RPM to g-force calculator
Relative centrifugal force from rotor radius and speed, in either direction.
Relative centrifugal force from rotor radius and speed, in either direction.
A centrifuge separates things by spinning them, and what does the separating is the outward acceleration at the sample, measured in multiples of gravity and written × g orRCF, relative centrifugal force. Two rotors turning at the same rpm produce different forces if their radii differ, which is why protocols quote g, not rpm. This calculator converts between the two for any rotor once you know its radius.
Worked example. A microcentrifuge rotor with rmax 8.4 cm at 13,000 rpm gives 1.118 × 10⁻⁵ × 8.4 × 13,000² = 15,870 × g, close to the 16,000 × g that most miniprep protocols ask for. The constant comes from ω²r ÷ 9.81 with ω converted from rpm to rad/s; if the radius is in millimetres use 1.118 × 10⁻⁶ instead, which the unit menu handles for you. Force scales with the square of the speed, so doubling the rpm quadruples the g.
The radius is the distance from the centre of the rotor shaft to the point in the tube you care about. rmax is the bottom of the tube in its running position and is the figure protocols mean unless they say otherwise, because pelleting happens there. rmin is the top of the liquid, where the force can be a third lower. Fixed-angle rotors hold tubes at 25–45°, so rmax is shorter than the tube; swinging-bucket rotors extend the tubes horizontally and reach a larger radius at the same rpm. Manufacturers list both values in the rotor table, and the quick-pick menu above carries approximate rmax values for common rotors.
| Task | Force |
|---|---|
| Pelleting mammalian cells | 300–500 × g |
| Pelleting yeast | 1,000–3,000 × g |
| Pelleting bacteria | 4,000–6,000 × g |
| Plasmid miniprep spins | 12,000–16,000 × g |
| Nucleic acid ethanol precipitation | 12,000–20,000 × g |
| Clearing lysates, membranes | 20,000–100,000 × g |
| Ultracentrifugation of ribosomes, viruses | 100,000–200,000 × g |
At 10,000 rpm a 7 cm microcentrifuge rotor delivers about 7,800 × g while a 15 cm swinging bucket delivers 16,800 × g, more than double. Copying an rpm from a colleague's protocol on a different centrifuge therefore under- or over-spins the sample, which is the usual reason a pellet is loose or a gradient is crushed. Convert the protocol's g-force to rpm for your rotor instead, and never exceed the rotor's rated maximum speed or the tube's rated g.
Multiply the rotor radius in centimetres by the square of the speed in rpm, then by 1.118 × 10⁻⁵. At 13,000 rpm in a rotor of 8.4 cm radius that is 1.118 × 10⁻⁵ × 8.4 × 13,000² ≈ 15,900 × g.
Rearrange the same formula: rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r)). For 16,000 × g in an 8.4 cm rotor that gives √(16,000 ÷ (1.118 × 10⁻⁵ × 8.4)) ≈ 13,050 rpm.
Protocols almost always quote the g-force at the bottom of the tube, rmax, because that is where the pellet forms. Use rmin only when a protocol says so, for example for gentle handling of cells near the top of a gradient. The radius is measured from the centre of the rotor shaft to the point of interest in the tube, not the tube length.
Because the same rpm gives a different force in every rotor: a large fixed-angle rotor at 10,000 rpm pulls far harder than a microcentrifuge at 10,000 rpm. Relative centrifugal force is what actually moves particles, so it transfers between instruments and rpm does not.
On the rotor itself, in its manual, or in the manufacturer’s rotor table, listed as rmax and rmin in millimetres or centimetres. If nothing is available, measure from the centre of the drive shaft to the bottom of a tube in its running position with a ruler.