Application Note - A Primer on RPM vs. RCF

• RPM - Stands for revolutions per minute -- the speed you set your centrifuge to spin your rotor.
• RCF - Stands for relative centrifugal force, also referred to as g-force. The applied force resulting from the spinning action that is perpendicular to the axis of rotation. Force is relative to the earth's gravitational force and has no units, but is simply identified as xg (times gravity) or RCF.
• g - Stands for gravity. A universal constant that represents the natural pull or force of objects towards the earth.

Is there really a difference between RPM and RCF? YES! Since clean sample separation is dependent on the g-force applied, it is important to run at the proper speed that results in your desired g-force. The speed at which you spin your rotor will affect the amount of force applied to your sample. However, the relationship follows this equation

RCF = 0.00001118 x radius of rotation (cm) x RPM²

This means that RCF increases exponentially with an increase in RPM. This relationship is shown pictorially below.

It assumes that two different rotors are spinning at the same speed of 2500 rpm, however rotor 'A' has a rotating radius of 7.15 cm and rotor 'B' has a rotating radius of 25.75 cm. The resulting g-force at the furthest most point of the tube from the axis of rotation is 500xg and 1800xg respectively. Thus selecting a rotor with a larger radius of rotation will provide higher g-forces at a given speed.

Alternately, assume a rotor with a radius of 22.36 cm, running at 4000 rpm results in a RCF of 4000 xg. Changing the speed to 4500 rpm results in a g-force of 5060 xg.

To explore this relationship more, experiment with our G-Force Calculator.

G-Force Calculator Explained