Centrifugation RCF Calculator

Calculate relative centrifugal force from RPM and rotor radius for cell and organelle fractionation.

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Why RPM Is Not a Protocol

RPM describes how fast the rotor spins. What actually separates particles is relative centrifugal force, which depends on both speed and radius — so the same RPM in a different rotor delivers a different force.

RCF = 1.118 × 10−5 × RPM² × r

Because RCF scales with the square of RPM but only linearly with radius, doubling the speed quadruples the force while doubling the radius merely doubles it. A protocol specifying only RPM is unreproducible on other equipment, which is why published methods should always state ×g.

Rotor radiusAt 10,000 RPMDifference
5 cm5,590 ×gBaseline
8 cm8,944 ×g60% higher
15 cm16,770 ×gThree times the force

Typical Separations

TargetRCFNotes
Whole cells, nuclei500–1,000 ×gGentle — avoids lysis
Mitochondria10,000–20,000 ×gStandard benchtop range
Microsomes, membranes100,000 ×gUltracentrifuge required
Ribosomes150,000–200,000 ×gUltracentrifuge

Differential centrifugation exploits this ordering: spin at low force first to pellet the largest structures, keep the supernatant, then increase force stepwise. Each pellet enriches a different fraction, which is the basis of classical subcellular fractionation.

Fixed-angle rotors have a radius gradient along the tube, so RCF differs between the top and bottom of the sample. Protocols normally quote the maximum, at rmax, but average RCF is lower — worth knowing when a separation is marginal.

Worked Examples

Example 1: 10,000 RPM, r=8cm
RCF=1.118e-5x100,000,000x8
Result: RCF=8,944xg
Pellets bacteria, mitochondria, yeast
Example 2: Need 100,000xg, r=8cm
RPM=sqrt(100000/(1.118e-5x8))
Result: RPM=33,438 — ultracentrifuge required
For ribosomes, viruses, membranes
Example 3: Same RPM, different rotor
10,000 RPM at r = 5 cm versus r = 15 cm
Result: 5,590 versus 16,770 ×g
Three times the force from the identical speed setting. This is exactly why protocols specified only in RPM fail to reproduce.
Example 4: Differential fractionation
Sequential spins at 1,000, then 20,000, then 100,000 ×g
Result: Nuclei, then mitochondria, then microsomes
Each step pellets progressively smaller structures from the previous supernatant — the classical route to subcellular fractions.

Common Mistakes

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Copying a protocol by RPM instead of ×g

Different rotors have different radii, so the same RPM produces different force. Always convert to RCF when transferring a method between centrifuges.

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Forgetting that radius is measured to the sample

Rotor radius is to the point of interest in the tube, not the rotor edge. Using the wrong reference gives an incorrect RCF.

⚠️
Spinning cells too hard

Above roughly 1,000 ×g many cell types lyse or become damaged. Gentle pelleting preserves viability and morphology.

⚠️
Ignoring the RCF gradient in fixed-angle rotors

Force varies along the tube. Quoted values are usually maximum RCF, while the average across the sample is appreciably lower.

Frequently Asked Questions

Centrifugation protocols?
300xg 5min: cells. 1000xg 10min: nuclei. 3000xg: mitochondria crude. 10,000xg: lysosomes, chloroplasts. 100,000xg 60min: microsomes (ER), ribosomes.
k-factor?
k=2.53e11xln(rmax/rmin)/RPMmax². Smaller k=more efficient rotor. Time=k/s where s=sedimentation coefficient in Svedbergs.
What is the difference between RPM and RCF?
RPM is rotational speed; RCF is the force experienced by the sample, which depends on both speed and rotor radius. Only RCF is transferable between centrifuges.
Why do protocols specify ×g rather than RPM?
Because the same RPM gives different forces in different rotors. Stating RCF makes a method reproducible on any equipment.
What force is needed for mitochondria?
Typically 10,000 to 20,000 ×g, achievable on a standard benchtop centrifuge. Microsomes and ribosomes require an ultracentrifuge at 100,000 ×g or more.
Does RCF vary within the tube?
Yes, in fixed-angle rotors the radius differs between the top and bottom of the sample. Quoted values are usually the maximum at rmax.