RCF to RPM Conversion Calculators

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Converting between RCF (relative centrifugal force, expressed as × g) and RPM (revolutions per minute) is a routine task in laboratory work. Because centrifugal force depends on both rotation speed and rotor radius, RPM alone cannot specify the force experienced by a sample — the rotor radius is required. RCF is the scientifically correct way to report centrifuge conditions in publications and protocols, ensuring reproducibility across laboratories using different centrifuges and rotors.

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Conversion Formulas

RPM to RCF

RCF (× g) = 1.118 × 10⁻⁵ × r × N²
r = rotor radius in centimeters; N = speed in RPM.

RCF to RPM

N (RPM) = √(RCF / (1.118 × 10⁻⁵ × r))

Worked Examples

RPM → RCF: r = 12 cm, N = 3,000 RPM:
RCF = 1.118 × 10⁻⁵ × 12 × 9,000,000 = 1,207 × g

RCF → RPM: Target 2,000 × g, r = 10 cm:
N = √(2,000 / (1.118 × 10⁻⁵ × 10)) = √17,889,107 = 4,229 RPM

Why RCF Over RPM?

Two centrifuges at 5,000 RPM with r = 8 cm and r = 16 cm:
Centrifuge 1: RCF = 2,236 × g; Centrifuge 2: RCF = 4,472 × g — same RPM, double the force. Reporting RCF eliminates this ambiguity.

Quick Reference

  • 300 × g: sediments whole mammalian cells
  • 3,000 × g: sediments bacteria
  • 10,000 × g: sediments mitochondria
  • 100,000 × g: ultracentrifuge — sediments ribosomes

Glossary

RCF (Relative Centrifugal Force)
Centrifugal acceleration as a multiple of Earth's gravity (g). RCF = 1.118 × 10⁻⁵ × r × N². The correct unit for reporting centrifuge conditions — reproducible across different rotors and instruments.
RPM (Revolutions Per Minute)
Rotational speed of a centrifuge rotor. Does not specify centrifugal force without rotor radius. Convert to RCF for reproducible methods reporting. RPM = √(RCF / (1.118 × 10⁻⁵ × r)).
Rotor Radius (r)
The distance from the center of rotation to the sample position in cm. Required to convert between RPM and RCF. Listed in centrifuge manuals as minimum (r_min), average (r_av), and maximum (r_max) values.

Frequently Asked Questions

RCF = 1.118 × 10⁻⁵ × r × N², where r = rotor radius in cm and N = speed in RPM. Example: r = 10 cm, N = 6,000 RPM: RCF = 1.118 × 10⁻⁵ × 10 × 36,000,000 = 4,025 × g. The rotor radius is found in the centrifuge manual. Most manufacturers also label rotors with their maximum radius.

N = √(RCF / (1.118 × 10⁻⁵ × r)), where r = rotor radius in cm. Example: need 5,000 × g with r = 15 cm: N = √(5,000 / (1.118 × 10⁻⁵ × 15)) = √(29,814,550) = 5,460 RPM. Round to the nearest 100 RPM for setting on the centrifuge control panel.

RPM alone does not specify centrifugal force — force also depends on rotor radius. Two centrifuges at the same RPM but different rotor sizes produce different g-forces. Specifying RCF (× g) ensures any laboratory can replicate the exact sedimentation conditions by calculating the correct RPM for their own rotor. This is essential for reproducibility in differential centrifugation protocols.

Use the average radius (r_av) for a representative force value — most appropriate for reporting in methods. Use the maximum radius (r_max) to determine the highest force experienced at the bottom of tubes. Both values are listed in the centrifuge/rotor manual. For differential centrifugation protocols where precise force matters, confirm which radius the original protocol specified before calculating your RPM.