Traditionally UK bicycle maximum recommended tyre pressures would have been displayed on the sidewall of the tyre in psi (pounds per square inch) and/or bar. More recently we are seeing tyres showing pressures in kPa or even kgf/cm2. So what are these units of pressure?
kPa - kilopascal
1kPa ≡ 1000Pa. The pascal (symbol: Pa) is the SI derived unit of pressure named after the French physicist Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre.
kgf/cm2 - Kilogram-force per square centimetre
A kilogram-force per square centimeter (kgf/cm2), often just kilogram per square centimeter (kg/cm2), or kilopond per square centimeter is a unit of pressure using metric units. Its use is now deprecated; it is not a part of the International System of Units (SI).
psi - Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in2 or lbf/sq in) is a unit of pressure based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch.
The bar is a non-SI unit of pressure, defined by the IUPAC as exactly equal to 100,000 Pa. The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909.
So what if my pump gauge is only marked in psi and Bar?
No problem. As long as your gauge has a Bar scale you can easily work out the right pressure even if the tyre is only marked in kPa or kgf/cm2.
Take a tyre marked 500kPa / 5.1kgf/cm2. The most accurate way is to divide 500kPa by 100. This would give you 5 bar as 1 bar = 100,000 Pa or 100 kPa.
Another way would be to say that 5.1 kgf/cm2 is roughly the same as 5 bar. There's not a lot in it. Check out the table below or try the pressure conversion utility.
Remember the figure quoted on the tyre wall is the maximum pressure you should inflate the tyre to.
Pressure conversion factors