Calculator
Example data
| Mode | Radius | RPM | RCF (×g) | Acceleration (m/s²) |
|---|---|---|---|---|
| RPM → RCF | 10 cm | 5000 | 2795 | 27,407.2 |
| RCF → RPM | 8 cm | 14,954 | 20000 | 196,133.0 |
| Acceleration → RCF | — | — | 20 | 196.13 |
| RCF → Acceleration | — | — | 5000 | 49,033.3 |
Formula used
- RCF from RPM (radius in cm): RCF = K × r(cm) × RPM², where K = 4π²/(360000×g₀)
- RPM from RCF (radius in cm): RPM = √(RCF / (K × r(cm)))
- RCF from acceleration: RCF = a / g₀
- Acceleration from RCF: a = RCF × g₀
Here, r is the distance from the rotor center to the sample tip, g₀ is standard gravity, and RCF is expressed as “×g”.
How to use this calculator
- Select the conversion mode you need for your run.
- For RPM↔RCF, enter rotor radius and choose the unit.
- Enter the known value (RPM, RCF, or acceleration).
- Keep g₀ as default unless your protocol specifies otherwise.
- Press Convert to view results and steps.
- Use CSV or PDF to archive the calculation with notes.
Understanding RCF and “×g”
Relative centrifugal force (RCF) describes how strongly a sample is pulled outward during spinning. It is expressed as “×g”, meaning multiples of standard gravity (g₀ ≈ 9.80665 m/s²). Two rotors at the same RPM can produce very different RCF because the sample sits at different radii.
When “g” already equals RCF
Many protocols state a target like “5,000 g”. In practice, that number is the same as 5,000 ×g (RCF = 5,000). The difference is mainly wording: “g” is the multiplier, while RCF is the formal reporting term. If you also know acceleration in m/s², convert by RCF = a/g₀.
Rotor radius drives the conversion
For RPM-based settings, you must know the radius from the axis to the sample tip. Use the maximum radius, not the tube length. Swing-bucket and fixed-angle rotors can have different radii for the same tube. A change from 8 cm to 10 cm increases RCF by 25% at the same RPM.
The standard conversion equation
RCF is computed from RPM using: RCF = 1.118×10⁻⁵ × r(cm) × RPM² (with standard gravity). This calculator also shows the physics form: ω = 2π·RPM/60, a = ω²·r(m), and RCF = a/g₀. Both methods should match within rounding, so discrepancies usually indicate a unit error.
Quick reference data points
At r = 10 cm and 5,000 RPM, RCF is about 2,795 ×g. At r = 8 cm, reaching 20,000 ×g requires roughly 14,954 RPM. If your instrument only accepts RPM, these comparisons help you sanity-check settings before a run and confirm you typed the radius correctly.
Selecting settings for common workflows
Gentle clarification spins are often 200–500 ×g for 3–10 minutes. Cell pelleting may use 3,000–6,000 ×g, while microcentrifuge protein precipitations can be 10,000–20,000 ×g. High-speed separations and ultracentrifugation can exceed 100,000 ×g. Always follow your SOP time and temperature, then adjust RPM using your rotor radius.
Documenting runs and troubleshooting
Report both the target (RCF or RPM) and the rotor radius used. If pellets smear or resuspend poorly, confirm the radius unit and that you used the sample-tip radius. Also verify tube ratings and balance mass pairs. Exporting CSV/PDF results makes audits easier and improves reproducibility between runs and different lab sites worldwide too.
1) Is “g” the same as RCF?
In most lab protocols, yes. A setting written as 5,000 g means the sample experiences about 5,000 times gravity, which is reported as 5,000 ×g (RCF).
2) What rotor radius should I use?
Use the distance from the rotor axis to the sample tip at maximum fill. For swinging buckets, measure to the tube tip when fully extended. Using a smaller radius will understate the true RCF.
3) Why do some centrifuges accept only RPM?
Many instruments control speed directly and leave force reporting to the user. Because RCF depends on rotor radius, RPM alone is not enough unless the rotor geometry is known.
4) Will swapping rotors change my results?
It can. At the same RPM, a larger radius produces a higher RCF. If you change rotor type or bucket position, recalculate RPM to match the same target ×g and keep conditions consistent.
5) How do I convert acceleration (m/s²) to RCF?
Divide acceleration by standard gravity: RCF = a / g₀. With g₀ = 9.80665 m/s², an acceleration of 98.07 m/s² corresponds to about 10 ×g.
6) Should I change the g₀ value?
Leave it at 9.80665 m/s² unless your method specifies a different convention. Changing g₀ slightly changes RCF and can matter for strict SOPs, validation reports, or cross-lab comparisons.