Centrifuge Calculator

Convert centrifuge rotor speed (RPM) to relative centrifugal force (RCF, in x g), or find the RPM needed for a target RCF, using your rotor's radius.

Quick Facts

RCF formula
RCF = 1.118 x 10^-5 x r x RPM^2
r is the rotor radius in centimeters; RCF is in multiples of g.
Angular velocity
ω = 2π x RPM / 60
Converts revolutions per minute to radians per second.
Standard gravity
g = 9.80665 m/s²
The reference value used to express RCF as "x g".

Your Results

Calculated
Relative Centrifugal Force
-
RCF, in multiples of standard gravity (x g)
Rotor Speed
-
RPM (revolutions per minute)
Angular Velocity
-
ω = 2π x RPM / 60, in rad/s
Tangential Velocity
-
Linear speed at the rotor radius, v = ωr

Ready

Enter rotor radius and either RPM or a target RCF, then press Calculate.

Formula and Method for Centrifuge Speed (RPM to RCF)

A centrifuge spins samples around a fixed axis so that inertia pushes material outward, away from the axis, at a force much greater than normal gravity. That force is expressed as relative centrifugal force (RCF), a multiple of standard gravity (x g). RCF depends on two things: how fast the rotor spins (RPM) and how far the sample sits from the axis of rotation (the rotor radius). This calculator converts between rotor speed and RCF for a given radius, and also reports the angular velocity and the tangential (linear) speed at the rotor tip.

How the calculation works

A point moving in a circle of radius r at angular velocity ω experiences centripetal acceleration a = ω²r. Converting rotor speed from RPM to radians per second gives ω = 2π × RPM / 60. Dividing the acceleration by standard gravity (g = 9.80665 m/s²) expresses it as a multiple of g: RCF = ω²r / g. Substituting the RPM-to-ω conversion and using r in centimeters simplifies to the widely used shortcut formula RCF = 1.118 × 10⁻⁵ × r × RPM². To go the other direction — finding the RPM needed for a target RCF — the formula is rearranged to RPM = √(RCF / (1.118 × 10⁻⁵ × r)). The tangential velocity at the sample, v = ωr, shows how fast the outer edge of the sample is physically moving.

Common mistakes

  • Using the wrong radius: manufacturers list rmin (top of the tube), rmax (bottom of the tube), and ravg (the midpoint). Most protocols intend ravg unless they say otherwise — using rmax will overstate the RCF, rmin will understate it.
  • Assuming RPM alone reproduces a protocol: the same RPM setting gives a different RCF on every rotor because radius varies by machine. Always convert to RCF (or supply the exact rotor's radius) when following a published protocol.
  • Mixing radius units: the shortcut constant 1.118 × 10⁻⁵ assumes the radius is in centimeters — convert millimeters or inches to centimeters (or use the ω²r/g form with meters) before applying it.

Real-world applications

  • Molecular biology protocols (DNA/RNA extraction, plasmid prep) almost always specify a target RCF in x g, since that force — not the RPM — determines pelleting behavior.
  • Clinical and blood-bank centrifuges use RCF settings to consistently separate plasma, serum, and cellular components regardless of which rotor is installed.
  • Industrial and food-processing separators use the same RPM-to-RCF relationship to size equipment for a target separation force at a given rotor diameter.

Frequently Asked Questions

What is RCF and how is it different from RPM?
RPM (revolutions per minute) is how fast the rotor spins. RCF (relative centrifugal force), expressed in multiples of g, is the actual force that spinning applies to a sample. RCF depends on both RPM and the rotor's radius, so the same RPM setting produces a different RCF on a large rotor than on a small one.
How do I convert RPM to RCF (g-force)?
Use RCF = 1.118 x 10⁻⁵ x r x RPM², where r is the rotor radius in centimeters. This comes from RCF = ω²r/g, with angular velocity ω = 2π x RPM / 60 and standard gravity g = 9.80665 m/s².
How do I find the RPM needed for a target RCF?
Rearrange the RCF formula and solve for RPM: RPM = √(RCF / (1.118 x 10⁻⁵ x r)), with r in centimeters. This tells you what speed to set on a centrifuge with a given rotor radius to reach a protocol's specified g-force.
Why do lab protocols specify RCF instead of RPM?
RCF is rotor-independent, so a protocol that says "centrifuge at 3,000 x g" gives the same physical force on any machine. An RPM value alone is not enough to reproduce results unless the rotor radius is also specified, since the RPM-to-force conversion changes with radius.

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