Centrifugation: RCF ↔ RPM conversion
Relative centrifugal force depends on rotor radius as well as speed, so the same rpm on two centrifuges is not the same spin. Convert either way, or rescale a protocol.
RCF ↔ RPM × g
Enter the rotor radius and either the speed or the force — the other is filled in.
Move a protocol to a different rotor
A published method says “spin at 12 000 rpm” but does not say in what. Convert through × g so the sample sees the same force.
Clearing factor (k-factor) and pelleting time
The k-factor describes how quickly a rotor pellets a particle of a given sedimentation coefficient. Lower k means faster.
| Particle | Sedimentation coefficient |
|---|---|
| Serum albumin | 4.5 S |
| IgG | 7 S |
| Catalase | 11.3 S |
| Ribosomal subunit (bacterial, small) | 30 S |
| Ribosomal subunit (bacterial, large) | 50 S |
| Bacterial ribosome | 70 S |
| Eukaryotic ribosome | 80 S |
| Polysome (5 ribosomes) | ≈ 200 S |
| Tobacco mosaic virus | 185 S |
| Mitochondrion | ≈ 10 000 S |
Typical spins
| Purpose | Force | Time | Notes |
|---|---|---|---|
| Pellet bacterial cells | 4 000 × g | 10 min | Gentle enough to resuspend |
| Pellet mammalian cells | 200–300 × g | 5 min | Higher force damages membranes |
| Clear a lysate (small scale) | 16 000 × g | 15–30 min | Standard benchtop microfuge maximum |
| Nuclei | 600 × g | 10 min | 4 °C, in hypotonic buffer |
| Mitochondria | 10 000 × g | 15 min | After a 600 × g pre-clear |
| Microsomes / membranes | 100 000 × g | 60 min | Ultracentrifuge required |
| Ribosomes | 150 000 × g | 2–3 h | Through a sucrose cushion |
| Exosomes | 100 000–120 000 × g | 70–90 min | After serial pre-clears |
| Plasmid miniprep clearing | 16 000 × g | 10 min | Room temperature is fine |
Where the constant comes from
Relative centrifugal force is the centripetal acceleration divided by standard gravity:
RCF = ω²r / g, where ω = 2πN/60 rad s⁻¹.
Substituting and expressing r in centimetres gives
RCF = (2π/60)² × r/100 ÷ 9.80665 × N² = 1.11846 × 10⁻⁵ × r × N². The calculator
above works in SI internally rather than using the rounded constant, so results agree with
rotor manuals to four figures.
Choosing which radius to use
- rmax — the bottom of the tube. Use this when you want to know whether something will pellet.
- rav — halfway down the sample column. This is what most published protocols and rotor specification sheets quote.
- rmin — the meniscus. Relevant for gradient work, where you care about the force at the top of the sample.
The difference is not trivial: a fixed-angle microfuge rotor might have rmin = 38 mm and rmax = 85 mm, so the force at the bottom of the tube is more than twice that at the top.
Common rotor radii
| Rotor type | rmax (approx.) | Max speed | Max RCF |
|---|---|---|---|
| Benchtop microfuge, 24 × 1.5 mL | 85 mm | 14 000 rpm | ≈ 18 600 × g |
| Benchtop microfuge, 18 × 2 mL (compact) | 73 mm | 13 300 rpm | ≈ 14 400 × g |
| Swing-out, 4 × 100 mL | 180 mm | 4 500 rpm | ≈ 4 100 × g |
| Fixed-angle, 6 × 50 mL | 108 mm | 15 000 rpm | ≈ 27 200 × g |
| Ultracentrifuge, fixed-angle 8 × 39 mL | 92 mm | 45 000 rpm | ≈ 208 000 × g |
| Ultracentrifuge, swinging bucket 6 × 38 mL | 153 mm | 32 000 rpm | ≈ 175 000 × g |
These are representative values only — always take the radius from your own rotor's manual.
Frequently asked questions
What is the formula for RCF?
RCF = 1.118 × 10⁻⁵ × r × N², with r the rotor radius in centimetres and N the speed in rpm. The constant comes from (2π/60)² / g expressed in cm.
Why should protocols specify × g rather than rpm?
The force a sample experiences depends on the rotor radius as well as the speed, so the same rpm on two different rotors gives different forces. Reporting × g makes a protocol reproducible on any centrifuge.
Which radius should I use — rₘᵢₙ, rₐᵥ or rₘₐₓ?
Use rₘₐₓ for pelleting (the force at the tube bottom) and rₐᵥ when comparing to published protocols, which usually quote average radius. Rotor manuals list all three.