How this calculation works
A centrifuge protocol specified in revolutions per minute does not imply the same force in every rotor. Distance from the rotation axis matters, and different positions within one sample can also experience different relative centrifugal forces. This page calculates the force range between entered minimum and maximum sample radii. It additionally inverts the maximum-radius relation for an entered target RCF, keeping the radius convention explicit.
RCF = 1.118×10⁻⁵ r_cm RPM²; target RPM = √(target RCF/(1.118×10⁻⁵ r_max))A worked example
At one thousand rpm with radii five and ten centimetres, the calculated range is 55.9 to 111.8 times gravity. A target of 111.8 times gravity at the ten-centimetre position returns one thousand rpm. If the maximum radius changes to twenty centimetres, the same target requires roughly 707.1 rpm. That is an arithmetic equivalence at one location, not proof that two separations will behave identically.
Good to know
The model does not determine sedimentation time, pellet recovery or a complete protocol transfer. Particle properties, path length, rotor geometry and fluid behavior are not entered. Never treat the accepted numeric range as a rotor or tube speed rating. Equipment limits and approved operating procedures must come from the applicable manufacturer documentation, not from this force conversion.
Reference: Eppendorf — rotor RCF and RPM converter (opens in a new tab) ↗
What this calculation answers
A centrifuge protocol specified in revolutions per minute does not imply the same force in every rotor. Distance from the rotation axis matters, and different positions within one sample can also experience different relative centrifugal forces.
This page calculates the force range between entered minimum and maximum sample radii. It additionally inverts the maximum-radius relation for an entered target RCF, keeping the radius convention explicit.
The Centripetal force and circular motion calculator uses force and circular-motion quantities. Rotor RCF instead expresses acceleration relative to standard gravity at specified sample radii.
Use the right input conventions
Enter radii in centimetres measured from the axis to the relevant sample positions. Tube length or diameter is not a substitute for rotor radius. Maximum radius must be at least the entered minimum radius.
Speed is revolutions per minute and RCF is a multiple of standard gravity. The conversion factor assumes these stated units; entering millimetres without converting changes the result by a factor of ten.
Interpret the result
The highest-radius sample position has the largest RCF at a fixed speed. The target-speed result is based specifically on that maximum radius and is not an average force across the tube.
RCF increases with the square of rotational speed. Doubling speed quadruples calculated force, whereas doubling radius at fixed speed only doubles it. This distinction explains why rotor substitutions need careful unit and radius matching.
Work through the example
At one thousand rpm with radii five and ten centimetres, the calculated range is 55.9 to 111.8 times gravity. A target of 111.8 times gravity at the ten-centimetre position returns one thousand rpm.
If the maximum radius changes to twenty centimetres, the same target requires roughly 707.1 rpm. That is an arithmetic equivalence at one location, not proof that two separations will behave identically.
Where the model stops
The model does not determine sedimentation time, pellet recovery or a complete protocol transfer. Particle properties, path length, rotor geometry and fluid behavior are not entered.
Never treat the accepted numeric range as a rotor or tube speed rating. Equipment limits and approved operating procedures must come from the applicable manufacturer documentation, not from this force conversion.
A few questions, answered
Is the target force averaged over the tube?
No. The target RPM uses the entered maximum sample radius; the separate minimum-radius result shows the lower endpoint.
What changes in the comparison?
Doubling maximum radius doubles force at fixed RPM and reduces the RPM required for the same target by the square root of two.