RPM to RCF Calculator (Centrifuge g-Force)
Radius = distance from the center of the rotor to the bottom of the tube (rmax), in centimeters.
How RPM and RCF relate
RPM (revolutions per minute) only tells you how fast the rotor turns. What actually separates your sample is the relative centrifugal force (RCF), expressed in multiples of gravity (× g). RCF depends on both speed and radius: the same 3000 RPM produces far more force in a large rotor than in a small one. That is why protocols should, and good ones do, specify × g rather than RPM.
The calculator uses the standard formula:
RCF = 1.118 × 10−5 × r × RPM², where r is the radius in centimeters.
Rearranged to find the speed for a target force: RPM = √(RCF ÷ (1.118 × 10−5 × r)).
Worked example
A protocol calls for 1,000 × g and your rotor's maximum radius is 10 cm. RPM = √(1000 ÷ (1.118 × 10−5 × 10)) ≈ 2,991 RPM, so set about 3,000 RPM. Run the same protocol in a rotor with a 15 cm radius and you only need about 2,442 RPM.
Where to find the rotor radius
Check the rotor specifications in your centrifuge manual. If it isn't listed, measure from the center of the rotor shaft to the bottom of a tube seated in its holder (for angle rotors, measure with the tube in place, along the line it sits). Many manufacturers also publish the maximum RCF at maximum speed; you can back-calculate the radius from those two numbers, which is how we derived the ≈11 cm value for the LC-6 Clinical Centrifuge (3074 × g at 5000 RPM).
Common mistakes
- Copying RPM between centrifuges. A setting that works in one machine can under- or over-spin in another. Convert through × g.
- Using the radius in millimeters. The formula expects centimeters; 110 mm is 11 cm.
- Mixing up rmax and ravg. Most protocols assume rmax (tube bottom). If yours specifies average radius, use that instead.
- Exceeding the rotor's rated speed. Never run above the maximum RPM in your manual, whatever the calculator says.
FAQ
Is RCF the same as g-force?
Yes. RCF expressed as "× g" means multiples of Earth's gravity; 1,000 × g is 1,000 times the force of gravity.
What RCF do I need for PRP?
PRP protocols vary by kit and clinic, using one or two spins that range widely, roughly 100 to 2,000 × g. Follow the protocol from your kit or training and use this calculator to translate it to your centrifuge's RPM.
Why does my centrifuge show RPM only?
Many benchtop centrifuges display RPM. Convert your target × g to RPM here, using your rotor radius.
Looking for a centrifuge? Compare our benchtop and PRP centrifuges, or see more free lab calculators.
