric pressure and the arithmetic is easy. Scientists use the pascal because it falls out of SI base units cleanly. Diving, aviation and vacuum work each kept their own conventions on top of that.

None of them are wrong; they are just anchored to different things. The converter holds the factors so you do not have to remember which is which.

A worked example: car tyre pressure

A European car door sticker specifies 2.3 bar for the front tyres. The forecourt air pump reads in psi. One bar is exactly 100,000 pascals, and one psi is 6,894.757 pascals, so:

• 2.3 bar × 100,000 = 230,000 Pa
• 230,000 ÷ 6,894.757 = 33.359 psi

So you would set the pump to about 33.4 psi. Going the other way, a US sticker calling for 32 psi is 32 × 6,894.757 = 220,632 Pa, or 2.2063 bar — near enough to 2.2 on a forecourt gauge.

What is standard atmospheric pressure in each unit?

One standard atmosphere is defined as exactly 101,325 pascals. That single definition is where most of the other numbers come from:

• 101,325 Pa
• 101.325 kPa
• 1.01325 bar
• 1,013.25 millibar
• 14.696 psi
• 760 mmHg (which is why a barometer reads "760" at sea level)
• 760 torr — the torr was defined to make this exact

If a reading looks wrong by a factor of roughly 14.7 or roughly 760, you have almost certainly mixed up absolute pressure with one of these references.

How do I use the converter?

1. Enter your figure in the left-hand box.
2. Choose the unit you currently have from the first list.
3. Choose the unit you want from the second list.
4. Read the converted value on the right. It updates as you type, so you can sweep through a range of inputs quickly.

Gauge pressure versus absolute pressure

This is the mistake that causes the most trouble, and no converter can catch it for you. A tyre gauge reading 33 psi is telling you the pressure above the surrounding air — that is gauge pressure, sometimes written psig. The actual absolute pressure inside the tyre is 33 + 14.7 = 47.7 psi, written psia.

Unit conversion is linear and handles either one correctly, but it cannot know which you meant. If you are working on vacuum systems, boiler ratings or anything where a specification mentions "absolute", check the reference before converting, not after.

Frequently asked questions

Is bar an SI unit?

No. The bar is accepted for use alongside SI but is not part of it. It is defined as exactly 100,000 pascals, so the conversion is exact rather than rounded.

What is the difference between torr and mmHg?

They are almost but not quite identical. The torr is defined as exactly 1/760 of a standard atmosphere; the millimetre of mercury is defined from the density of mercury at 0 °C under standard gravity. The two differ by less than one part in seven million, which matters in precision metrology and nowhere else.

Why is psi so awkward to convert?

Because the pound-force and the inch are defined independently of the metric system, the factor of 6,894.757 pascals per psi is not a round number in either direction. There is no shortcut — it is a factor worth letting a tool handle.

What pressure unit should I use in a technical document?

Kilopascals or megapascals if the audience is international or scientific, bar if it is European industrial, psi if it is North American. Whichever you pick, state it explicitly on first use and stay consistent; most pressure errors in documentation come from an unlabelled number, not a bad conversion.

Can I convert a pressure difference the same way?

Yes. Because every unit here is a simple multiple of the pascal with no offset, a difference of 2 bar converts exactly like an absolute value of 2 bar. This is unlike temperature, where the offsets mean differences and absolute values behave differently.

Related tools

• Power Converter — watts, horsepower, BTU per hour and the rest.
• Torque Converter — newton-metres, pound-feet and kilogram-force metres.
• Area Converter — useful when you need the area that a pressure is acting on.
• Temperature Converter — gas pressure and temperature usually travel together.