RCF vs. RPM in Centrifugation

RPM describes how fast a centrifuge rotor turns. RCF describes the centrifugal acceleration at a defined radius relative to Earth’s gravitational acceleration. They are related, but they are not interchangeable: the same RPM produces a different RCF in rotors with different radii.

For a protocol that must be reproduced or transferred, record RCF, time, temperature, rotor type, and the radius convention or rotor position used. RPM alone is incomplete unless the exact rotor is also fixed.

The practical difference

Quantity	What it describes	Depends on	Best use
RPM	Revolutions per minute of the rotor	Rotational speed only	Setting or documenting speed for a specific known rotor when the method requires it
RCF	Centrifugal acceleration relative to gravity, commonly written × g	RPM and distance from the axis of rotation	Comparing or transferring force conditions across compatible rotors

Two rotors at 10,000 RPM can produce different maximum RCF because their radii differ. Conversely, they need different RPM settings to produce the same RCF. This is why a protocol written only in RPM may not transfer correctly to another centrifuge.

The conversion

With radius in centimeters, a commonly used relationship is:

RCF = 1.118 × 10⁻⁵ × r × RPM²

where r is the selected rotor radius in centimeters. Thermo Fisher Scientific gives this relationship in its centrifuge best-practices guide 🔗.

The equation is simple; choosing the correct radius is the important part. Rotor documentation may report minimum, average, or maximum radius and corresponding RCF. A fixed-angle tube also spans a range of radii, while a swing bucket changes position during acceleration. Use the radius definition and RCF value supplied for the installed rotor and vessel position rather than measuring an approximate external dimension.

Maximum RCF is not the force everywhere in the sample

Manufacturers commonly publish maximum RCF at the greatest relevant radius. Material closer to the axis experiences less centrifugal acceleration. The sample therefore experiences a field across the tube, not one identical force at every point.

For routine protocols, the published rotor value and validated method may be sufficient. For density gradients, rate-zonal work, ultracentrifugation, or transfers where sedimentation path matters, rotor geometry and k-factor can become important. A single maximum-RCF number cannot describe the entire separation.

How to transfer a protocol

Identify the original rotor, vessel, radius convention, RCF, time, and temperature.
Confirm that the new rotor and vessel are compatible with the sample, force, fill, containment, and instrument.
Use the new rotor’s official documentation or centrifuge control to set the target RCF.
Do not exceed the speed or force limit of the rotor, bucket, adapter, vessel, lid, or centrifuge; the lowest applicable limit governs.
Verify separation quality with the method’s acceptance criteria rather than assuming equal maximum RCF guarantees an identical result.

Eppendorf’s centrifugation basics 🔗 recommends retaining rotor type, RCF, temperature, and run time when following a protocol. Rotor geometry, acceleration and braking, sedimentation path, tube fill, sample viscosity, and time can still affect the outcome.

When time may also need reconsideration

Matching RCF does not necessarily make two rotors equivalent. Fixed-angle and swing-bucket rotors can differ in sedimentation path and pellet location. Rotor k-factor is used in some applications to compare pelleting efficiency and estimate time changes, particularly in high-speed and ultracentrifuge work.

Do not apply a generic time-scaling equation without the installed rotor documentation and an appropriate protocol. The sample, particle, viscosity, rotor geometry, acceleration profile, and separation endpoint determine whether such scaling is valid.

Common documentation failures

Writing 10,000 g when the method actually means 10,000 RPM.
Reporting RPM without the exact rotor.
Copying the centrifuge’s maximum RCF rather than the run setting.
Converting with an assumed rotor radius.
Ignoring a lower limit for the tube, adapter, bucket, lid, or aging rotor.
Omitting temperature, time, acceleration, brake setting, or fill conditions when they affect the result.
Treating equal RCF as proof that pellet position, gradient shape, recovery, or sample integrity will match.

Safety and operating limits

RCF conversion is not permission to run a rotor faster. Use only approved rotor, bucket, adapter, lid, and vessel combinations, and follow the current manuals. Balance loads as specified and stop operation according to the instrument procedure if unusual vibration or noise occurs.

For hazardous or potentially infectious material, containment follows the laboratory risk assessment. CDC guidance emphasizes sealed rotors or safety cups for aerosol control in relevant diagnostic work and requires manufacturer instructions for rotor selection and care to be followed. CDC: safe work practices in diagnostic laboratories 🔗

A complete centrifuge method record

Centrifuge and rotor identification.
Rotor type and installed buckets or adapters.
Vessel, fill volume, sample balance, and closure.
Target RCF and whether the value is maximum, average, or another defined radius.
Corresponding RPM if required by the instrument or record.
Time, temperature, acceleration, and braking settings.
Containment configuration and handling controls where applicable.
Separation acceptance criteria and any transfer verification.