RCF vs RPM: What Every Lab Needs to Know About Centrifuge Speed
RCF vs RPM: What Every Lab Needs to Know About Centrifuge Speed
It is one of the most common sources of irreproducible results in the lab: a protocol calls for a spin at "5,000 × g," the operator dials in "5,000 RPM," and the separation quietly fails. RCF and RPM are not the same thing — and confusing them can cost you an experiment. Here is what the difference is and how to get it right every time.
What RPM Measures
RPM (revolutions per minute) describes how fast the rotor turns. It says nothing, by itself, about the force your sample experiences — because that force also depends on how far the sample sits from the center of rotation.
What RCF Measures
RCF (relative centrifugal force), expressed as a multiple of gravity (× g), is the actual force applied to your sample. This is what protocols specify, because it is reproducible across different instruments. The relationship is:
RCF = 1.118 × 10⁻⁵ × r × (RPM)²
where r is the rotor radius in centimeters. The key takeaway from that formula: because RCF depends on the radius, the same RPM produces different g-force on different rotors. A wide floor-standing rotor generates far more force at 5,000 RPM than a compact microcentrifuge rotor at the same RPM.
Why This Trips Up So Many Labs
Older protocols and lab notebooks often record RPM because that is what the dial showed on the original instrument. When the same protocol is run on a different centrifuge — with a different rotor radius — copying the RPM reproduces the wrong force. Always translate to RCF, then back to the RPM appropriate for your rotor.
Convert It Correctly (Without the Math)
You do not need to compute this by hand. Our free RCF ⇄ RPM calculator converts in both directions — enter your rotor radius and either value to get the other. Keep the RCF from your protocol constant, plug in your rotor's radius, and it returns the exact RPM to set.
A Quick Best-Practice Checklist
- Record spins in × g (RCF), not RPM, in every protocol you write.
- Note the rotor radius alongside — it is required to convert.
- When adopting a protocol from another lab, convert to RCF and re-derive the RPM for your rotor.
- Confirm your target RCF is within your rotor's rated maximum before spinning.
The Bottom Line
RPM is what the motor does; RCF is what your sample feels. Specify RCF, convert for your rotor, and your separations become reproducible across any instrument. New to centrifuge selection? Start with our complete centrifuge buying guide, or explore centrifuges built for consistent, protocol-accurate performance.