Centrifuge RCF ⇄ RPM Calculator

Understanding the centrifuge RCF and RPM calculator — concept and principle

A centrifuge protocol is only reproducible when it is written as relative centrifugal force (RCF, in multiples of gravity, × g), because the same rpm produces very different forces in rotors of different size. This calculator converts a speed in revolutions per minute into RCF for a given rotor radius, and works backwards to find the rpm that delivers a target RCF. Use it whenever you move a protocol to a different centrifuge or rotor.

At r = 10 cm, 3,000 rpm gives about 1,006 × g; 1,000 × g needs about 2,991 rpm.

How it is calculated

RCF = 1.118 × 10⁻⁵ × r × rpm², where r is the radius in centimetres from the centre of rotation to the point of interest in the tube.
rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r)) for a target force.

RCF has no unit of its own; it is a ratio to standard gravity and is written as "× g". Radius must be in cm for this constant to apply.

The principle

An object moving in a circle needs a centripetal acceleration a = ω²r, where ω is the angular speed in radians per second. One revolution is 2π radians, so ω = 2π × rpm ÷ 60. Dividing a by standard gravity (9.80665 m/s²) and converting r from metres to centimetres gives (2π/60)² ÷ 980.665 ≈ 1.118 × 10⁻⁵. Because force grows with the square of speed, doubling rpm quadruples RCF, while doubling the radius only doubles it.

Worked example

Inputs: r = 10 cm, speed = 3,000 rpm, target RCF = 1,000 × g.
rpm² = 3,000² = 9,000,000.
RCF = 1.118 × 10⁻⁵ × 10 × 9,000,000 = 1,006.2 × g.
For the target: 1.118 × 10⁻⁵ × 10 = 1.118 × 10⁻⁴; 1,000 ÷ 1.118 × 10⁻⁴ = 8,944,544.
√8,944,544 = 2,990.74 rpm, which you would set as about 2,990 rpm.

Each point is the calculator re-run with only “Speed (rpm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Speed (rpm)	RCF at this rpm (× g)
750	62.8875
1,500	251.55
2,250	565.9875
3,000	1,006.2
3,750	1,572.1875
4,500	2,263.95
6,000	4,024.8

How to read the result

Rough orders of magnitude help when checking a protocol: pelleting intact mammalian cells is usually done at a few hundred × g, bacteria at several thousand × g, and organelles or fine precipitates at higher forces, while ultracentrifugation of membranes and macromolecules reaches around 100,000 × g. These are typical ranges only; follow the force stated in your protocol. Remember that RCF varies along the tube: it is highest at the bottom (r max) and lowest at the top (r min).

Common mistakes

Using the rotor diameter instead of the radius, which doubles the computed force.
Entering radius in mm or inches; the constant assumes centimetres.
Copying an rpm value from a protocol written for a different rotor.

Keep in mind: Never exceed the maximum speed, RCF or load rating of the rotor and tubes given by the manufacturer, and always balance the rotor. The calculated value is a physics conversion, not a safety rating.

Frequently asked questions

Which radius should I use, r max or r average?

Protocols usually quote RCF at r max (tube bottom), which is what pellets experience. The rotor manual lists r min, r avg and r max; use the same one the protocol used.

Is × g the same as RCF?

Yes. "1,000 × g" means a relative centrifugal force of 1,000, that is 1,000 times standard gravity.

Why does a small change in rpm change RCF so much?

RCF depends on rpm squared, so a 10% increase in speed raises the force by about 21%.

Sources: Beckman Coulter, rotor and centrifuge user manuals (RCF = 1.118 × 10⁻⁵ r N²) · Halliday, Resnick and Walker, Fundamentals of Physics, chapter on uniform circular motion

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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