RPM to RCF (×g)
Use this when your protocol gives speed in RPM and you need relative centrifugal force.
RCF (×g) to RPM
Use this when your method is in ×g and your centrifuge accepts RPM.
Equivalent Speed Between Rotors
Estimate the RPM needed on a second rotor to match the same RCF at a different radius.
Note: This tool is for planning and education. Always confirm rotor limits, tube ratings, and official manufacturer guidance before running a centrifuge.
What this Beckman Coulter rotor calculator helps you do
A rotor calculator converts between RPM (revolutions per minute) and RCF (relative centrifugal force, shown as ×g). In real lab work, protocols often specify one unit while your instrument interface may require the other. This page helps you switch quickly and consistently.
The core reason this matters is simple: sedimentation depends on force, not just speed. Two rotors can spin at the same RPM and still produce very different g-forces because their radii differ.
The core formula
The calculator uses the standard equation:
- RCF = 1.118 × 10-5 × r × RPM2
Where:
- RCF is relative centrifugal force (×g)
- r is radius in centimeters
- RPM is revolutions per minute
To solve for RPM, the formula is rearranged:
- RPM = √(RCF / (1.118 × 10-5 × r))
How to choose the right radius value
For rotor calculations, radius is usually measured from the center of rotation to a specific point in the tube. Depending on your protocol, that may be:
- rmax (bottom of sample column)
- ravg (middle of sample column)
- rmin (top of sample column)
Many methods and rotor manuals reference one of these explicitly. If your SOP does not specify, ask your lab lead and use the same convention consistently across experiments.
Practical workflow for centrifuge method transfer
1) Start from protocol requirements
If the protocol says “spin at 18,000 ×g for 20 minutes,” use the RCF-to-RPM section with your rotor radius.
2) Verify rotor and tube limits
Even if the math says a speed is possible, your rotor may have a maximum permitted RPM lower than the calculated value. Tube and cap ratings can also be limiting factors.
3) Keep documentation clean
Record both the target RCF and the RPM actually programmed on the instrument. This makes troubleshooting and cross-lab replication much easier.
Common mistakes to avoid
- Using radius in millimeters instead of centimeters
- Applying one rotor’s RPM directly to a different rotor
- Ignoring maximum rated rotor speed
- Mixing fixed-angle and swinging-bucket assumptions without checking geometry
- Skipping calibration and maintenance checks
Quick example
Suppose you need 12,000 ×g with a rotor radius of 10.0 cm. Enter those values into the RCF-to-RPM calculator. The result is approximately 10,363 RPM. You would then verify that this speed is below the rotor’s rated maximum before starting the run.
Final reminder
This calculator is designed for rapid planning, protocol conversion, and training. For production workflows, regulated environments, and critical samples, always use current manufacturer documentation and your lab’s approved procedures.