What is the difference between RPM and RCF in centrifugation?

RPM vs RCF: The #1 Mistake New Lab Users Make

RPM and RCF are two critical parameters for centrifugation, yet they are frequently confused by beginner lab technicians. Using the wrong setting can ruin your sample and deliver inconsistent experimental results.

RPM (Revolutions Per Minute) RPM describes how fast the rotor spins, counting the number of full rotations per minute. It is a machine‑side speed value. RPM tells you nothing about the actual force acting on your sample.

Key limitation: The same RPM will generate different centrifugal force if you use rotors of different radii.

RCF (Relative Centrifugal Force), commonly referred to as ×g RCF is the actual centrifugal force exerted on your sample, expressed as multiples of gravitational force (g‑force). RCF depends on two factors: rotor speed and rotor radius.

Best lab practice: All standard experimental protocols should specify RCF (g), not RPM. When you change rotors, you must recalculate RPM to keep the same RCF.

Simple real‑world example If you run two different rotors at 5 000 RPM:

• A short‑radius rotor may produce 2 000 × g
• A long‑radius rotor can reach 4 000 × g

Same RPM, completely different force on your sample. That is why copying RPM values from a paper may lead to failed separation on your centrifuge.

Quick practical tip Most modern centrifuges support direct input of RCF (g). Enter your target g‑value directly, and the instrument will auto‑convert to corresponding RPM for the installed rotor. If your device only accepts RPM, use an RCF‑RPM conversion calculator.

⚠️ Common beginner pitfall: Directly copy RPM numbers from published protocols without checking rotor size. This is one of the top causes of unreproducible lab results.