Centrifuge Calculator: RCF, RPM, and Rotor Radius
Enter any two of RPM, RCF (g-force), and rotor radius and this calculator solves for the third. Use the rotor preset menu to fill in a standard radius instantly, switch the solve mode to convert in any direction, and check the reference table for typical g-forces used in common lab protocols. The step-by-step panel shows the exact arithmetic so you can reproduce the number in a methods section.
Formula
Worked example
A rotor with a 5.4 cm radius spinning at 10 000 RPM: RCF = 1.118e-5 x 5.4 x 10 000^2 = 6 037 x g. Converting back: RPM = sqrt(6 037 / (1.118e-5 x 5.4)) = 10 000 RPM. The angular velocity is 2 x pi x 10 000 / 60 = 1 047 rad/s and the centrifugal acceleration at the tip is 1 047^2 x 0.054 = 59 200 m/s^2, which is 6 037 times Earth gravity.
What is RCF and why does it matter?
Relative Centrifugal Force (RCF), usually expressed as x g, is the centrifugal force experienced by a sample relative to Earth's standard gravitational acceleration (9.81 m/s^2). It is the scientifically meaningful number for comparing centrifugation protocols across different instruments, because two rotors spinning at the same RPM but with different radii exert very different forces on the sample. Lab protocols almost always specify RCF rather than RPM for this reason, and most journals now require RCF in methods sections. RPM is only unambiguous when the exact rotor and radius are also specified.
How to measure rotor radius
The rotor radius used in the RCF formula is the distance from the centre of the rotor axle to the farthest point of the sample, which is the bottom of the tube when the rotor is spinning. This is sometimes called the rmax. Many manufacturers publish a table of rmax values for each tube position. If no table is available, you can measure it physically: open the centrifuge lid (with the rotor stopped and power off), place a ruler from the centre of the spindle to the outermost edge of the tube holder, then subtract the space above the bottom of the tube. Using the rotor centre or a mid-tube measurement gives a smaller, incorrect radius and will produce an RCF that is lower than the actual force.
Three-way solve: RCF, RPM, and radius
The RCF formula has three unknowns, so knowing any two lets you calculate the third. The most common use is calculating RCF from RPM and radius when following a protocol and converting it to your own rotor. The reverse, finding the RPM that delivers a target RCF on your rotor, is equally important when adapting published methods to a new instrument. Solving for radius given a known RCF and RPM is useful when you want to verify whether a particular rotor is even capable of reaching a target force at a given speed. Use the "Solve for" selector at the top of this calculator to switch between the three modes.
Common mistakes and safety
Using the wrong radius is the most frequent calculation error: always use the rmax (distance to tube bottom), not the average radius or the radius printed on the rotor lid. Exceeding a rotor's rated maximum RPM is dangerous and can cause catastrophic rotor failure, so always check the maximum speed specification before running. High-speed and ultracentrifuge runs above 50 000 x g typically require a rotor that has passed the manufacturer's interval-service inspection. Adapting protocols between angle-head and swing-bucket rotors needs care: the effective radius differs between rotor types, and pellet compaction characteristics change too.
Typical centrifugation conditions by application
| Application | Typical RCF | Time | Notes |
|---|---|---|---|
| Cell debris removal | 200 - 600 | 5 min | Pellets unbroken cells and large debris |
| Whole cells / yeast | 500 - 1000 | 5 min | Low-speed pelleting of intact cells |
| Nuclei | 600 - 1000 | 10 min | Nuclear fraction isolation |
| Mitochondria | 1 000 - 10 000 | 15 min | Crude mitochondrial pellet |
| Cell membranes | 10 000 - 40 000 | 30 min | Large membrane fragments |
| Microsomes / ribosomes | 100 000+ | 60 - 120 min | High-speed ultracentrifugation |
| Plasmid DNA (CsCl) | 150 000 - 200 000 | 12 - 24 h | Equilibrium density gradient |
These are approximate starting points. Optimal conditions depend on your specific rotor, buffer, and sample type.
Frequently asked questions
What is the difference between RCF and RPM?
RPM (revolutions per minute) is the rotational speed of the motor. RCF (relative centrifugal force, in x g) is the actual force experienced by the sample. Two rotors at the same RPM exert different forces if their radii differ. Because protocols are designed around a specific force rather than a speed, RCF is the correct value to specify and match when reproducing or adapting a protocol.
Why does my calculated RCF differ from the centrifuge display?
Most centrifuge control panels display RPM, not RCF. Some modern models show an RCF estimate, but they typically use the manufacturer's average radius for the selected rotor, which may not match your tube position. If you enter the exact rmax for your tube position and the RPM from the display, the result from this calculator will be the most accurate estimate of the force at the sample tip.
How do I convert a protocol from one rotor to another?
Find the RCF in the original protocol (or calculate it using the original rotor's radius and the specified RPM). Then switch this calculator to "Solve for RPM," enter that target RCF and your new rotor's radius, and read off the RPM you need to set on your centrifuge. Always check that the resulting RPM is within the rated maximum for your rotor.
What g-force do I need to pellet bacteria versus mammalian cells?
Mammalian cells pellet readily at 200-600 x g in 5 minutes. Yeast cells typically need 500-1000 x g. Bacteria are smaller and usually require 1000-5000 x g for 5-10 minutes. These are starting points: actual conditions depend on cell density, buffer viscosity, and the required degree of pellet compaction.
What is the 1.118e-5 constant in the RCF formula?
The constant 1.118 x 10^-5 comes from unit conversion in the full angular-velocity derivation. The centrifugal acceleration in SI units is omega^2 x r, where omega = 2 x pi x RPM / 60 rad/s and r is in metres. Dividing by 9.81 m/s^2 (Earth gravity) and collapsing the unit conversions for r in centimetres gives the compact form: RCF = 1.118e-5 x r_cm x RPM^2.
Does an angle-head rotor give a different RCF than a swing-bucket rotor?
The formula applies to both rotor types, but the effective rmax differs. In a fixed-angle rotor, the tube sits at an angle and the rmax is measured to the outermost point of the tube at that angle. In a swing-bucket rotor, the buckets hang perpendicular to the axis during spinning and the rmax is the full radial distance to the tube bottom. Manufacturers list these values separately in their rotor specification tables.