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Centrifuge RCF / RPM Converter

Converts between rotor speed (rpm) and relative centrifugal force (RCF, in multiples of g) for a given rotor radius with RCF = 1.118 × 10⁻⁵ × r_cm × rpm², solving whichever of the two is not given.

When to use

A protocol specifies a centrifugation force in ×g and you need the rpm for your rotor, or you know the rpm and radius and need the force applied to the sample.

Do not use when: You need the centripetal force in newtons on a specific mass (use centripetal-force), or sedimentation times and k-factors of ultracentrifuge rotors.

Formula

RCF (×g) = 1.118 × 10⁻⁵ × radius_cm × rpm²; rpm = √(RCF / (1.118 × 10⁻⁵ × radius_cm)); ω = 2π × rpm / 60

RCF = ω² r / g with g = 9.80665 m/s², which reduces to the 1.118 × 10⁻⁵ constant for r in cm and speed in rpm. Because RCF is proportional to the radius, the same rpm gives different forces at the top and bottom of a tube; use the radius your protocol refers to (usually r_max).

Inputs

ParameterTypeUnitRequiredDescription
rpmnumberrpmnoRevolutions per minute. Give rpm or rcf_g, not both. Range: > 0, ≤ 200000
rcf_gnumber×gnoForce in multiples of standard gravity (×g). Give rcf_g or rpm, not both. Range: > 0, ≤ 2000000
radius_cmnumbercmyesDistance from the rotor axis to the point of interest in the tube (r_max = bottom of the tube, r_min = top; protocols usually mean r_max or r_average). Range: > 0, ≤ 100

Outputs

OutputTypeUnitDescription
rcf_gnumber×g1.118 × 10⁻⁵ × radius_cm × rpm².
rpmnumberrpm√(rcf_g / (1.118 × 10⁻⁵ × radius_cm)).
radius_cmnumbercmRotor radius the conversion refers to.
angular_velocity_rad_snumberrad/srpm × 2π / 60.
notestringWhich quantity was solved and how the radius affects the result.

Example

3,000 rpm at 10 cm radius: {"rpm":3000,"radius_cm":10} → {"rcf_g":1006.2,"rpm":3000,"angular_velocity_rad_s":314.16}

1,000 ×g needed at 8 cm radius: {"rcf_g":1000,"radius_cm":8} → {"rpm":3344,"rcf_g":1000}

GET https://tttkmbb.com/api/v1/calculate/centrifuge-rcf-rpm?rpm=3000&radius_cm=10

Machine access

Sources

FAQ

Why do protocols give ×g instead of rpm?

The force on the sample depends on both speed and rotor radius, so only RCF (×g) is transferable between centrifuges; convert it to rpm with your rotor's radius.

Which radius should I use?

The maximum radius (bottom of the tube) is the usual reference for pelleting; some manufacturers quote the average radius. Check the rotor specification sheet.

Where does 1.118 × 10⁻⁵ come from?

RCF = ω² r / g with ω = 2π × rpm / 60 and r in metres; converting r to centimetres and dividing by 9.80665 m/s² gives (2π/60)² / (100 × 9.80665) = 1.118 × 10⁻⁵.

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