Free lab tool
RPM to RCF Calculator (g-force)
To convert RPM to RCF (g-force), multiply the rotor radius in centimetres by the square of the speed in RPM and by 1.118 × 10⁻⁵. This free calculator also works backwards, giving the RPM needed for a target g-force, and accepts the radius in centimetres or millimetres.
Calculator
Distance from the rotor axis to the sample. See your rotor data sheet.
Relative centrifugal force
Enter your values to see the result.
Results are a guide. Check them against your protocol, the reagent data sheet and your instrument manual.
How it works
RPM counts how many times the rotor turns each minute. RCF, the relative centrifugal force, is the acceleration the sample actually feels, written as a multiple of Earth's gravity (× g). The two are linked by the rotor radius, so the same RPM gives a different RCF in a different rotor.
Formula
RCF = 1.118 × 10⁻⁵ × r × RPM²
RPM = √( RCF ÷ (1.118 × 10⁻⁵ × r) )
Where
- RCF
- relative centrifugal force, in multiples of g (× g)
- r
- rotor radius in centimetres (cm)
- RPM
- rotor speed in revolutions per minute
- 1.118 × 10⁻⁵
- constant equal to 4π² ÷ (3600 × 980.665), which folds the conversion to radians per second and standard gravity into one number
Good to know
- RCF is highest at the bottom of the tube, where the radius is largest. Use the radius your protocol or rotor data sheet refers to.
- Quote protocols in × g rather than RPM so they work on any centrifuge and rotor.
Worked example
Example: 5,000 RPM in a rotor with a 10 cm radius
- 01
Radius r = 10 cm and speed = 5,000 RPM.
- 02
RPM² = 5,000 × 5,000 = 25,000,000.
- 03
RCF = 1.118 × 10⁻⁵ × 10 × 25,000,000 = 2,795.
The sample feels 2,795 × g. Going the other way, 10,000 × g at an 8 cm radius needs about 10,574 RPM.
Frequently asked questions
What is the difference between RPM and RCF?
RPM is how fast the rotor spins, in revolutions per minute. RCF is the force the sample experiences, as a multiple of Earth's gravity. RCF depends on both the speed and the rotor radius, so one RPM setting can give very different g-forces in different rotors. That is why protocols are best written in × g.
How do I convert RPM to RCF?
Use RCF = 1.118 × 10⁻⁵ × r × RPM², with the radius r in centimetres. For a 10 cm radius at 5,000 RPM that gives 2,795 × g. To go from RCF to RPM, divide RCF by 1.118 × 10⁻⁵ × r and take the square root.
Which rotor radius should I use?
Use the distance from the rotor axis to the point in the tube your protocol refers to. Rotor data sheets often list a minimum, an average and a maximum radius, because RCF rises towards the bottom of the tube. If you are unsure, use the same radius the protocol's authors used, or the maximum radius for the peak force.
Can I convert RPM to RCF without knowing the radius?
No. The radius is part of the formula, so the conversion is only valid for a specific rotor. Find the radius in the rotor's data sheet or manual, or measure from the axis of rotation to the sample position.
Why does my centrifuge show a different RCF?
A centrifuge usually calculates RCF from the radius of the rotor fitted, often the maximum radius, and may round the value. If you enter a different radius here, the result will differ. Check which radius your rotor and protocol use and enter that.
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