RPM to RCF (x g) Calculator - Centrifuge Speed Converter
Convert centrifuge speed to relative centrifugal force and back, for any rotor radius.
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A protocol that says 12,000 x g and a centrifuge dial that reads RPM are not the same instruction, and the bridge between them is your rotor's radius - so the same RPM is a different force in every rotor you own. The conversion itself is exact physics, RCF = omega squared times r over g, and the only judgement in it is which radius to use: a protocol's x g conventionally means the rotor's maximum radius, the distance from the axis to the bottom of the tube. Using the minimum or average radius instead scales the force by the ratio of the two, which is how a spin that should pellet cells ends up leaving half of them in the supernatant.
The rotor's maximum radius — it is in the rotor manual.
RCF = 1.118 × 10⁻⁵ × r(cm) × RPM²
- Radius is the rotor's maximum unless you say otherwise; a protocol's ×g is conventionally at rmax. A swing-out rotor's radius differs between its bucket positions and its rest position.
How to use the RPM to RCF Converter tool
- 1Choose which way you are converting - a dial reading to x g, or a protocol's x g to a dial reading.
- 2Enter your rotor's maximum radius in centimetres. It is in the rotor manual.
- 3Enter the speed or the force you have.
- 4Read the converted value.
Frequently asked questions
What is the formula for RPM to RCF?
RCF = 1.118 x 10^-5 x r x RPM squared, with r the rotor radius in centimetres. That constant is not arbitrary - it is what you get by collecting the units in RCF = omega squared times r over g, where omega is 2 pi RPM over 60 and g is 9.80665 m/s squared. This tool computes it from that derivation rather than storing the rounded number.
Which radius should I use - maximum, minimum or average?
Maximum, unless the protocol says otherwise. A published x g figure conventionally refers to the rotor's maximum radius, the distance from the axis of rotation to the bottom of the tube, because that is where the pellet forms. A swing-out rotor's radius differs between its resting and its swung-out position, so use the figure the manufacturer quotes for the spinning geometry.
Why do two centrifuges at the same RPM give different results?
Because force depends on the radius as well as the speed, and rotors differ. A microfuge rotor at around 8.5 cm and a large swing-out rotor at 15 cm produce very different forces at the same dial setting - which is exactly why protocols are written in x g and why converting for your own rotor matters.
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