Centrifuge RPM ↔ RCF Converter
Convert between RPM and relative centrifugal force (RCF, × g).
Distance from the centrifuge axis to the bottom of the tube (use the rotor's maximum radius for the worst-case g-force).
Formula: RCF = 1.118 × 10⁻⁵ × r (cm) × RPM²
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What it does
RPM (revolutions per minute) tells you how fast the rotor spins; RCF (relative centrifugal force, × g) tells you the actual g-force acting on your sample. RCF is what matters biologically, and it depends on rotor radius. Protocols that specify a g value are portable between rotors; RPM values are not, because the same RPM produces very different g-forces in a mini centrifuge and a floor-standing machine.
How it works
RCF = 1.118 × 10⁻⁵ × r × RPM², where r is the rotor radius in cm. Rearranged: RPM = √( RCF / (1.118 × 10⁻⁵ × r) ). Use the rotor's maximum radius (from the manual) for the worst-case g-force. If the protocol gives r_max (spindle axis to the bottom of the tube), the conversion is exact for that position; material higher in a swinging-bucket tube sees less force, which is why journals ask you to quote RCF rather than RPM.
Worked example
Radius 10 cm at 3000 RPM → RCF = 1.118×10⁻⁵ × 10 × 3000² ≈ 1006 × g. To get 1000 × g at the same radius, set RPM ≈ 2993. A protocol calling for 12,000 × g can be reproduced on any rotor once you know its radius: at 8 cm the RPM would be √(12000 / (1.118×10⁻⁵ × 8)) ≈ 11,590 RPM.
When to use it
Use it to reproduce a protocol on a different centrifuge, convert a published RCF into your instrument's RPM, and compare spins across rotors of different sizes. It is also handy when adapting spin conditions between fixed-angle and swinging-bucket rotors, or when a rotor manual only lists RPM for a given g value.
FAQ
- How do I convert RPM to RCF (g-force)?
- RCF (relative centrifugal force, × g) is calculated from RPM and the rotor radius (cm) using the formula: RCF = 1.118 × 10⁻⁵ × r × RPM². Enter the rotor radius and RPM above; the converter computes RCF instantly.
- How do I convert g-force (RCF) to RPM?
- Rearranging the formula: RPM = sqrt( RCF / (1.118 × 10⁻⁵ × r) ), where r is the rotor radius in cm. Switch the converter to RCF → RPM mode, enter the radius and the g-force value, and the RPM is calculated.
- Why is rotor radius required for the conversion?
- RCF depends on how far the sample is from the axis of rotation. Two rotors running at the same RPM produce different g-forces if their radii differ. Always use the rotor's maximum radius (found in the rotor manual) for the worst-case g-force.
- What is the difference between RCF and g-force?
- They are the same quantity. RCF (relative centrifugal force) is expressed in multiples of Earth gravity (× g), so '1000 × g' and 'RCF 1000' mean the same g-force. RPM is different — it is only the spin speed.