What it does
A centrifuge protocol should give the force in multiples of gravity (× g, also called relative centrifugal force or RCF), because the same rpm gives very different forces in different rotors. This tool converts in both directions:
RCF = ω²r / g = 1.118 × 10⁻⁵ × r (cm) × rpm²
It also does two things that are hard to do by hand.
- k-factor and pelleting time. The k-factor sums up how quickly a rotor pellets at a given speed. The time in hours to pellet particles with a sedimentation coefficient S, in svedbergs, is roughly k / S. It applies to particles that start spread through the tube and must travel from the minimum radius to the maximum.
- Moving a protocol to another rotor. If a protocol says “60 min in rotor A”, the same separation in rotor B takes 60 × kB / kA minutes. This is more accurate than matching × g, because it also counts how far the particles must travel.
How to use it
- rpm and × g. Enter the radius and choose its unit. Type a speed to get the force, or type a force to get the speed. Use the maximum radius (to the tube bottom) unless a protocol names another. The facts show the speed that gives the same force on a 7 cm and a 15 cm rotor.
- Rotor k-factor. Enter the minimum and maximum radius of the rotor (the rotor manual lists both), the speed, and a sedimentation coefficient, or pick a particle from the list. You get the k-factor, the force at the minimum, average and maximum radius, the time to pellet, and a chart of pelleting time against speed.
- Same run in another rotor. Enter the protocol time and its rotor’s k-factor, then your rotor’s radii and speed. The k-factor from the card above is filled in until you type your own.
Always stay within the rotor’s and the tubes’ rated speed and force, and balance the rotor.
Worked example
A microcentrifuge with a 9.5 cm radius at 13,000 rpm gives 17,953 × g. The same force needs 15,145 rpm on a 7 cm rotor and 10,346 rpm on a 15 cm rotor.
An ultracentrifuge rotor with radii from 4.0 to 8.5 cm, at 40,000 rpm, has a k-factor of 119.3. The force is 71,568 × g at the minimum radius and 152,081 × g at the maximum. Bacterial 70S ribosomes pellet in about 102 minutes.
A protocol written for that rotor says 60 min. A rotor with radii from 3.5 to 7.5 cm, at 45,000 rpm, has k = 95.3, so the same separation takes about 48 min.
Questions
Why does the k-factor time differ from what my protocol says? k / S assumes water at 20 °C and particles that start evenly spread through the tube. Dense sucrose or glycerol, cold runs, and part-filled tubes all change the real time. Use it to compare rotors, not as an exact time.
Which radius should I use? For pelleting, use the maximum radius, since that is where the pellet forms. Some protocols quote the average radius instead. The rotor card shows all three.
Is anything sent anywhere? No. Everything is worked out in your browser, and the page works offline once loaded.
Where the numbers come from
- RCF and k-factor: standard physics of circular motion and sedimentation (RCF = ω²r/g; k = ln(rmax/rmin)·10¹³ / (3600·ω²)).
- Standard gravity 9.80665 m/s²: an exact defined value (CGPM).
- Sedimentation coefficients: rough, rounded orientation values; ribosome names are their S values by definition.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.