Centrifugation is one of the most common operations in life science laboratories. Cells are pelleted, lysates are clarified, precipitates are collected, and many purification workflows depend on centrifugation.
However, centrifugation protocols are commonly described using two different quantities: RPM and RCF, usually expressed as ×g.
They are related, but they are not the same thing.
The same RPM can generate different centrifugal forces when the rotor radius changes. Therefore, transferring an RPM value directly from one centrifuge to another does not necessarily reproduce the same experimental condition.
What is RPM?
RPM means revolutions per minute.
It describes how many complete rotations the centrifuge rotor makes in one minute.
For example, 5,000 RPM means that the rotor completes 5,000 revolutions per minute.
RPM therefore describes rotational speed. It does not directly describe the centrifugal acceleration experienced by the sample.
What is RCF?
RCF means relative centrifugal force.
It describes the centrifugal acceleration applied to a sample relative to gravitational acceleration and is commonly expressed as ×g.
For example, 1,000 ×g means that the sample experiences an acceleration approximately 1,000 times gravitational acceleration.
Unlike RPM alone, RCF depends on both rotational speed and the radial distance from the axis of rotation.
How are RPM and RCF related?
A commonly used relationship is:
RCF = 1.118 × 10−5 × r × RPM2
where r is the rotor radius in centimeters.
This relationship shows two important features.
First, RCF increases in proportion to rotor radius.
Second, RCF increases with the square of RPM.
Therefore, doubling the RPM does not double the centrifugal force. At the same rotor radius, it increases RCF approximately four-fold.
The same RPM can produce very different ×g values
Consider two centrifuge rotors operating at the same speed of 5,000 RPM.
For a rotor radius of 5 cm:
RCF ≈ 1,400 ×g
For a rotor radius of 10 cm:
RCF ≈ 2,800 ×g
The RPM is identical, but the centrifugal force is approximately two-fold different.
This is why a protocol written only as:
“Centrifuge at 5,000 RPM for 10 min.”
does not fully define the centrifugal force unless the rotor radius is also known.
Why does rotor radius matter?
The sample travels in a circular path around the centrifuge axis. The farther the sample is from that axis, the greater the centrifugal acceleration at the same rotational speed.
This is why centrifuge and rotor manuals often provide a maximum rotor radius, commonly written as rmax.
The Current Protocols in Protein Science appendix on centrifuges and rotors provides rmax values for commonly used rotors and nomograms for calculating RCF from RPM and rmax, or RPM from RCF and rmax.
Therefore, when converting RPM to RCF for routine laboratory work, the rotor radius specified by the manufacturer should be used.
Why RPM alone can reduce reproducibility
If two laboratories use different centrifuge rotors, the same RPM may expose their samples to different centrifugal forces.
This matters because centrifugation is a physical separation process. Changes in centrifugal force can change sedimentation efficiency and, depending on the biological material, may also affect sample quality.
Sharifian Gh and Norouzi highlighted that arbitrary differences in centrifugal force and incomplete understanding of centrifugation conditions can threaten reproducibility in cell-separation procedures.
They also emphasized that centrifugation outcome depends on more than centrifugal force alone, including variables such as temperature, osmolarity, fluid volume, viscosity, and centrifugation time.
Therefore, when transferring a centrifugation protocol between laboratories or instruments, RCF is generally more informative than RPM alone.
Why is RPM still commonly reported?
RPM has historically been a common way to describe centrifuge operation because centrifuges were often controlled and displayed directly in rotational speed.
Many older protocols were developed using a particular centrifuge and rotor, so an RPM value became associated with that specific experimental setup.
When the same rotor is used, such a protocol may work perfectly well.
The problem appears when the protocol is transferred to another rotor with a different radius.
In that case, the same RPM no longer represents the same centrifugal force.
Which rotor radius should be used?
RCF varies along the length of a centrifuge tube because the distance from the axis of rotation changes.
For routine conversion, manufacturers commonly specify a rotor radius or maximum radius that can be used to calculate maximum RCF.
