Centrifuge Radius Calculator

Article

Use this page to translate between radius, RPM, and ×g, then compare settings across rotors. Always confirm your instrument’s maximum speed and rated radius before spinning.

1) What “radius” means in a rotor

In centrifugation, radius is the distance from the rotation axis to the sample’s effective position. Fixed-angle rotors use a shorter mean radius than swing-bucket rotors at full extension. Even a 1 cm change can noticeably shift the achieved force at the same speed.

2) Typical lab ranges you can sanity-check

Common microcentrifuges run about 6,000–15,000 RPM and reach roughly 2,000–21,000 ×g depending on rotor geometry. Benchtop swing buckets often operate around 500–4,000 RPM for 50–3,000 ×g. Use these bands to spot typos in inputs before you export results.

3) Why RPM alone is not enough

RPM measures rotational speed, but force depends on both RPM and radius. Two rotors at 10,000 RPM can differ by thousands of ×g if one has a 6 cm radius and the other 12 cm. That is why protocols specify RCF (×g) when reproducibility matters.

4) The key relationship used here

This calculator uses RCF = 1.118×10⁻⁵ × r(cm) × RPM². Because RPM is squared, small speed increases have large effects. For example, raising RPM from 8,000 to 10,000 increases RPM² by 56.25%, so RCF rises by the same percentage when radius stays constant.

5) Solving for radius when RCF is known

If a protocol asks for 5,000 ×g and your instrument shows only RPM, solving for radius helps compare rotors. With 8,000 RPM and 5,000 ×g, r ≈ 6.99 cm. Enter the same values to confirm your rotor’s effective radius aligns with manufacturer geometry.

6) Conversions and reporting

Laboratory specs often list radius in millimeters, while the formula needs centimeters. This tool converts mm, cm, meters, and inches automatically and displays all four for quick cross-checks. Use the CSV for spreadsheets or the PDF for method documentation and audit trails.

7) Practical tips for better accuracy

Use the mean sample radius, not the rotor’s outer diameter, especially for adapters. If tubes sit at different depths, calculate the midpoint between minimum and maximum radius. For sensitive separations, keep speed ramps consistent and avoid overfilling tubes, which can shift the effective radius.

FAQs

1) What is RCF and why is it written as ×g?

RCF is relative centrifugal force, expressed as multiples of Earth’s gravity. Writing ×g makes protocols comparable across rotors because it reflects actual acceleration at the sample.

2) Which radius should I enter for my rotor?

Use the effective sample radius: the distance from the axis to the sample’s midpoint during the run. If you only know min and max, enter their average for a practical estimate.

3) Can I convert a protocol from RPM to ×g?

Yes. Enter RPM and your rotor radius, then solve for RCF. If you switch rotors, recompute because a different radius will change the ×g at the same RPM.

4) Why do results change so much with small RPM changes?

The relationship uses RPM squared. A modest speed increase produces a much larger change in force, so fine adjustments are normal when targeting a specific ×g value.

5) Is the constant 1.118×10⁻⁵ always valid?

It is the standard lab constant when r is in centimeters and speed is in RPM, derived from unit conversions in the acceleration equation. Keep units consistent for correct results.

6) How should I document settings for reproducibility?

Record rotor model, effective radius, target RCF, run time, temperature, and acceleration/deceleration settings. Export the CSV or PDF from this tool to attach a clean calculation record.