Pressure Converter

About the Pressure Converter

Pressure is force spread over an area, and it is measured in a surprising number of units. Car tyres are inflated in psi in the US and bar or kPa elsewhere. Weather maps use hectopascals or millibars, American barometers inches of mercury, and doctors millimetres of mercury for blood pressure. Engineers use pascals, kilopascals and megapascals, and divers and scientists talk about atmospheres. A tyre, a pump, a gas bottle or a pressure washer bought abroad can easily be over- or under-filled if the units are confused.

This pressure converter converts between ten pressure units: pascals, kilopascals, megapascals, millibars (hectopascals), bar, standard atmospheres, psi, millimetres of mercury, inches of mercury and torr. It shows the result, the exact factor both ways, the value in every other unit, and how it compares with average air pressure at sea level.

How to Use the Pressure Converter

Enter the pressure.

Choose the unit to convert from and to.

The converted pressure appears with the factor, all the other units and a comparison with sea-level air pressure.

How Pressure Conversion Works

Every unit is converted through the pascal, one newton per square metre:

  1 kPa   = 1,000 Pa              1 bar  = 100,000 Pa
  1 MPa   = 1,000,000 Pa          1 mbar = 1 hPa = 100 Pa
  1 atm   = 101,325 Pa            (exactly, by definition)
  1 psi   = 6,894.757 Pa          (one pound-force per square inch)
  1 mmHg  = 133.322 Pa            1 torr = 1/760 atm = 133.322 Pa
  1 inHg  = 3,386.389 Pa

  result = value × (from-unit in Pa) ÷ (to-unit in Pa)

Step-by-Step Example

32 psi to bar.

  To pascals:  32 × 6,894.757 = 220,632 Pa
  To bar:      220,632 ÷ 100,000 = 2.20632 bar

So a tyre at 32 psi is at about 2.20632 bar, or 220.632 kPa. That is about 2.18 times average air pressure at sea level — as a gauge reading, on top of the air pressure already there.

One standard atmosphere is 101,325 Pa, which is 14.6959 psi, 1.01325 bar, 1,013.25 mbar, 760 mmHg and about 29.9213 inHg.

Quick Reference

  1 bar   = 14.5038 psi = 100 kPa
  1 psi   = 0.0689476 bar = 6.89476 kPa
  1 atm   = 1.01325 bar = 14.6959 psi = 760 mmHg
  1 kPa   = 0.145038 psi = 7.50062 mmHg
  1 inHg  = 33.8639 mbar

A rough rule for tyres is that 1 bar is about 14.5 psi, so 2.2 bar is about 32 psi and 2.5 bar about 36 psi.

Gauge and Absolute Pressure

Most everyday gauges read gauge pressure, the pressure above the surrounding air. A tyre gauge showing 32 psi means the air inside is 32 psi above atmospheric pressure; the absolute pressure is about 32 + 14.7 = 46.7 psi. An empty tyre reads zero on a gauge, but still contains air at atmospheric pressure. Engineering and science often use absolute pressure, sometimes marked psia or bar(a), while gauges are marked psig or bar(g). When converting, keep the same kind: this converter changes units, not between gauge and absolute.

Weather and Altitude

Average air pressure at sea level is 1,013.25 hPa, but it varies with the weather, typically between about 980 and 1,040 hPa. Low pressure usually brings cloud, wind and rain, and high pressure settled weather. Pressure also falls with height: at 1,500 metres it is about 850 hPa, and at the top of Everest roughly a third of sea-level pressure. Aircraft altimeters work by measuring this change, which is why pilots set them to the local sea-level pressure.

Blood Pressure

Blood pressure is measured in millimetres of mercury, a unit from the days of mercury manometers. A reading of 120/80 means the pressure peaks at 120 mmHg above atmospheric pressure when the heart beats and falls to 80 mmHg between beats. In kilopascals that is about 16.0/10.7 kPa, a form used in some medical texts.

Pressure in Engineering

In engineering, pressures quickly become large, so megapascals are common. Mains water in a home is usually 2 to 4 bar, 200 to 400 kPa. A pressure washer might deliver 100 to 150 bar, and hydraulic systems in machinery 200 bar or more, 20 megapascals. A scuba cylinder is filled to about 200 to 300 bar. The strength of materials is also given in megapascals: ordinary concrete is rated around 20 to 40 MPa, the pressure it can take before crushing. Because a pascal is so small — about the pressure of a sheet of paper lying on a table — raw pascals are rarely used outside science.

Vacuum

Vacuum is pressure below atmospheric. It is often described in torr or millimetres of mercury, where 760 torr is normal air pressure and 0 torr a perfect vacuum. A household vacuum cleaner lowers the pressure by only a small fraction, while the vacuum pumps used in laboratories and electronics manufacturing reach a millionth of a torr or less. Vacuum gauges may read either absolute pressure or how far below atmospheric the pressure has dropped, so check which before converting.

Understanding Your Result

The headline is the converted pressure.

The conversion factor shows the exact factor in both directions.

The in every unit line lists the pressure in all ten units.

The also line compares the pressure with average sea-level air pressure.

When Should You Use This Calculator?

Use it to set tyre pressures given in a different unit from your gauge or pump.

Use it for weather, barometers and altitude.

Use it with pumps, compressors, pressure washers, gas bottles and hydraulic equipment.

Use it for physics, chemistry and engineering calculations.

Common Mistakes

Mixing gauge and absolute pressure. Add about 14.7 psi, or 1.013 bar, for absolute.

Confusing bar and atm. They are close but not equal: 1 atm is 1.01325 bar.

Mixing millibars and hectopascals. They are the same size, so no conversion is needed.

Using psi for kPa. 32 kPa is a very flat tyre; 32 psi is 220 kPa.

Forgetting pressure changes with temperature. Check tyres cold, before driving.