🌀 Laboratory
G-Force to RPM Calculator
Convert centrifuge speed in RPM to relative centrifugal force, or the RCF you need into RPM. Enter the rotor radius, then press Calculate.
About the G-Force to RPM Calculator
Relative centrifugal force (RCF, or ×g) depends on both rotor speed and radius. The same 10,000 RPM produces twice the force at twice the radius. That is why two centrifuges set to the same RPM can separate a sample very differently.
The standard formula is RCF = 1.118 × 10⁻⁵ × r × RPM², with r in centimetres. To go the other way, RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)). Measure the radius from the centre of the rotor to the bottom of the tube, not to the top of the bucket.
How the calculation works
- RCF (×g) = 1.118 × 10⁻⁵ × radius (cm) × RPM².
- RPM = √(RCF ÷ (1.118 × 10⁻⁵ × radius)).
- Doubling RPM quadruples RCF.
- Doubling radius doubles RCF at the same RPM.
Reference values
| RPM | 10 cm radius | 15 cm radius | 20 cm radius |
|---|---|---|---|
| 1,000 | 112 ×g | 168 ×g | 224 ×g |
| 3,000 | 1,006 ×g | 1,509 ×g | 2,012 ×g |
| 5,000 | 2,795 ×g | 4,193 ×g | 5,590 ×g |
| 10,000 | 11,180 ×g | 16,770 ×g | 22,360 ×g |
| 15,000 | 25,155 ×g | 37,733 ×g | 50,310 ×g |
Worked example: 15 cm rotor at two speeds
A 15 cm radius rotor at 4,000 RPM gives RCF = 1.118 × 10⁻⁵ × 15 × 16,000,000 = 2,683 ×g. At 8,000 RPM the same rotor gives 10,733 ×g—four times as much, because RCF scales with the square of the speed. To reach 10,000 ×g at 15 cm, the required speed is √(10,000 ÷ (1.118 × 10⁻⁵ × 15)) = 7,722 RPM.
Measure the radius to the bottom of the tube at full swing-out, not to the top of the bucket. For a fixed-angle rotor, the manufacturer's nomogram is more reliable than a single radius, because the tube sits at an angle and the effective radius depends on the tube position.
Balancing, rating and safety
A centrifuge rotor is rated for a maximum speed and a maximum density, and the two limits interact. Tubes must be loaded symmetrically and matched by weight, because an unbalanced rotor at high speed vibrates violently and can damage the drive or release the lid. Most modern machines detect imbalance and abort, but older ones may not.
Never exceed the rotor's rated speed, and derate it for dense solutions: a rotor approved for 1.2 g/mL may need a lower speed when filled with a denser gradient. Check the rotor log for age and previous runs. The g-force calculation tells you what the sample experiences; it does not tell you whether the rotor is safe to spin.