Why can't I just compare RPM between two centrifuges?

RPM only describes how fast the rotor spins. The actual force a sample feels also depends on how far it sits from the axis. A wide rotor at the same RPM produces far more g than a narrow one, so protocols must always be matched on RCF, not RPM.

What rotor radius should I use, r-max or r-min?

Use r-max, the distance to the bottom of the tube, for the strongest force your pellet experiences, which is what most pelleting protocols specify. Use r-min, the distance to the liquid surface, only when a protocol calls out the minimum force.

Where does the 1.118e-5 constant come from?

It folds together the conversion from RPM to angular velocity, the radius in millimetres, and division by standard gravity. The full relation is RCF equals omega-squared times radius divided by g, which simplifies to 1.118 times ten to the minus five when radius is in millimetres.

Is RCF the same as g-force?

Yes. RCF is expressed in multiples of standard gravity, so an RCF of 15000 means the sample experiences 15000 times its normal weight. The units x g and RCF are used interchangeably on centrifuge protocols.

Does this work for ultracentrifuges?

Yes, the formula holds at any speed. Ultracentrifuges simply reach very high RPM and large RCF values, often hundreds of thousands of x g, but the same radius-based relationship applies.

Centrifuge RCF / RPM Converter

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.

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.

Tips

Always confirm whether your protocol quotes r-max or r-min and match the same basis. When transferring a method between rotors, convert the source RCF to RPM on the new rotor rather than copying the RPM. Keep the rotor manual handy, as swing-bucket and fixed-angle rotors of the same machine often have different radii.