RPM to RCF Calculator: Convert Centrifuge rpm and × g

A protocol says "spin at 500 × g for 5 minutes", and the centrifuge on your bench only shows rpm. The two are linked by one number you have to look up: the radius of the rotor. This converter does the rest.

RCF and rpm are different things

Rpm (revolutions per minute) is how fast the rotor turns. Relative centrifugal force (RCF) is how hard the sample is pushed outward, given as a multiple of the pull of Earth's gravity, g. The sample feels the force; the rotor only provides the speed.

"× g" and RCF are the same quantity written two ways. "RCF 10,000", "10,000 × g" and "10,000 g" all mean a force 10,000 times gravity. Protocols quote × g because it holds on any machine, while the same rpm gives different forces on different rotors.

How to use the converter

Pick the direction under Convert: rpm to × g (the default) or × g to rpm.
Type the speed in rpm (default 13000) or the force in × g (default 10000).
Type the rotor radius (default 7.3) and choose cm or mm. The tool converts mm to cm before calculating.

The answer is rounded to a whole number. The formula line underneath shows the values that went in and ends with the same result.

The formula

RCF = 1.118 × 10⁻⁵ × r × rpm²
rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r))

r is the radius in centimetres, from the centre of the rotor to the sample. The constant comes from physics: the outward acceleration is ω² × r, where ω = 2π × rpm ÷ 60 is the angular speed in radians per second. Dividing by standard gravity (980.665 cm/s²) gives (2π ÷ 60)² ÷ 980.665, which is about 1.118 × 10⁻⁵.

Worked example with the default values

At 13000 rpm on a rotor with a 7.3 cm radius:

RCF = 1.118 × 10⁻⁵ × 7.3 × 13,000²
= 1.118 × 10⁻⁵ × 7.3 × 169,000,000
= 13,792.8, which the tool rounds to 13793 × g

Switch to × g to rpm and the default 10000 × g on the same rotor needs √(10,000 ÷ (1.118 × 10⁻⁵ × 7.3)), which the tool shows as 11069 rpm.

Why the radius matters so much

Force grows with the radius and with the square of the speed. Double the rpm and the force goes up four times; double the radius and it doubles. Run the same 13,000 rpm on a rotor with a 5 cm radius and you get about 9,447 × g instead of 13,793 × g, roughly a third less. That is why copying an rpm value from a protocol written for someone else's centrifuge is a common source of loose pellets or crushed cells.

Where to find the rotor radius

The rotor's manual or specification sheet. Look for r max (to the bottom of the tube), often listed with r min (to the top of the liquid) and sometimes r av (the average).
The rotor itself. Some rotors and lids are engraved with the maximum radius and the maximum speed.
The centrifuge manual, which often lists the rotors it accepts and tables of × g against rpm.
If nothing is listed, measure from the centre of the rotor spindle to the bottom of a tube seated in the rotor. For a swing-out rotor, measure with the bucket swung out horizontally, as it is during the run.

Use r max unless the protocol says otherwise. The tube bottom is where pellets form, and it is the radius manufacturers normally quote with × g figures. Small fixed-speed mini centrifuges usually print their × g in the manual, since you cannot change the speed anyway.

Typical forces, for orientation

Spins vary by protocol, so treat these as broad patterns rather than settings. Mammalian cells are usually pelleted gently, often a few hundred × g for about 5 minutes; you can check the cell count afterwards with the hemocytometer calculator. DNA and RNA spin column kits often call for 10,000 × g or more, and the purified sample then goes to the A260 panel of the Beer-Lambert calculator. Density gradient separation of blood or marrow cells, described on our stem cell technology page, runs at low force with the brake off so the layers are not disturbed.

Common mistakes

Typing the radius in mm while the unit says cm. 73 instead of 7.3 gives a force ten times too high, or a speed about three times too low.
Using r min or a guessed radius when the protocol was written with r max.
Going past the rotor's rated maximum speed. The formula will give you any number; the rotor will not survive it. The rated limit in the manual always wins.
Unbalanced loads. Opposite positions need tubes of matching mass, whatever the speed.

FAQ

Is RCF the same as g?

Yes. RCF is expressed in multiples of g, so "RCF 5,000" and "5,000 × g" describe the same force. Rpm is the one that differs, because it measures rotor speed, not force.

How do I convert rpm to g?

Multiply 1.118 × 10⁻⁵ by the rotor radius in cm and by the rpm squared. For 13,000 rpm at a 7.3 cm radius that gives about 13,793 × g.

Why does the rotor radius matter?

The outward force on a sample grows in direct proportion to its distance from the centre of rotation. Two centrifuges spinning at the same rpm with different rotors apply different forces, so the radius has to be in every conversion.

Where do I find my rotor's radius?

Check the rotor's manual or specification sheet for r max, or look for it engraved on the rotor or lid. Failing that, measure from the centre of the spindle to the bottom of a seated tube, with buckets swung out on a swing-out rotor.

Can I use the same rpm on a different centrifuge?

Only if the rotor radius is the same. Otherwise convert the protocol's × g with the new rotor's radius, and stay within that rotor's rated maximum speed.