RCF from RPM calculator
What force a speed actually produces on your rotor, in × g.
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Relative centrifugal force (× g)
12879
Radius is to the tube bottom at its spun position, from the rotor manual.
The formula
RCF = 1.118 × 10⁻⁵ × r × rpm², with the radius in centimetres. The force goes up with the square of the speed, so a 20% faster spin is not 20% harder — it is about 44% harder.
Why a protocol should never say rpm
The same rpm on two different rotors is two different forces. A benchtop microfuge at 8 cm and a larger rotor at 16 cm, both at 12,000 rpm, differ by a factor of two. A method that says “spin at 13,000 rpm” is only reproducible by somebody with the same centrifuge; one that says “16,000 × g” is reproducible by anyone.
If you are writing a protocol, publish the × g figure. If you are following one that gives rpm, this is the conversion you need — and you need their rotor radius to do it, which is usually the missing piece.
Which radius
Use the distance from the centre of the rotor to the bottom of the tube in its spun position, which is what the rotor manual calls r-max. A fixed-angle rotor and a swing-out rotor of the same nominal size give different numbers, because the tube ends up somewhere different.