Centrifuge RPM and RCF: Use the Right Unit for Protocols
Many lab protocols specify centrifugation as xg rather than RPM because the force on the sample depends on rotor radius. A small microcentrifuge rotor and a large swinging bucket rotor can produce very different forces at the same RPM.
This calculator uses the standard relationship RCF = 1.118 x 10^-5 x r x RPM^2, where radius is measured in centimetres.
That squared RPM term is important. Increasing speed from 6,000 RPM to 12,000 RPM does not merely double the force; it produces four times the RCF for the same rotor radius. This is why changing centrifuges or rotors without converting the protocol can lead to under-spinning or over-spinning a sample.
How to use this calculator
- Choose whether to solve for RCF or RPM.
- Enter the rotor radius from the centre of rotation to the sample.
- Enter either the RPM or target RCF.
- Review the result before setting up the centrifuge.
Rotor radius tip
Use the radius listed in the rotor manual when available. For swinging bucket rotors, the effective radius may change between rest and spin, so manufacturer data is better than a rough ruler measurement.
Why protocols should use xg
RCF is more transferable because it describes the force experienced by the sample. RPM describes how fast the rotor turns, but not how far the sample is from the centre of rotation. If one protocol says 10,000 xg for 10 minutes, you can calculate the correct RPM for your rotor. If it only says 10,000 RPM, the actual force depends on the rotor geometry.
Practical lab checks
Before converting a protocol, confirm the tube type, maximum rated speed, rotor limit, temperature requirement, and whether the protocol expects fixed-angle or swinging-bucket behavior. Pelleting cells, clearing lysates, spinning columns, and collecting precipitates can all respond differently to force and time. This calculator gives the speed-force conversion, but the biology or chemistry still depends on the sample and method.
Worked example
With a 7.5 cm rotor radius, 12,000 RPM gives about 12,074 xg. If a protocol asks for 16,000 xg on that same rotor, the required speed is about 13,800 RPM. Always check that the calculated RPM is within the rotor and tube safety ratings.