The Current Protocols reference defines rmax as the maximum rotating radius between the material being centrifuged and the axis of rotation and uses it for RPM-to-RCF conversion.
However, specialized experiments may require more precise definition of where RCF is being calculated.
For example, in platelet-rich fibrin research, reporting standards have distinguished RCF at different positions within the tube because rotor radius, tube angle, and sample position can all affect the force experienced by the sample.
This specialized example illustrates a broader principle: RCF should always be interpreted together with the radius at which it is defined.
What happens if RPM and ×g are confused?
RPM and ×g should never be treated as interchangeable numerical values.
Suppose a protocol specifies:
12,000 ×g
but the centrifuge is accidentally set to:
12,000 RPM
For a rotor radius of 8 cm, 12,000 RPM corresponds to approximately 12,900 ×g.
For a rotor radius of 5 cm, the same 12,000 RPM corresponds to only about 8,050 ×g.
The RPM value is identical, but the applied centrifugal force is substantially different.
This is why RPM-to-RCF conversion requires the rotor radius.
Is RCF alone enough for complete reproducibility?
No.
RCF removes one important source of ambiguity by accounting for rotor radius, but centrifugation outcome can still depend on several other parameters.
Depending on the experiment, relevant factors can include:
• centrifugation time
• temperature
• rotor geometry
• sample volume
• medium density and viscosity
• tube geometry
• acceleration and braking conditions
For cell centrifugation in particular, methodological work has emphasized that temperature, viscosity, osmolarity, sample volume, RCF, and centrifugation time can all influence sedimentation behavior.
Therefore:
Matching RCF is important, but matching RCF alone does not guarantee an identical biological outcome.
How should centrifugation conditions be reported?
For many routine experiments, reporting RCF and centrifugation time is more transferable than reporting RPM alone.
For example:
Samples were centrifuged at 2,800 ×g for 10 min.
If temperature is relevant:
Samples were centrifuged at 2,800 ×g for 10 min at 4°C.
When rotor geometry or equipment differences may influence the result, reporting the centrifuge model and rotor can further improve reproducibility.
In highly specialized centrifugation applications, additional parameters such as rotor radius, rotor angle, tube geometry, and the position at which RCF is defined may also be required.
Convert RPM and RCF with BioChemCalc
If a protocol reports RPM but your centrifuge uses RCF, or if you need to determine the RPM required to reach a specified ×g value, BioChemCalc provides a free conversion tool.
Centrifuge g ⇄ RPM Converter
https://biochemcalc.com/centrifuge_g_prm
Enter the rotor radius and either RPM or RCF to calculate the corresponding value.
This is especially useful when transferring protocols between centrifuges with different rotor sizes.
Conclusion
RPM and RCF describe different aspects of centrifugation.
RPM tells us how fast the rotor is rotating.
RCF tells us the centrifugal acceleration applied at a defined radius.
Because rotor radius differs among centrifuges and rotors, the same RPM can generate different centrifugal forces.
Therefore, RPM alone may be insufficient when centrifugation conditions need to be reproduced across different instruments.
RCF provides a more transferable description of centrifugal force, but reproducibility can also depend on centrifugation time, temperature, rotor geometry, sample properties, and other experimental conditions.
So when an old protocol says:
“Centrifuge at 5,000 RPM.”
one useful question is:
“What rotor radius was used?”
References
1. Sharifian Gh M, Norouzi F. Guidelines for an optimized differential centrifugation of cells. Biochemistry and Biophysics Reports. 2023;36:101585. doi:10.1016/j.bbrep.2023.101585.
2. Centrifuges and rotors. Current Protocols in Protein Science. 2001;Appendix 2:Appendix 2C. doi:10.1002/0471140864.psa02cs01.
3. A low-cost, open-source centrifuge adaptor for separating large volume clinical blood samples. 2022. PMCID: PMC9269434.
4. Miron RJ, Pinto NR, Quirynen M, Ghanaati S. Standardization of relative centrifugal forces in studies related to platelet-rich fibrin. Journal of Periodontology. 2019;90(8):817-820. doi:10.1002/JPER.18-0553.