Centrifuge Speed Calculator - RPM and RCF Conversion

A centrifuge speed calculator is a lab-methods tool that converts between rotor speed in RPM and relative centrifugal force in x g, so you can reproduce a protocol on any rotor once you know its radius and the target x g.

• • Match a published protocol on a new rotor: Translate a paper's x g value into the RPM your specific rotor needs, or compare your benchtop microcentrifuge's RPM to the protocol's stated x g.
• • Set the same spin on a shared lab centrifuge: Reproduce a colleague's pellet on a different rotor by matching RCF rather than blindly copying the RPM they used.
• • Pick a microcentrifuge for a quick spin: Estimate the x g produced by a benchtop microcentrifuge at a given RPM and rotor radius before buying or borrowing one.

Centrifuges are normally sold and operated in RPM, but biology and biochemistry protocols are written in x g because the force on the sample, not the rotor speed, drives sedimentation. The same RPM on a larger rotor produces a larger x g, which is why RPM alone is not portable between machines.

Enter either RPM or RCF plus the rotor radius in centimetres, and the tool reads out the matching value plus centripetal acceleration, angular velocity, tangential speed, and rotation period.

If you want to translate the rotor's x g into the actual newton force on a sample of known mass, the centrifugal force calculator uses the same omega and radius to solve for F directly.

The calculator takes your rotor radius and rotation input, converts RPM to angular velocity in rad/s, computes centripetal acceleration omega^2 r, then expresses the result in x g by dividing by standard gravity.

• RCF: Relative centrifugal force, a unitless multiple of standard gravity (x g).
• RPM: Rotor speed in revolutions per minute.
• r_cm: Rotor radius in centimetres, measured from the rotation axis to the sample.
• r_m: Same radius converted to metres (r_cm / 100) so omega is in SI units.
• omega: Angular velocity in radians per second, equal to 2*pi*RPM/60.
• g: Standard gravity, 9.80665 m/s^2, used to express the centripetal acceleration as a multiple of g.

Because the centripetal acceleration scales linearly with radius at fixed omega, doubling the rotor radius doubles the x g produced at the same RPM. That is why swapping a microcentrifuge for a large benchtop rotor at the same RPM produces a much stronger pellet and why protocols always quote x g rather than RPM.

According to Wikipedia - Centripetal force, the centripetal acceleration of any object in circular motion is a_c = omega^2 * r with omega in rad/s, which is the same omega^2 r term that drives every RCF calculation once you divide by standard gravity.

According to Wikipedia - Standard gravity, the standard gravity used to normalise RCF is g = 9.80665 m/s^2, so the conversion factor from centripetal acceleration in m/s^2 to x g is 1 / 9.80665.

Once you have the centripetal acceleration in m/s^2, the Newton's laws calculator turns it back into a newton force for any sample mass you multiply by.

Four ideas from classical mechanics and lab practice that make RCF more than a single number on a screen.

These four ideas reappear in any rotating-frame problem, from particle accelerators to the spin-stabilisation of satellites. The centrifuge is the everyday laboratory version of that circular-motion physics.

Centripetal acceleration in uniform circular motion also governs satellite orbits, so the orbital period calculator uses the same omega^2 r relation between rotation rate and radius that drives this calculator.

Use the centrifuge speed calculator in five steps.

1. 1 Open Solve For and pick RCF or RPM: Choose RCF if you know the protocol's x g and want the matching RPM, or choose RPM if you are staring at the dial on a working centrifuge.
2. 2 Enter the rotor radius in centimetres: Measure from the rotation axis to the loaded sample. Most rotor manuals list the maximum radius, and a typical benchtop rotor runs 7-10 cm.
3. 3 Type the known rotation value: For Solve For = RCF, type the rotor's RPM. For Solve For = RPM, type the target RCF in x g. The calculator uses the rotor radius plus the known value to recover the missing one.
4. 4 Read the primary output in its natural unit: The headline panel shows RCF in x g or RPM in rev/min depending on which Solve For mode you picked. Supporting rows show centripetal acceleration, angular velocity, tangential speed, and rotation period so you can sanity-check the conversion.
5. 5 Reset before the next sample: Press Reset to restore the default rotor radius and rotation inputs. The defaults (10 cm, 3000 RPM, 1000 x g) are a useful benchtop quick-spin reference.

For a published protocol that calls for 12,000 x g on a 8.5 cm rotor, set Solve For to RPM, rotor radius to 8.5, and RCF to 12000. The calculator returns about 11,237 RPM with a centripetal acceleration near 117,680 m/s^2.

When you need to check how much torque and motor power the spin is drawing from the rotor, the torque, power and speed calculator converts between rotational speed, torque, and mechanical power for the same RPM and radius.

Five practical reasons to use this calculator instead of carrying the conversion factor in your head.

• • Portable protocols across rotors: Convert any published x g value into the RPM your specific rotor needs without re-deriving the formula each time.
• • Bidirectional in one screen: Switch the Solve For menu between RCF and RPM and the same inputs give the matching rotation in the other unit, with no separate conversion table needed.
• • Auditable centripetal acceleration: The centripetal acceleration in m/s^2 is reported alongside RCF so you can verify the omega^2 r step by hand when you are double-checking a service log.
• • Cross-machine pellet matching: When two lab members use different rotors, matching RCF gives the same pellet instead of fighting over whose dial setting is correct.
• • Unit reminders for new techs: The display lists x g, m/s^2, rad/s, m/s, and seconds side-by-side, which helps anyone learning the difference between rotational and linear units.

The calculator is intentionally narrow: it solves the RCF-RPM-radius triangle. Rotor imbalance, sample viscosity, and temperature rise are handled by separate lab procedures, not by this rotation conversion tool.

If you want to translate that centripetal acceleration into the kinetic energy and work done on the spinning sample, the work, energy and power calculator covers the energy side of the rotation conversion.

What changes the RCF the calculator returns, and which assumptions it does not make for you.

• • The calculator assumes rigid-body rotation, so it does not model the radial pressure gradient inside a partially filled tube or the velocity profile of a swinging-bucket rotor swinging into the spin plane.
• • The Solve For menu uses a static RCF-to-RPM relation, so it does not account for ramp-up and brake-down periods, which dominate short spins on a microcentrifuge.
• • The radius input is a single number, so multi-stage spins with different radii in the same rotor run need separate calculations.

These caveats explain why bench scientists quote RCF with time and temperature rather than just RPM: rotor radius, sample position, and rotor age all quietly change the x g the sample actually feels, even when the dial reads the same RPM.

According to Wikipedia - Centrifuge, the relative centrifugal force of a rotor is RCF = omega^2 * r / g with omega in rad/s, which is why RCF scales linearly with rotor radius and quadratically with rotation rate.

When a worn rotor changes the stress the material sees, the fatigue life calculator estimates how many spin cycles the rotor alloy can take before it needs scheduled replacement.