Centrifugation speed: rpm to g conversion

Convert centrifuge speed between RPM (revolutions per minute) and RCF (relative centrifugal force, × g) in either direction. Enter your rotor radius and type a value in the RPM or the RCF field — the other updates instantly.

Why RPM and g are not the same

Many benchtop microcentrifuges only display speed in RPM, but protocols specify spins in relative centrifugal force (RCF), measured in units of gravity (× g), because RCF is what actually determines how your sample pellets. Two centrifuges set to the same RPM can apply very different forces if their rotors have different radii. Reporting and reproducing a protocol correctly means converting to g.

The RPM ↔ g formula

The relationship between speed and force depends on the rotor radius:

RCF = 1.118 × 10−6 × r × RPM²

where RCF is in × g and r is the rotor radius in millimetres. Rearranged to go from force back to speed:

RPM = √( RCF / (1.118 × 10−6 × r) )

Because RCF scales with the square of the speed, small changes in RPM produce large changes in force.

How to find your rotor radius

The radius that matters is the distance, in millimetres, from the centre of the rotor axis to the point of interest in the tube — usually the bottom of the tube during the spin (the maximum radius, r_max). You’ll find it in your rotor’s manual or printed on the rotor itself. If a protocol reports r_max, r_min and r_avg, use the same one the protocol used. A typical fixed-angle microcentrifuge rotor has a radius of roughly 80–100 mm.

Worked example

A microcentrifuge with an 85 mm rotor spinning at 13,000 RPM generates:

RCF = 1.118 × 10−6 × 85 × 13000² ≈ 16,060 × g

So “spin at 13,000 RPM” on that machine is about 16,000 × g. On a centrifuge with a larger rotor, the same RPM would give a higher force — which is exactly why protocols should state g, not RPM.

Tips

• Always report g (with the radius if possible) so others can reproduce your spin on any machine.
• Use r_max for pelleting unless a protocol says otherwise.
• Double-check high-speed spins — because force grows with the square of RPM, a small speed change can exceed a tube’s rated maximum.

Frequently asked questions

How do I convert RPM to g?

Multiply 1.118 × 10⁻⁶ by the rotor radius in mm and by the square of the RPM. Enter your radius and RPM above and the g value is calculated for you.

How do I convert g to RPM?

Divide the RCF by (1.118 × 10⁻⁶ × radius in mm) and take the square root. Type the RCF in the tool and it returns the required RPM.

Why do I need the rotor radius?

RCF depends on how far the sample sits from the axis of rotation. Without the radius, an RPM value cannot be converted to a force, and the same RPM gives different g values on different rotors.

Should I use radius in mm or cm?

This tool uses millimetres. If your rotor radius is in centimetres, multiply by 10 before entering it.

What radius should I use if my protocol gives a range?

Use the same radius the protocol used (often r_max, the bottom of the tube). For pelleting, r_max is the usual choice.