Why × g
RPM is how fast the rotor turns. What matters for pelleting is the force on the sample, the relative centrifugal force (RCF), written as × g. The same RPM gives a larger force in a larger rotor, so protocols give × g to work on any centrifuge.
The formula
RCF = 1.118 × 10−5 × r × RPM², with the radius r in centimetres.
A rotor with a 10 cm radius at 13,000 RPM gives 1.118 × 10−5 × 10 × 13,000² ≈ 18,900 × g. Going the other way, RPM = √(RCF ÷ (1.118 × 10−5 × r)).
Measuring the radius
Use the distance from the centre of the rotor to the bottom of the tube in its spinning position, the maximum radius. It is usually printed on the rotor or in its manual. Force rises with the square of the speed but only in proportion to the radius, so small errors in the radius matter less than errors in speed.
Calculator
The RCF to RPM Converter converts either way from the rotor radius.