Pressure Converter

What this calculates

Enter a pressure in pascals, kilopascals, megapascals, bar, pounds per square inch, standard atmospheres, millimetres of mercury or inches of mercury, and this shows it in all eight at once, with each conversion worked through. It also places the pressure on a scale running from 0.1 kPa to 10,000 kPa, each band shown in bar as well, so the size of a pressure can be read at a glance.

It converts the number you enter as it is. It does not convert between gauge and absolute pressure.

The formula

Every conversion goes through pascals: into pascals with one unit's size, then out again with another's.

Each unit's size comes from its definition.

A psi is a pound-force — the pound's mass under standard gravity — on a square inch, and a millimetre of mercury is a 760th of a standard atmosphere. Neither comes out as a terminating decimal in pascals, so the working shows such a figure to ten significant figures, marked ≈, while every result is worked out from the exact value. The inch of mercury is the one conventional figure rather than one derived from the others.

Worked example

Some conversions come out exact. 1 atm is 101,325 Pa, which is exactly 1.01325 bar and exactly 760 mmHg. 100 kPa is exactly 1 bar.

When to use it, and the mistakes to avoid

Use it when a pressure is given in a unit you do not think in, when comparing a figure in psi with one in bar or kilopascals, and when a reading is given in atmospheres or in millimetres or inches of mercury.

The mistakes that cost the most:

• Treating bar and atmospheres as the same. They are close but not equal: 1 atm is 1.01325 bar, and 1 bar is 0.9869233 atm. At 10 bar the difference is already visible — 9.869233 atm.
• Using ÷ 14.5 as if it were exact. A bar is 14.50377 psi, not 14.5. The rough rule turns 32 psi into about 2.207 bar against the true 2.206322 bar, and 100 psi into about 6.897 bar against the true 6.894757 bar. Close for a rough sense, but not an answer.
• Mixing up kPa and bar. A bar is exactly 100 kPa, so the kPa figure is always a hundred times the bar figure: 250 kPa is 2.5 bar, not 250 bar or 25 bar.
• Expecting gauge and absolute pressure to be converted. This converter changes the unit of the number you enter; it does not add or remove the pressure of the atmosphere. If a figure is a gauge reading, the result is a gauge reading in the new unit, and the same for an absolute reading.
• Expecting every digit of every result. Results are shown to seven significant figures — 1 bar shows as 14.50377 psi — while each is worked out from the exact value. A pressure far too small for a unit, such as 0.000000000000001 psi in megapascals, bar or atmospheres, reads as 0, and a note under the result says so rather than leaving a silent zero.
• Entering a negative pressure. This version does not convert between gauge and absolute pressure, where a reading below zero would make sense, so it refuses a negative value with a message instead of converting it.

FAQ