Rotor Radius Calculators

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Rotor radius is the distance from the center of the centrifuge rotor's rotation axis to the sample position in the tube, measured in centimeters. It is a critical parameter in converting between RPM (revolutions per minute) and RCF (relative centrifugal force in × g): RCF = 1.118 × 10⁻⁵ × r × RPM². Different rotors at the same RPM produce very different RCFs — larger rotors generate much greater centrifugal force. This is why centrifuge protocols must always specify RCF (× g) rather than RPM to ensure reproducibility across different centrifuge models and rotor types.

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RCF Calculation Using Rotor Radius

RCF (× g) = 1.118 × 10⁻⁵ × r (cm) × RPM²

r = rotor radius in cm (distance from rotation axis to sample midpoint). Example: r = 10 cm; 4,000 RPM: RCF = 1.118 × 10⁻⁵ × 10 × 16,000,000 = 1,789 × g.

RPM to RCF Conversion

RPM = √(RCF / (1.118 × 10⁻⁵ × r))

Example: achieve 2,000 × g with r = 12 cm: RPM = √(2000 / (1.118 × 10⁻⁵ × 12)) = √(14,903,129) = 3,860 RPM.

Finding Rotor Radius

Methods: Centrifuge manual: rotor specification sheet lists rmax (bottom of tube), rmin (top of liquid), and ravg (midpoint). Manufacturer website: search by centrifuge model + rotor model. Direct measurement: ruler from rotation axis to midpoint of tube. Use ravg for calculations.

Why Rotor Radius Matters

Same 5,000 RPM: r = 5 cm → RCF = 1,398 × g; r = 20 cm → RCF = 5,590 × g. Four-fold RCF difference from the same RPM setting! Always report RCF (× g) in protocols for reproducibility.

Glossary

Rotor Radius (r)
Distance (cm) from centrifuge rotation axis to sample midpoint; determines RCF from RPM: RCF = 1.118×10⁻⁵ × r × RPM²; use ravg (midpoint of sample column); found in rotor manual or by direct measurement.
RCF (Relative Centrifugal Force)
Centrifugal force in multiples of gravity (× g); RCF = 1.118×10⁻⁵ × r × RPM²; preferred unit for centrifuge protocols because it is independent of rotor type; ensures reproducibility.
rmax / ravg / rmin
Maximum (tube bottom), average (sample midpoint), and minimum (top of liquid) rotor radii; ravg used for standard RCF calculations; listed in rotor specifications or measured directly.

Frequently Asked Questions

Rotor radius (r) is the distance from the center of the centrifuge rotation axis to the sample position — typically the midpoint of the centrifuge tube. It determines how much centrifugal force is generated at a given RPM: RCF = 1.118 × 10⁻⁵ × r × RPM². Because r appears in the formula, different-sized rotors at the same RPM produce very different RCFs. Example: at 5,000 RPM: small rotor (r = 5 cm): RCF = 1,398 × g. Large rotor (r = 20 cm): RCF = 5,590 × g. A 4-fold RCF difference from the same speed setting — which is why protocols must specify RCF, not RPM.

Three methods: (1) Centrifuge/rotor manual: look up rmin (minimum radius — top of liquid in tube), rmax (maximum radius — bottom of tube), and ravg (average radius — midpoint of sample column). Use ravg for standard calculations. (2) Manufacturer website: find your centrifuge model → rotor specifications → radius values. (3) Physical measurement: open centrifuge; place tube in rotor; measure distance with ruler from center of rotor spindle to the midpoint of the tube. Different tube positions in the rotor may have slightly different radii — always use the position you actually centrifuge in.

RPM to RCF: RCF = 1.118 × 10⁻⁵ × r(cm) × RPM². RCF to RPM: RPM = √(RCF / (1.118 × 10⁻⁵ × r)). Example 1: Beckman Allegra X-22R, rotor r = 12.4 cm, 3,500 RPM: RCF = 1.118 × 10⁻⁵ × 12.4 × 3,500² = 1.118 × 10⁻⁵ × 12.4 × 12,250,000 = 1,697 × g. Example 2: need 500 × g with r = 15 cm: RPM = √(500/(1.118 × 10⁻⁵ × 15)) = √(2,981,502) = 1,727 RPM.

RPM is centrifuge- and rotor-specific: the same RPM produces different centrifugal forces in different rotors. If a protocol specifies '3,000 RPM': a small rotor (r = 5 cm) gives 504 × g — may not pellet the target particles. A large rotor (r = 20 cm) gives 2,016 × g — may damage fragile cells or pellet unintended material. RCF (× g) is independent of the specific centrifuge or rotor: any combination of RPM and rotor radius that gives the specified RCF will produce the same separation result. This is why journals and protocols should always report speed as '× g' (e.g., '300 × g, 5 minutes') rather than 'X RPM'. Convert any RPM-specified protocol to RCF using your rotor's radius before following it.