Converter
Example data
| Radius (cm) | RPM | RCF (× g) | Notes |
|---|---|---|---|
| 10 | 5,000 | 2,795 | Typical benchtop spin for quick pellets. |
| 15 | 12,000 | 24,149 | Higher radius strongly increases g force. |
| 8 | 7,477 | 5,000 | RPM computed from a target g force. |
Formula used
This calculator uses the standard centrifuge conversion with radius in centimeters:
- RCF (× g) = 1.118 × 10⁻⁵ × r(cm) × RPM²
- RPM = √( RCF / (1.118 × 10⁻⁵ × r(cm)) )
It also computes angular velocity ω = 2π·RPM/60, tangential speed v = ω·r, and centripetal acceleration a = ω²·r.
How to use this calculator
- Select a conversion mode: RPM → RCF or RCF → RPM.
- Enter the radius to the sample and pick the correct unit.
- Enter either RPM or RCF (× g), depending on the mode.
- Choose decimal precision, then click Convert.
- Download a CSV or PDF report if needed.
Why RCF and RPM are not interchangeable
RPM is only a speed, while RCF is the effective acceleration on your sample. Two rotors running at 10,000 rpm can differ by thousands of ×g because the sample sits at different radii. Reporting RCF improves reproducibility between instruments and labs. When protocols specify only rpm, results may shift after rotor swaps silently.
Key constant in the conversion
The widely used relationship is RCF = 1.118×10⁻⁵ × r(cm) × RPM². The 1.118×10⁻⁵ factor comes from ω = 2π·RPM/60 and dividing centripetal acceleration by standard gravity. Keep radius in centimeters to avoid unit mistakes. Always verify your radius unit before calculating anything.
Radius measurement that matters
Use the distance from the rotation axis to the center of the liquid column, not the rotor edge. For swing-bucket rotors, the radius changes slightly as buckets swing out. Many manuals list a minimum and maximum radius; the maximum radius gives the highest RCF. If unsure, measure to the tube midline carefully.
How fast RCF grows with RPM
Because RPM is squared, small speed changes are amplified. Increasing RPM by 10% raises RCF by about 21%. Doubling RPM increases RCF fourfold. This is why staying within rotor limits is critical when chasing higher g force. Even 500 rpm can matter greatly.
Solving for RPM from a target RCF
Rearranging gives RPM = √(RCF /(1.118×10⁻⁵ × r(cm))). For example, with r = 8 cm and target 5,000 ×g, the required speed is about 7,477 rpm. Larger radii need less RPM to reach the same RCF. Use the rotor’s radius for planning.
Derived motion values help troubleshooting
The calculator also reports ω (rad/s), tangential speed v (m/s), and centripetal acceleration a (m/s²). These values explain why long-radius rotors feel “stronger” at the same RPM. They also help estimate mechanical stress and vibration sensitivity. High speed imbalances amplify.
Practical reporting and documentation
Record radius, RPM/RCF, temperature, and run time in your protocol. If you share methods, state both RPM and RCF plus the rotor model. Exporting CSV or PDF results makes it easy to attach conversions to batch records and training notes. Use consistent rounding precision.
FAQs
What is g force (RCF) in centrifuges?
RCF is relative centrifugal force, expressed as multiples of gravity (×g). It represents the acceleration acting on the sample at a given radius, which is what drives pelleting and separations.
Which radius should I use for the conversion?
Use the distance from the axis of rotation to the sample’s effective position. For liquid samples, the tube midline is a good estimate. If your manual lists min/max radius, pick the value that matches your method.
Why do manufacturer tables sometimes disagree with my calculation?
Tables may assume a specific radius, bucket angle, or tube height. Some also round constants differently. If you match the same radius in centimeters and the same g0 convention, your computed RPM–RCF pair should align closely.
Can this be used for microcentrifuges and large floor models?
Yes. The equation scales across instruments as long as you enter the correct radius for the rotor and the value you want to convert. Always confirm the rotor’s maximum rated RPM and RCF.
Does temperature change the RPM to RCF conversion?
Not directly. The conversion depends on geometry and rotational speed. Temperature affects viscosity, density, and separation performance, so protocols still specify temperature even though the RPM–RCF math is unchanged.
How accurate is the result compared with my instrument display?
Most displays are accurate enough for routine work, but they may use a stored radius or rounding. Measuring radius carefully and using consistent units usually keeps differences small. For critical assays, follow the rotor’s certified chart.