Centrifuge Calculator

The centrifuge calculator lets you convert between relative centrifugal force (RCF) and revolutions per minute (RPM) of a motor in a centrifuge.

Speed required4,000 rpmas entered

Relative centrifugal force1,789.19 × gω²r ÷ g at 4,000 rpm and 100 mm

Acceleration at the rotor tip17,545.96 m/s²

Angular velocity418.879 rad/s

Tip speed41.8879 m/s

By the 1.118 × 10⁻⁶ shortcut1,788.8 × gthe standard rule of thumb. Its constant is (2π/60)²/1000g, whose exact value is 1.118243 × 10⁻⁶ — so the shortcut runs about 0.02% low, which never matters in practice

Spin duration10 minutes

RCF × time17,891.9 g·minthis product, not the rpm, is what determines separation — it is how a protocol transfers between rotors of different radius

At twice the speed7,156.76 × gfour times the force — RCF goes as the square of rpm

At twice the radius3,578.38 × gdouble, at the same rpm — which is why rpm alone never specifies a protocol

The formula

RCF = ω²r ÷ g, with ω = 2π × rpm ÷ 60

Never write a protocol in rpm

What separates a sample is the acceleration it experiences, and that depends on the rotor radius as well as the speed. Two centrifuges spun at the same rpm with different rotors deliver different forces — double the radius and you double the force at identical rpm. A method specified only in rpm is therefore not reproducible on other equipment, which is why relative centrifugal force, quoted in multiples of g, is the correct unit.

The widely used shortcut RCF = 1.118 × 10⁻⁶ × r(mm) × rpm² is the same relation with the unit conversions folded into the constant, which is (2π/60)² divided by 1000g. That constant is really 1.118243 × 10⁻⁶, so the rounded version runs about 0.02% low — irrelevant in any real protocol, but it is why the two figures above differ in the fourth digit rather than matching exactly.

Radius means the distance to the part of the tube you care about, and that is usually the maximum radius at the bottom of the tube rather than the middle. Different sources quote r_min, r_av and r_max, so a protocol should say which — the difference across a long tube can be substantial.