RPM is not the same as g-force
A centrifuge spins samples so that denser particles settle faster. What matters for separation is the acceleration the sample feels, the relative centrifugal force (RCF), expressed as a multiple of Earth’s gravity, × g. The rotor speed in revolutions per minute (RPM) only tells part of the story, because the same speed produces more force in a larger rotor.
That is why published protocols give spin conditions in × g. To reproduce them on your centrifuge, you need to convert to RPM using your rotor’s radius.
The formula
RCF = 1.118 × 10⁻⁵ × r × RPM²
with r the radius in centimeters from the center of the rotor to the point of interest, usually the bottom of the tube. Rearranged:
RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r))
The constant comes from ω²r ÷ g, with ω = 2π × RPM ÷ 60 and g = 9.81 m/s².
Example
A rotor with a 10 cm radius at 3,000 RPM:
- RCF = 1.118 × 10⁻⁵ × 10 × 3,000² = 1,006 × g
- Acceleration: about 9,870 m/s²
- To spin at exactly 1,000 × g on this rotor: 2,991 RPM
On a microcentrifuge with a 7 cm radius, 1,000 × g needs about 3,580 RPM; on a large floor centrifuge with a 20 cm radius, about 2,115 RPM.
Good practice
- Find r max, the radius to the bottom of the tube, in the rotor documentation. Some protocols use r average instead.
- Force grows with the square of the speed: doubling RPM gives four times the force.
- Never exceed the rated speed of the rotor, the buckets or the tubes, and derate as the manufacturer requires for dense solutions.
- Balance tubes by mass, not by volume, opposite each other.
- Note both the × g value and the time, and the temperature if it matters, when you write up a method.