What this calculator finds: Relative Centrifugal Force (RCF, g-force)

RCF expresses the centrifugal acceleration on the sample as a multiple of gravitational acceleration (g). For example, “1000 × g” means the sample experiences 1000 times the force of gravity. Since a larger rotor radius gives a higher RCF at the same RPM, protocols must be matched between machines using RCF rather than RPM.

Why it matters

• Converting a protocol’s specified g-force into the correct RPM for your rotor
• Reproducing identical separation conditions across machines with different rotors
• Avoiding excessive force that could damage delicate samples such as cells

Relative Centrifugal Force (RCF)

The constant 1.118 × 10⁻⁵ combines the unit conversions so that, with radius in cm and speed in RPM, the result comes out as a multiple of gravity (g).

How the formula works

• RCF is proportional to the square of speed N — doubling the RPM quadruples the force.
• RCF is directly proportional to radius r — a larger rotor produces more force at the same RPM.
• The radius is normally taken as the maximum radius (r_max) from the rotor center to the bottom of the tube.

Worked example

Spinning a rotor of radius r = 10 cm at N = 5,000 RPM gives RCF = 1.118 × 10⁻⁵ × 10 × 5000² = 1.118 × 10⁻⁵ × 10 × 25,000,000 ≈ 2,795 × g — about 2,800 times gravity on the sample.

Practical tips

• When a protocol gives g, rearrange for speed: RPM = √(RCF / (1.118×10⁻⁵ × r)).
• The radius depends on the specific rotor and tube position — check the manufacturer’s r_max/r_avg values.
• Always balance tubes symmetrically to prevent vibration and rotor damage.

Limitations & cautions

• RCF describes only the force on the sample; actual separation also depends on density, viscosity, and time.
• The real radius (and thus RCF) varies along the tube, so a single value is only an approximation.
• Every rotor and tube has a maximum rated RCF — never exceed it.