Centrifuge RCF to RPM Calculator
Centrifuge RCF to RPM Calculator
Convert a required force in × g into the rotor speed to dial up on your own centrifuge — the direction you need when transferring a published method.
RCF to RPM
× g + radius → RPMA method calling for 1,500 × g, run in a rotor of radius 10 cm
Formula
with r in centimetres
- RCF
- the force the method requires, in multiples of standard gravity (× g)
- r
- rotor radius in CENTIMETRES, conventionally r-max, measured from the axis of rotation to the bottom of the tube
- 1.118 × 10⁻⁵
- the same constant as the forward calculation — (2π/60)², ÷ 100 for centimetres to metres, ÷ 9.80665 m/s²
- √
- force rises with the square of speed, so the inverse takes a square root: halving the force needs only about a 29% reduction in RPM
Worked example
A method calling for 1,500 × g, run in a rotor of radius 10 cm
1.118 × 10⁻⁵ × 10 = 1.118 × 10⁻⁴
1,500 ÷ 1.118 × 10⁻⁴ = 13,416,816
√13,416,816 = 3,663 RPM
Between 3,000 and 6,000 → routine range; confirm against the rotor's rated maximum
RPM required for a target force, by rotor radius
| Rotor radius | 500 × g | 1,000 × g | 1,500 × g | 3,000 × g | 10,000 × g |
|---|---|---|---|---|---|
| 6 cm | 2,730 | 3,861 | 4,729 | 6,688 | 12,210 |
| 8 cm | 2,364 | 3,344 | 4,095 | 5,792 | 10,574 |
| 10 cm | 2,115 | 2,991 | 3,663 | 5,180 | 9,458 |
| 16 cm | 1,672 | 2,364 | 2,896 | 4,095 | 7,477 |
| 20 cm | 1,495 | 2,115 | 2,590 | 3,663 | 6,688 |
Checks before you dial the speed in
| Check | Why it matters |
|---|---|
| Rotor maximum rated speed | An absolute limit. If the calculated RPM exceeds it, the force cannot be achieved in that rotor — you need a rotor with a larger radius or a different instrument. |
| Derating for dense samples | Manufacturers derate rotors for gradients and dense media such as caesium chloride or sucrose. Follow the published derating rather than the nominal maximum. |
| Tube and adapter rating | Tubes and adapters have their own force limits, often well below the rotor’s. Clinical blood tubes are not rated for microfuge speeds. |
| Which radius the method used | If the source method quoted r-average and you measure r-max, your speed will be low. Recalculate whenever you change rotor, bucket or adapter. |
Transferring a published spin to your own centrifuge
Published methods specify force, because force is the reproducible quantity, but centrifuges are dialled in revolutions per minute. This calculation is the bridge, and it needs one piece of local information the paper cannot supply: the radius of your rotor. Take it from the rotor manual where you can — manufacturers publish r-max, r-min and r-average for each rotor and bucket combination — and measure only if you have to, from the axis of rotation to the bottom of the tube as it sits loaded.
Check every answer against the rotor’s maximum rated speed before you use it. A rotor is a stressed component with a finite fatigue life, and overspeeding is the one centrifuge error with a serious physical consequence rather than a merely analytical one. Manufacturers also derate rotors when the sample is dense — caesium chloride and sucrose gradients are the usual cases — because the load the rotor carries depends on the mass being spun, not just the speed. Follow the published derating, and never treat a calculated speed as authorisation to exceed the rated one.
The radius convention has to match at both ends of the transfer. If the source method quoted a force based on r-average and you compute your speed from r-max, you will run the sample slower than intended, and the discrepancy grows with tube length. The radius also changes whenever the geometry changes: a swing-out bucket has a different effective radius from a fixed-angle rotor, and dropping an adapter into the bucket to hold a smaller tube changes it again. Recalculate rather than reusing the old figure.
Two practical points follow from the square-root relationship. First, force is far more sensitive to speed than the other way round, so a 5% error in speed is roughly a 10% error in force, and a centrifuge whose tachometer has drifted is worth having serviced. Second, the calculation says nothing about time, acceleration or braking, all of which affect the result: a short spin spends a meaningful fraction of its run accelerating, and a hard brake can resuspend a soft pellet or disturb a gel barrier that had formed properly.
Frequently asked questions
How do I convert × g to RPM?
Divide the required force by 1.118 × 10⁻⁵ times the rotor radius in centimetres, then take the square root. A method calling for 1,500 × g in a 10 cm rotor needs about 3,663 RPM.
What if the calculated RPM is above my rotor's rated speed?
You cannot run it. The rotor rating is an absolute limit and overspeeding risks rotor failure. Use a rotor with a larger radius, which reaches the same force at a lower speed, or a centrifuge rated for the force.
Why does the same protocol need a different RPM on a different centrifuge?
Because force depends on the rotor radius as well as the speed. A wide swing-out rotor reaches a given force at a much lower RPM than a compact fixed-angle rotor, which is exactly why methods specify force rather than speed.
Does using an adapter for smaller tubes change the answer?
Yes. Anything that moves the bottom of the tube nearer to or further from the axis of rotation changes r-max, and therefore the speed needed. Recalculate when you change rotor, bucket or adapter.
Why do rotors get derated for gradients?
The stress on a rotor depends on the mass it is carrying, so a dense medium such as caesium chloride or sucrose loads it more heavily than water at the same speed. Manufacturers publish a reduced maximum speed for these, and it should be followed.
Related calculators
References
- Beckman Coulter. Rotors and Tubes for Beckman Coulter Centrifuges — rotor speed ratings, derating for dense media, and r-max data.
- Clinical and Laboratory Standards Institute. Procedures for the Handling and Processing of Blood Specimens for Common Laboratory Tests. CLSI guideline GP44.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.