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RCF ↔ RPM Converter

Converts between RCF and RPM using rotor radius.Enter the rotor radius and either RCF or RPM to solve for the other.

Always enter the radius in mm. Enter only one of RCF or RPM.

Result

💡 Fill in the values and press Calculate to see the result here.

How to use

  1. Enter the rotor radius (mm) — the distance from the centre of rotation to the point in the tube you care about.
  2. Enter whichever you know, RCF (× g) or RPM.
  3. The other value is calculated. The conversion works in both directions.

💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.

Formula & notes

RPM is how many turns the rotor makes in a minute. Rotors differ in size from instrument to instrument, so the same RPM does not put the same force on the sample.

RCF, the relative centrifugal force, states that force as a multiple of gravity. It does not depend on the rotor, which is why protocols should be written in RCF.

The two are connected by this.

Relative centrifugal force
RCF = 1.118 × 10⁻⁵ × r(mm) × RPM²
Rotational speed
RPM = √( RCF ÷ (1.118 × 10⁻⁵ × r) )

The constant 1.118 × 10⁻⁵ is what is left after tidying up the angular velocity and the acceleration due to gravity into units of millimetres and minutes.

The square on RPM matters. Doubling the speed quadruples the force.

Practical notes

  • Where you measure the radius to changes the answer. From the axis to the bottom of the tube is the maximum radius (r_max); halfway along is the average (r_av). Check which one the protocol means — for pelleting it is normally r_max.
  • The radius is in the instrument manual. It differs between rotors, and changing the rotor on the same instrument changes it. Use the published figure rather than a ruler.
  • Record protocols in RCF. An RPM alone cannot be reproduced on another instrument. Where a paper's methods give only RPM, the rotor has to be tracked down alongside it.
  • Fixed-angle and swing-out rotors hold the sample in different places, so their effective radii differ.
  • This calculation covers speed only. Acceleration and braking time, temperature and brake settings are separate and affect the separation.

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