Centrifuge RPM to RCF Calculator
Centrifuge RPM to RCF Calculator
Convert rotor speed in revolutions per minute to relative centrifugal field in × g, using your own rotor radius — the number a method should actually specify.
RPM to RCF
RPM + radius → × g3,000 RPM in a rotor of radius 10 cm
Formula
with r in centimetres
- r
- rotor radius in CENTIMETRES, measured from the axis of rotation to the point of interest — conventionally r-max, the bottom of the tube. Entering millimetres gives an answer ten times too large.
- RPM
- rotor speed in revolutions per minute, as read from the instrument
- 1.118 × 10⁻⁵
- folds together (2π/60)² for revolutions per minute to radians per second, ÷ 100 for centimetres to metres, and ÷ 9.80665 m/s² for standard gravity
- RCF
- relative centrifugal field, a multiple of standard gravity, written × g
Worked example
3,000 RPM in a rotor of radius 10 cm
r is in centimetres: 10 cm, not 100 mm
1.118 × 10⁻⁵ × 10 × 3000² = 1.118 × 10⁻⁵ × 10 × 9,000,000
= 1,006 × g
Between 1,000 and 3,000 × g → routine clinical range
The same RPM in different rotors (× g)
| Rotor radius | 2,000 RPM | 3,000 RPM | 4,000 RPM | 10,000 RPM |
|---|---|---|---|---|
| 6 cm | 268 | 604 | 1,073 | 6,708 |
| 8 cm | 358 | 805 | 1,431 | 8,944 |
| 10 cm | 447 | 1,006 | 1,789 | 11,180 |
| 16 cm | 716 | 1,610 | 2,862 | 17,888 |
Which radius to measure
| Radius | Measured to | Used for |
|---|---|---|
| r-max | The bottom of the tube | Pelleting — the force experienced at the pellet. The usual default, and the value this calculator expects. |
| r-min | The surface of the liquid | The minimum force anywhere in the sample; relevant to gradient work and to whether the top of the sample sediments at all. |
| r-average | The mid-point of the liquid column | Sedimentation coefficient work and some manufacturer tables. Quoted forces can differ substantially from r-max in a long tube. |
Why a method should specify × g, not RPM
A centrifugation step is defined by the force applied to the sample, not by the speed of the motor. Relative centrifugal field, expressed as a multiple of standard gravity, is a property of what happens to the sample; revolutions per minute is a property of the machine. Because the force depends on how far the tube sits from the axis of rotation, the same 3,000 RPM produces about 600 × g in a compact microfuge rotor and over 1,600 × g in a wide swing-out rotor. A method that says spin at 3,000 RPM is therefore not reproducible between laboratories; one that says spin at 1,000 × g is.
The radius in this equation is measured in centimetres, and that is where most errors enter. A radius entered in millimetres inflates the answer tenfold, and some published tables quote the radius in millimetres with a correspondingly different constant, so the two cannot be mixed. Rotors also have three radii: r-max at the bottom of the tube, r-min at the meniscus, and r-average between them. The convention for pelleting is r-max, because that is the force at the pellet, but any method worth reproducing states which one it used.
The constant is not empirical. Relative centrifugal field is ω²r divided by standard gravity, so converting revolutions per minute into radians per second contributes (2π/60)², converting the radius from centimetres to metres contributes a division by 100, and dividing by 9.80665 m/s² completes it. Multiplied out, those three steps give 1.118 × 10⁻⁵. Change any of the assumptions — a radius in millimetres, a speed in revolutions per second — and the constant changes with them.
The consequences at the bench are concrete. A serum separator tube spun below the force its gel needs leaves the barrier incompletely formed, and fibrin strands and platelets carry over into the plasma, blocking probes and raising potassium and lactate dehydrogenase as platelets continue to release their contents. Spin too hard and red cells shear: visible haemolysis makes potassium, lactate dehydrogenase and aspartate aminotransferase uninterpretable and the sample has to be redrawn. Both failures arise the same way, from copying an RPM figure written for somebody else’s rotor.
Frequently asked questions
What is the difference between RPM and RCF?
RPM is how fast the rotor turns; RCF is the force the sample actually experiences, expressed as a multiple of gravity. RCF depends on both the speed and the rotor radius, so the same RPM gives different forces in different rotors. Protocols should specify RCF.
Is the rotor radius in centimetres or millimetres?
Centimetres, for this form of the equation. The constant 1.118 × 10⁻⁵ assumes a radius in centimetres, so entering millimetres makes the answer ten times too large. If a table you are copying from quotes millimetres, it will be using a different constant.
Should I measure to the bottom of the tube or the middle?
To the bottom of the tube for pelleting work, which gives r-max — the force at the pellet, and the usual convention. r-average and r-min give lower figures and are used for gradient and sedimentation work, so a method should say which it means.
Where does the constant 1.118 × 10⁻⁵ come from?
It combines three conversions: revolutions per minute to radians per second, which contributes (2π/60)²; centimetres to metres, a division by 100; and division by standard gravity, 9.80665 m/s². It is arithmetic, not an empirical fitting constant.
How hard should I spin a serum separator tube?
Follow the tube manufacturer’s stated force and time, not another laboratory’s RPM. Too little force leaves an incomplete gel barrier with fibrin and platelets in the plasma; too much causes haemolysis and makes potassium, LDH and AST unreportable.
Related calculators
References
- Beckman Coulter. Rotors and Tubes for Beckman Coulter Centrifuges — rotor r-max, r-min and relative centrifugal field data.
- Clinical and Laboratory Standards Institute. Procedures for the Handling and Processing of Blood Specimens for Common Laboratory Tests. CLSI guideline GP44.
- Clinical and Laboratory Standards Institute. Collection of Diagnostic Venous Blood Specimens. CLSI guideline GP41.