Centrifuge RCF ↔ RPM Calculator (g-force)

How it works

RCF (× g) = 1.118 × 10⁻⁵ × r (cm) × rpm² · rpm = √( RCF ÷ (1.118 × 10⁻⁵ × r) ) · t₂ = t₁ × RCF₁ ÷ RCF₂ · k = 2.533 × 10¹¹ × ln(r-max ÷ r-min) ÷ rpm² · t (h) = k ÷ S · t₂ = t₁ × k₂ ÷ k₁

A centrifuge only spins at a set number of revolutions per minute, but the force a sample feels depends on how far it sits from the axis: centripetal acceleration is ω²r, and with ω in rpm and r in centimetres, dividing by standard gravity gives the familiar 1.118 × 10⁻⁵ constant. Because different rotors put tubes at different radii, protocols should quote relative centrifugal force (× g), and this converter gives the speed that reproduces it on your rotor — type the radius in cm, mm or inches. If your rotor cannot reach the protocol's g-force, "Adjust run time" keeps the g × time product constant, which is a good rule of thumb for pelleting; when you know both rotors' k-factors, the more exact t₂ = t₁ × k₂ ÷ k₁ is shown too. The k-factor itself depends on the ratio of maximum to minimum radius, so a rotor with a long liquid column is slower to clear than a shallow one at the same g; with a sedimentation coefficient in Svedbergs it gives the pelleting time in hours. Optional fields under More options compare a second rotor and warn when you exceed the rotor's rated speed or the tube's rated g.

Worked example

A microcentrifuge rotor with r-max 8 cm spun at 10,000 rpm produces 1.118 × 10⁻⁵ × 8 × 10,000² ≈ 8,944 × g. To reach a protocol's 12,000 × g on the same rotor you would set √(12,000 ÷ (1.118 × 10⁻⁵ × 8)) ≈ 11,583 rpm; with r-min 4.5 cm the k-factor at that speed is about 1,086. If you can only run 10,000 rpm, a 10-minute 12,000 × g step becomes 10 × 12,000 ÷ 8,944 ≈ 13.4 minutes.

Sources: Beckman Coulter — Relative centrifugal field · Eppendorf — Transferring centrifugation parameters from a protocol

Frequently asked questions