Pressure Units

Scientists work in SI Units. For pressure this means newtons per square meter (Nm-2 ). SI units are named

after famous scientists and  the pressure unit of 1 Nm-2 is named after Blaise Pascal, so 1 Nm-2 = 1 Pa

1 Pa is a tiny pressure equivalent to one newton (the weight of a small apple) over one square metre.

To keep life simple, the kilopascal (kPa) is normally used.

1 kPa = 1000 Pa = 1000 Nm-2

By a lucky coincidence 100 kPa is very close to the pressure of the atmosphere at sea level. That pressure is caused by over 10 km of atmosphere sitting on top of us.

100 kPa ≈ 1 atm  (1 atmosphere)

Another name for 100 kPa is 1 bar, so

100 kPa = 1 bar ≈ 1 atm

The bar is a useful unit because it relates to normal atmospheric pressure. A tyre with 3 bar is 3 times atmospheric pressure.
In this unit we will use bar for the pressure in an aerosol can.

Download our Aerosols and Pressure poster here


This pressure gauge is calibrated in kPa. Look at the black text.
But what is the red text? This is not an SI unit!
Where was the gauge made? The Americans still use pounds per square inch and so do many other countries. Even in Europe you will still find psi on gauges for cycle and car tyres and most engineering uses.


It gets worse! This gauge is calibrated in kg/cm
and psi.

This is a simple, honest gauge calibrated in bar, but can we trust it?


When this gauge reads zero, what it means is 1 bar because the atmospheric pressure all around us is 1 bar.

Gauges tell us what the pressure is ABOVE atmospheric pressure. For normal things like tyres we can ignore the extra bar from the atmosphere, but scientists and engineers have to know exactly what the pressure is and not forget the extra bar.

The pressure in this tyre is 1 bar, but so is the pressure outside, so the tyre is flat. A pressure gauge will read zero.
Scientists and engineers need to know the difference bewteen gauge pressure and absolute pressure. Gauge pressure is often identified with the letter g after the unit. For example, psig.