Protocols state g-force, centrifuges run in rpm. Enter your rotor’s radius and convert either way — speed to relative centrifugal force, or the speed you need for a given × g.
Enter the rotor radius, choose a direction and the result updates as you type.
Use the radius your protocol refers to. The maximum radius (tube bottom) is the usual convention; the average or minimum radius gives a lower force at the same speed.
RCF states the acceleration at a point in the rotor as a multiple of Earth’s gravity: at 10 000 × g a sample feels ten thousand times its weight. It is what actually moves particles through a liquid, which is why protocols specify it rather than a speed.
The acceleration of circular motion is ω²r, with ω the angular speed and r the radius. Converting rpm to radians per second and dividing by g gives RCF = 1.118 × 10⁻⁵ × r[cm] × rpm². At 10 000 rpm with a 10 cm radius that is 1.118 × 10⁻⁵ × 10 × 10⁸ ≈ 11 180 × g. Going the other way, rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r)).
The force grows with the distance from the axis, so one rotor has a range of values. Rotor specifications list the maximum radius (the bottom of the tube), the minimum (the top of the liquid) and the average. Most protocols mean the maximum; if a method is sensitive to the gradient, such as density centrifugation, it will say which one.
The same speed gives a very different force in a wide fixed-angle rotor and a small microcentrifuge rotor. Stating the force makes a protocol reproducible on any centrifuge: you convert it to the rpm your own rotor needs, which is exactly what this calculator does.
RCF = ω² r ÷ g with ω = 2π × rpm ÷ 60, the radius r in meters and standard gravity g = 9.80665 m/s². Collecting the constants gives RCF = 1.118 × 10⁻⁵ × r[cm] × rpm²; the reverse direction takes the square root. A radius entered in millimeters is converted to centimeters first.
Some older online calculators use a constant near 1.19 × 10⁻⁵, which overstates the force by about 7%; this page uses the value that follows from the definition. The result assumes the radius you enter is measured to the point of interest and ignores the time the rotor spends accelerating.
This calculator is for general laboratory reference. Never exceed the rated speed of your rotor or tubes; check the manufacturer’s specifications.
Short answers to the questions people ask most about centrifuge speed and g-force.