Centrifuge RCF and RPM converter
Convert between relative centrifugal field and rotor speed, for a stated radius.
Nominal figures for the class, not a specification. Rotors within a class vary by roughly a tenth, and so does the field they produce — take the radii from your rotor’s manual where accuracy matters.
Axis to the bottom of the tube.
Axis to the top of the liquid. Optional.
Published protocols almost always mean the bottom of the tube, which is what rotor specifications quote.
Set the centrifuge to
Across the tube
Liquid top
5647× g
Average
8824× g
Tube bottom
12000× g
11236 rpm · 2.12× from top to bottom · k = 1510
The bottom of the tube sees 2.1× the force at the top of the liquid. A protocol quoting a single number almost always means the maximum, which is what a rotor's specification sheet gives.
Transfer a spin from another rotor
Matching × g between rotors does not reproduce a separation, because a longer tube gives a particle further to travel. The clearing factor accounts for that: run time scales with k.
Equivalent time on this rotor, at 11236 rpm
13.6min
k 2220 → 1510
The clearing factor describes an ideal particle sedimenting through water at 20 °C, all the way from the top of the liquid to the bottom of the tube. Viscous media, partly filled tubes and anything that pellets by aggregating rather than sedimenting will all depart from it, so treat the result as a starting point to check rather than a substitution.
RCF = ω²r ÷ g, ω = 2πN ÷ 60; k = ln(r_max/r_min) × 10¹³ ÷ (3600 ω²)- Sedimentation in a centrifugal field, and the clearing factor derived from it — Journal of the American Chemical Society, 1924
- Standard gravity, 9.80665 m/s², as fixed by the CGPM — 3rd General Conference on Weights and Measures, Resolution 2 (BIPM), 1901
When to use this
Use this when a protocol specifies a force in × g and your centrifuge is set in rpm, or the reverse. The conversion depends entirely on your rotor’s radius, so the same rpm is a different force on every machine.
Worked example
A protocol says 12,000 × g and your microcentrifuge rotor has an 8.5 cm maximum radius.
- Solve for
- Speed
- Target field
- 12,000 × g
- Maximum radius
- 8.5 cm
Result
That is the field at the tube bottom. Nearer the top of the tube the sample sees appreciably less.
What people get wrong
- Using the centrifuge’s radius rather than the rotor’s. The number belongs to the rotor, and swapping rotors changes it.
- Matching rpm when moving a protocol between machines. Match the force instead — matching rpm on a larger rotor turns a gentle spin into a hard one.
- Treating × g as a single number. A fixed-angle rotor can deliver more than twice the field at the tube bottom as at the liquid surface.
Questions
+Which radius should I enter?
The maximum, unless the protocol says otherwise. Manufacturers quote maximum RCF and protocols almost always mean it — but the tool shows the minimum and average too, so you can see the spread.
+Where do I find my rotor’s radius?
In the rotor manual or stamped on the rotor itself. The tool offers nominal values by rotor class if you cannot find it, but they are starting points, not specifications.
+Does time convert too?
Not by this tool. Keep the time and match the force; for a rigorous transfer between rotors of different geometry you need the k-factor, which is not implemented here.
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Science last reviewed .