Relative Centrifugal Force Calculators

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Relative centrifugal force (RCF), measured in multiples of gravitational acceleration (× g), is the actual separating force experienced by particles in a centrifuge. RCF = 1.118 × 10⁻⁵ × r × RPM², where r is the rotor radius in centimeters and RPM is the speed in revolutions per minute. The same RPM produces very different RCF values in different rotors — a key reason why scientific protocols must always specify RCF (× g) rather than RPM for reproducibility. To convert RPM to RCF: RPM = √(RCF / (1.118 × 10⁻⁵ × r)).

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RCF Formula

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

r = rotor radius in cm (distance from rotation axis to midpoint of sample). Example: r = 10 cm; 5,000 RPM: RCF = 1.118 × 10⁻⁵ × 10 × 25,000,000 = 2,795 × g.

RPM to RCF Conversion

RPM = √(RCF / (1.118 × 10⁻⁵ × r))

Example: need 500 × g; r = 15 cm: RPM = √(500 / (1.118 × 10⁻⁵ × 15)) = √(500/0.0001677) = √2,981,503 = 1,727 RPM.

Finding Rotor Radius

From centrifuge manual (rmax, ravg, rmin for each rotor); from manufacturer website; or measure directly with ruler. Use ravg (average radius = midpoint of sample tube) for standard calculations.

Why Use RCF Not RPM?

Same 5,000 RPM: small rotor (r=5 cm) → 1,398 × g; large rotor (r=20 cm) → 5,590 × g. A 4-fold difference from the same RPM. Always specify RCF in protocols → ensures same separation regardless of centrifuge model.

Glossary

RCF (Relative Centrifugal Force)
The separating force in multiples of gravity (× g); RCF = 1.118×10⁻⁵ × r(cm) × RPM²; always specify RCF in protocols, not RPM — same RPM gives very different RCF in different rotors.
Rotor Radius (r)
Distance (cm) from rotation axis to midpoint of sample tube; use ravg; found in rotor manual or measured directly; determines RCF at a given RPM: larger r = higher RCF at same RPM.
RPM to RCF Conversion
RCF = 1.118×10⁻⁵ × r × RPM²; or RPM = √(RCF/(1.118×10⁻⁵ × r)); use rotor radius from manufacturer specifications; convert published RPM protocols to RCF for use on different centrifuges.

Frequently Asked Questions

RPM (revolutions per minute): the rotational speed of the centrifuge rotor — how fast it spins. RCF (relative centrifugal force, × g): the actual force experienced by the sample, expressed as multiples of gravity (g = 9.81 m/s²). RCF = 1.118 × 10⁻⁵ × r × RPM². The key difference: the same RPM produces very different RCF in rotors with different radii. Example: 3,000 RPM: rotor r = 8 cm → RCF = 1.118 × 10⁻⁵ × 8 × 9,000,000 = 805 × g. Rotor r = 15 cm → RCF = 1.118 × 10⁻⁵ × 15 × 9,000,000 = 1,509 × g. Nearly double the force from the same RPM! This is why protocols must specify × g, not RPM.

RPM → RCF: RCF = 1.118 × 10⁻⁵ × r (cm) × RPM². Example: Eppendorf 5415D (r_avg = 7.87 cm), 13,000 RPM: RCF = 1.118 × 10⁻⁵ × 7.87 × 169,000,000 = 14,874 × g ≈ 14,900 × g. RCF → RPM: RPM = √(RCF / (1.118 × 10⁻⁵ × r)). Example: need 1,000 × g; rotor r = 12 cm: RPM = √(1,000 / (1.118 × 10⁻⁵ × 12)) = √(1,000/0.0001342) = √7,452,308 = 2,730 RPM.

Rotor radius (r) = distance from the center of the rotor's rotation axis to the midpoint of the sample in the centrifuge tube. Finding r: (1) Centrifuge manual: look up your rotor model; specifications list rmax (tube bottom), rmin (top of liquid), and ravg (midpoint of sample). (2) Manufacturer website: search 'Eppendorf rotor F-45-30-11 specifications' etc. (3) Direct measurement: open the centrifuge; place a tube in the rotor; measure from center of spindle to midpoint of tube with a ruler. Use ravg for calculation. Note: different rotor positions may have slightly different radii in some rotors.

Common centrifugation protocols: Cell pelleting (mammalian cells): 300–500 × g, 5 min (gentle — doesn't damage cells). Bacterial pelleting (E. coli): 4,000–6,000 × g, 10–15 min. Yeast: 3,000–5,000 × g, 5 min. Platelet-rich plasma preparation: 150–200 × g, 15–20 min. Red blood cell sedimentation: 2,000 × g, 10 min. Mitochondria isolation: 600 × g (nuclear spin-down) → 10,000 × g (mitochondria). DNA precipitation: 12,000–16,000 × g, 15 min. Microcentrifuge max (typical): 13,000–21,000 × g. Ultracentrifugation: 100,000–500,000 × g (ribosomes, membrane vesicles, lipoprotein isolation, virus purification).