RPM to RCF calculator
Convert a centrifuge's speed in RPM to the force it applies in × g, or find the speed that gives a force.
RCF = 1.118 × 10⁻⁵ × radius (cm) × RPM²
RPM and RCF: what's the difference?
RPM is how fast the rotor spins, in revolutions per minute. RCF, relative centrifugal force, is the force on the sample, given as a multiple of Earth's gravity (× g, or g-force). The force depends on both the speed and how far the sample is from the centre, so the same RPM gives a larger force in a larger rotor. That is why methods give spins in × g, which carries across centrifuges.
The RCF formula
RCF = 1.118 × 10⁻⁵ × r × RPM², where r is the radius in centimetres. So 13,000 RPM in a rotor with an 8.4 cm radius gives 1.118 × 10⁻⁵ × 8.4 × 13,000² ≈ 15,870 × g.
Converting RCF (× g) to RPM
Rearranged, RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)). To spin at 10,000 × g in the same 8.4 cm rotor, set √(10,000 ÷ (1.118 × 10⁻⁵ × 8.4)) ≈ 10,320 RPM.
Which radius to use
Use the maximum radius, from the centre of the rotor to the bottom of the tube, which is how most rotor manuals and methods quote RCF. It is printed on the rotor or in its manual. Fixed-angle and swing-out rotors have different radii, so check the one you are using.