Why × g
RPM is how fast the rotor turns. What matters for pelleting is the force on the sample, the relative centrifugal force (RCF), written as × g. The same RPM gives a larger force in a larger rotor, so protocols give × g to work on any centrifuge.
The formula
RCF = 1.118 × 10−5 × r × RPM², with the radius r in centimeters.
A rotor with a 10 cm radius at 13,000 RPM gives 1.118 × 10−5 × 10 × 13,000² ≈ 18,900 × g. Going the other way, RPM = √(RCF ÷ (1.118 × 10−5 × r)).
Measuring the radius
Use the distance from the center of the rotor to the bottom of the tube in its spinning position, the maximum radius. It is usually printed on the rotor or in its manual. Force rises with the square of the speed but only in proportion to the radius, so small errors in the radius matter less than errors in speed.
Is RCF the same as g?
Yes. RCF, relative centrifugal force, is the force on the sample as a multiple of Earth's gravity, so 1,500 RCF, 1,500 × g and 1,500 g all mean the same thing. RPM is not the same: it is the rotor's speed, and the force it produces depends on the rotor.
Common settings
For an 8.5 cm rotor, the usual size in a microcentrifuge. Each link has a table for other radii.
| × g | RPM at 8.5 cm | Typical use |
| 300 × g | 1,780 | pelleting mammalian cells |
| 500 × g | 2,290 | gentle spins, washing cells |
| 1,000 × g | 3,240 | blood cells, large debris |
| 1,500 × g | 3,970 | plasma separation |
| 3,000 × g | 5,620 | yeast and bacterial cells |
| 5,000 × g | 7,250 | bacterial pellets |
| 10,000 × g | 10,260 | cell debris, miniprep lysate |
| 14,000 × g | 12,140 | spin columns, DNA pellets |
| 16,000 × g | 12,980 | ethanol precipitation of DNA |
| 20,000 × g | 14,510 | protein precipitates, fine debris |
Going the other way: 3,000 RPM, 5,000 RPM, 10,000 RPM and 13,000 RPM in × g.
Calculator
The RCF to RPM Converter converts either way from the rotor radius.