Calculators
RCF and RPM
Convert between revolutions per minute and relative centrifugal force for a given rotor radius, with a list of common rotors and a helper that derives the radius from your own rotor manual.
Formula: RCF = 1.118 × 10⁻⁵ × r(cm) × RPM². Fill either the speed or the force field and the other one follows.
Radii in the list are back-calculated from each manufacturer’s published maximum speed and force pair. Check them against your own rotor manual before critical work.
Rotor not in the list?
Enter the maximum speed and maximum force printed on the rotor or in its manual to work out the maximum radius.
Formula
RCF = 1.118 × 10⁻⁵ × r × RPM²
- RCF
- relative centrifugal force, in multiples of gravity
- r
- spin radius in centimetres
- RPM
- revolutions per minute
Worked example
What is 14,000 rpm in a microcentrifuge with an 8.4 cm rotor?
- r = 8.4 cm
- RPM = 14000
- Square the speed: 14000² = 1.96 × 10⁸
- Multiply by the radius: 1.96 × 10⁸ × 8.4 ≈ 1.65 × 10⁹
- Multiply by the constant: 1.65 × 10⁹ × 1.118 × 10⁻⁵
Answer about 18,400 × g
Common mistakes
- Reporting rpm in a methods section instead of g. Revolutions per minute mean nothing without the rotor radius, and your protocol will not be reproducible — always report the g value.
- Using the maximum radius when the pellet forms partway up the tube. Particles at different distances experience different forces; in a fixed-angle rotor the spread can exceed twofold.
- Running one rotor's protocol on a rotor of a different radius without converting. The same rpm on a larger rotor produces far more force and may exceed the rating of the tube or the rotor.
- Entering the radius in millimetres into a formula that expects centimetres — a tenfold error.
Frequently asked
Where do I find my rotor radius?
It is printed on the rotor or given in its manual. If neither is at hand, use the section further down this page: enter the manufacturer's rated maximum speed and force and the maximum radius is derived for you.
What is the difference between rmax and rmin?
rmax is the distance from the axis to the bottom of the tube, rmin the distance to the liquid surface. A pellet at the tube bottom sees the highest force and molecules near the surface the lowest.
Which radius should I report for a gradient?
Gradient work usually quotes the force at rmax, but since behaviour across the whole tube matters, it is better to give both values in your methods.