Centrifuge RPM to RCF Calculator
Enter a speed in rpm and your rotor's radius to get the relative centrifugal force in × g, or enter the force a protocol asks for and get the rpm to set. The radius can be in centimetres, millimetres or inches, and a table of common speeds for your rotor is included. Runs entirely in your browser. Free, no sign-up.
Maximum radius (rmax) from the rotor manual, to the bottom of the tube.
Distance from the axis to the top of the liquid; shows the lower end of the range.
Results
18,626
RCF at the bottom of the tube (× g)
The working
RCF = 1.118 × 10⁻⁵ × r × rpm² = 1.118 × 10⁻⁵ × 8.5 × 14,000² = 18,626 × g
Common speeds for this rotor
| Target (× g) | Speed (rpm) |
|---|---|
| 300 | 1,777 |
| 500 | 2,294 |
| 1,000 | 3,244 |
| 3,000 | 5,619 |
| 5,000 | 7,254 |
| 10,000 | 10,258 |
| 12,000 | 11,237 |
| 16,000 | 12,976 |
The constant 1.118 × 10⁻⁵ applies to a radius in cm: it is (2π ÷ 60)² ÷ g with g = 980.665 cm/s², rounded to four digits. Rotors and tubes have a maximum rated speed and force. Check the rotor manual before running at a computed setting.
How to Use the RPM to RCF Calculator
- 1. Choose the direction. RPM to RCF if you know the speed on the dial, RCF to RPM if a protocol gives you a force in × g.
- 2. Enter the rotor radius. Use the maximum radius, rmax, from the rotor's manual or label: the distance from the centre of rotation to the bottom of the tube. Pick cm, mm or inches.
- 3. Enter the speed or the force. The result appears immediately with the working.
- 4. Use the table. It lists the speed to set for common forces, such as 300, 1,000 or 16,000 × g, for your rotor.
The RCF Formula
A tube spinning at a radius r from the axis feels a centripetal acceleration of ω²r, where ω is the angular speed. Dividing by the acceleration of gravity expresses that as a multiple of g. With the speed in rpm and the radius in centimetres, the standard form is:
RCF (× g)
RCF = 1.118 × 10⁻⁵ × r (cm) × rpm²
Speed
rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r))
The constant is (2π ÷ 60)² ÷ 980.665, where 980.665 cm/s² is standard gravity, which comes to 1.1183 × 10⁻⁵ and is rounded to 1.118 × 10⁻⁵. The same relationship with the radius in millimetres, the form Beckman Coulter prints on its relative centrifugal field page, uses 1.118 × 10⁻⁶ because a millimetre is a tenth of a centimetre. For inches the constant is 2.84 × 10⁻⁵. The calculator applies the right one for the unit you choose.
Worked Examples
- RPM to RCF. A microcentrifuge rotor with rmax = 8.5 cm at 14,000 rpm: 1.118 × 10⁻⁵ × 8.5 × 14,000² = 18,626 × g. These are the calculator's default inputs.
- RCF to RPM. A protocol asks for 1,000 × g in the same rotor: rpm = √(1,000 ÷ (1.118 × 10⁻⁵ × 8.5)) = 3,244 rpm. Most machines step in tens, so the closest setting is 3,240 rpm, which gives 998 × g.
- Why radius matters. The same 1,500 rpm is 377 × g in a swinging-bucket rotor with a 15 cm radius, and only 214 × g at 8.5 cm.
Because speed is squared, small changes in rpm matter: doubling the speed quadruples the force, and 10% more speed gives 21% more force.
Which Radius Should I Use?
A rotating tube does not feel one force: the force rises from the liquid surface to the tube bottom. The published rating of a rotor, and the number a protocol usually means, is the maximum force at rmax, the distance from the axis to the bottom of the rotor cavity or bucket during centrifugation. Manufacturers list rmax in the instruction manual. If you want the force at the top of the liquid, enter that distance in the optional surface field; the result shows both. Some manuals also give an average radius, ravg; use the one your protocol or lab convention names, and say which in your methods.
Report × g, Not Just rpm
Two centrifuges at the same rpm exert different forces if their rotors differ, so a methods section that says only "spun at 10,000 rpm" cannot be reproduced. Report the force in × g (written "× g"), along with the rotor type, the temperature and the time. If your protocol was written in rpm, convert it for the rotor you actually use, and keep within the maximum speed printed on the rotor and the force rating of the tubes.
Frequently Asked Questions
- How do you convert RPM to RCF?
- Square the rpm, multiply by the rotor radius in centimetres, then multiply by 1.118 × 10⁻⁵. For 14,000 rpm at 8.5 cm that is 14,000² × 8.5 × 1.118 × 10⁻⁵ ≈ 18,626 × g. If your radius is in millimetres use 1.118 × 10⁻⁶ instead.
- How do you convert RCF to RPM?
- Divide the force by 1.118 × 10⁻⁵ times the radius in centimetres, then take the square root. For 1,000 × g at 8.5 cm: √(1,000 ÷ (1.118 × 10⁻⁵ × 8.5)) ≈ 3,244 rpm. Round to the speeds your centrifuge allows and check the force at that setting.
- Is RCF the same as g-force?
- Yes. Relative centrifugal force (RCF), relative centrifugal field and g-force all describe the acceleration as a multiple of Earth's standard gravity, written × g. It is dimensionless: 18,626 × g means 18,626 times the force of gravity.
- Why do protocols give × g instead of rpm?
- Rpm describes only how fast the rotor turns. The force on the sample also depends on the radius, so the same rpm gives different forces in different rotors. × g is the quantity that pellets cells or precipitates DNA, and it transfers between instruments.
- What radius do I use for RCF?
- Normally the maximum radius, rmax, from the rotor manual — the distance from the axis of rotation to the bottom of the tube. That gives the maximum RCF, which is what manufacturers quote. For fixed-angle rotors the radius varies along the tube, so be explicit about which one you used.
- Does the size of the tube change the RCF?
- Not for a given radius, but the tube changes where the sample sits. A longer tube or a fuller one puts liquid at a different radius, so the force at the surface and at the bottom differ. The optional surface radius shows that range.