Centrifuge protocols are written in RCF (relative centrifugal force, in multiples of gravity) precisely because RPM alone is meaningless without knowing the rotor. This converter moves between the two using your rotor radius so you can reproduce a protocol on any machine.
How it works
The governing relationship is RCF = 1.118 x 10^-5 x r x RPM^2, where r is the
rotor radius in millimetres. The constant bundles the conversion from
revolutions per minute to angular velocity, the radius unit, and division by
standard gravity, 9.80665 m/s².
To go the other way, solve for speed: RPM = sqrt( RCF / (1.118 x 10^-5 x r) ).
Because RCF scales with the square of RPM, doubling the speed quadruples the
force — a relationship that catches out many researchers moving between machines.
Worked example
A microcentrifuge rotor with r-max of 80 mm spun at 12,000 RPM produces
1.118e-5 x 80 x 12000^2 = 12,874 x g. If a protocol instead asks for 15,000 x
g on that same rotor, you need sqrt(15000 / (1.118e-5 x 80)) ≈ 12,950 RPM.
A different rotor with r-max 55 mm achieves 12,874 x g at a higher RPM:
sqrt(12874 / (1.118e-5 x 55)) ≈ 14,496 RPM. Moving the protocol from the 80 mm
rotor to the 55 mm rotor requires spinning faster — not the same RPM — to
deliver the same pelleting force.
Why this matters in practice
Two centrifuges labeled “12,000 RPM max” can deliver wildly different g-forces if their rotors have different radii. A tabletop unit with an 80 mm rotor at 12,000 RPM produces roughly the same RCF as a larger floor centrifuge at 10,000 RPM on a 105 mm rotor. Copying the RPM number gives a different result; converting to RCF and back gives the correct speed on the new machine.
Common centrifuge steps and their typical RCF targets:
| Step | Typical RCF | What pellets |
|---|---|---|
| Low-speed clarification | 300 – 600 x g | Cell debris, large aggregates |
| Cell pelleting | 500 – 1,500 x g | Mammalian or yeast cells |
| Organelle isolation | 3,000 – 15,000 x g | Mitochondria, nuclei |
| Membrane vesicles | 50,000 – 150,000 x g | Microsomes, membranes |
| Ultracentrifugation | 100,000 – 600,000 x g | Ribosomes, viruses |
Tips
- Always confirm whether your protocol quotes r-max or r-min and match the same basis. r-max gives the force at the tube bottom (the pellet), r-min at the meniscus.
- When transferring a method between rotors, convert the source RCF to RPM on the new rotor — never copy the RPM number directly.
- Swing-bucket and fixed-angle rotors of the same machine often have very different radii; check the rotor manual for each separately.
- For sensitive preparations, also consider the time × force integral (k-factor), not just the peak RCF.