Relative centrifugal force (RCF)
RCF is the force a centrifuge applies to a sample, expressed as a multiple of Earth's gravity and written as g. It is what a spin protocol specifies. RCF depends on both rotor speed and rotor radius, so the same RPM produces different RCF in different machines.
Force is not one number
Figure. Relative centrifugal force grows with distance from the axis, so a single tube experiences a range of forces rather than one value. The original L-PRF protocol is 408 g at the clot, 653 g at the tube bottom, and 326 g at the top. Those three figures describe the same spin.
In practice
RCF is calculated as 1.118 x 10⁻⁵ x r x N², where r is the rotor radius in centimetres and N is speed in RPM. Some manuals write it as 11.18 x r x (N/1000)², which is the same relation. Because force grows with distance from the axis, there is no single RCF in a tube: papers distinguish RCF-max, RCF-min, RCF-average, and RCF at the clot.