RPM to RCF Calculators

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Centrifuge speed can be expressed as RPM (revolutions per minute) or as RCF (relative centrifugal force, measured in × g — multiples of gravitational acceleration). RCF (× g) is the preferred unit because it is independent of the centrifuge model and rotor — the same RCF achieves the same separation regardless of which centrifuge is used. RPM, by contrast, gives different RCF values for different rotor radii. The conversion formula is: RCF = 1.118 × 10⁻⁵ × r × RPM², where r = rotor radius in centimeters.

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RPM to RCF Conversion

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

r = radius from centrifuge axis to sample midpoint (cm); RPM = speed in revolutions per minute.

Example: centrifuge at 3,000 RPM, r = 10 cm: RCF = 1.118 × 10⁻⁵ × 10 × 3,000² = 1.118 × 10⁻⁵ × 10 × 9,000,000 = 1,006 × g.

RCF to RPM Conversion

RPM = √(RCF / (1.118 × 10⁻⁵ × r))

Example: achieve 500 × g with r = 15 cm: RPM = √(500 / (1.118 × 10⁻⁵ × 15)) = √(500 / 0.0001677) = √(2,981,502) = 1,727 RPM.

Common Centrifuge Protocols

  • Pelleting cells: 300–400 × g, 5 min (mammalian cells); 3,000–5,000 × g, 10 min (bacteria)
  • Platelet-rich plasma: 200 × g, 10 min (platelets remain in suspension)
  • Nuclei isolation: 600 × g, 10 min (pellets nuclei, not mitochondria)
  • Mitochondria: 3,000 × g, 15 min (pellets mitochondria)
  • Microsomal fractions: 100,000 × g, 60 min (ultracentrifuge)

Why RCF Is Important

A protocol specifying '3,000 RPM' is ambiguous — a rotor with r = 5 cm at 3,000 RPM gives 504 × g; a rotor with r = 20 cm gives 2,016 × g. Always specify RCF (× g) for reproducibility. When receiving protocols: convert any RPM specification to RCF using your rotor radius.

Glossary

RCF (Relative Centrifugal Force)
RCF = 1.118 × 10⁻⁵ × r × RPM²; centrifuge force in multiples of g; preferred over RPM as it is independent of rotor type; ensures reproducibility across different centrifuges.
RPM (Revolutions Per Minute)
Centrifuge rotation speed; does NOT indicate centrifugal force alone — must be combined with rotor radius to calculate RCF; different rotors at the same RPM produce very different RCFs.
Differential Centrifugation
Sequential centrifugation at increasing RCF to sequentially pellet cellular components by density/size: cells (400×g), nuclei (600×g), mitochondria (10,000×g), microsomes (100,000×g).

Frequently Asked Questions

RCF = 1.118 × 10⁻⁵ × r × RPM². r = rotor radius in cm (distance from rotation axis to center of sample tube). RPM = centrifuge speed in revolutions per minute. Example: tabletop centrifuge at 4,000 RPM, rotor radius = 12 cm: RCF = 1.118 × 10⁻⁵ × 12 × 4,000² = 1.118 × 10⁻⁵ × 12 × 16,000,000 = 2,146 × g. Rotor radius: check your centrifuge manual or manufacturer's website for your specific rotor. Many centrifuges have a dial or display that can be switched between RPM and × g directly if you enter the rotor radius.

RCF (× g) is independent of the centrifuge model and rotor used — it directly describes the centrifugal force applied to the sample. RPM is centrifuge- and rotor-specific: a protocol specifying '5,000 RPM' gives very different results depending on rotor size. Example: at 5,000 RPM, a small rotor (r = 5 cm): RCF = 1.118 × 10⁻⁵ × 5 × 25,000,000 = 1,397 × g. Large rotor (r = 20 cm): RCF = 1.118 × 10⁻⁵ × 20 × 25,000,000 = 5,590 × g. The difference is 4-fold! This is why publications and protocols should always report RCF (× g) rather than RPM to ensure reproducibility across different laboratories and centrifuge models.

Different cellular components pellet at different RCF values (differential centrifugation): Cells (mammalian): 200–400 × g, 5–10 min. Cells (bacteria): 3,000–5,000 × g, 10 min (dense cell wall needs more force). Platelets: 1,500–2,000 × g. Cell debris and nuclei: 600 × g, 10 min. Mitochondria: 3,000–10,000 × g, 15 min. Lysosomes/peroxisomes: 15,000–20,000 × g. Microsomes (ER fragments): 100,000 × g, 60 min (ultracentrifuge). Ribosomes: 100,000–200,000 × g, 2 hours. The classic subcellular fractionation procedure (differential centrifugation) uses successive low-to-high speed spins to sequentially pellet and collect each fraction.

The rotor radius needed for RPM↔RCF conversion is the distance from the rotation axis (center of rotor) to the midpoint of your sample tube. Three ways to find it: (1) Centrifuge display: many modern centrifuges allow you to enter the rotor's radius or k-factor, and then display RCF directly alongside or instead of RPM. (2) Centrifuge manual/manufacturer website: find your specific rotor model → look up rmax (maximum radius, at the bottom of the tube), rmin (minimum radius, at top of liquid), and ravg (average, at midpoint of sample) — use ravg for calculations. (3) Physical measurement: use a ruler to measure from the center of rotation (axis) to the midpoint of a centrifuge tube placed in the rotor. Different tube positions may have different radii — use the average distance for your sample volume.