Centrifuge RCF (× g) to RPM Rotor Speed Converter Tool
Convert between relative centrifugal force in multiples of standard gravity and rotational speed in revolutions per minute using an explicit rotor radius and radius convention. The interactive workspace loads a checked 7 cm rotor example so you can inspect how radius convention and speed squaring govern pelleting force.
Loads the independently checked preparation fixture case converting 1,956.5 × g at a 7 cm maximum radius to 5,000 RPM using the standard published coefficient 1.118 × 10⁻⁵.
Check rotor radius units and radial position conventions
A centrifuge protocol that specifies 300 × g for mammalian cell pelleting or 12,000 × g for plasmid clearing is only as accurate as the radius used to convert that force into RPM on instruments lacking automatic g-force control. Manufacturer rotor manuals typically list three radii in millimetres or centimetres: rmin at the top of the liquid column, ravg at the middle of the tube and rmax at the bottom tip of the cavity.
For routine pelleting at the bottom of a conical tube, rmax is the standard reference position, whereas density-gradient separations often reference ravg. Confirm whether your rotor table lists millimetres (such as 70 mm) or centimetres (7.0 cm) before typing the value, and record the chosen convention with the run sheet.
Respect mechanical speed ratings and biological shear limits
Mammalian cells, fragile organelles and lentiviral particles can be damaged by excessive g-force, while inadequate g-force leaves cells in the supernatant and biases downstream seeding counts. Conversely, high-speed fixed-angle runs must never exceed the maximum RPM stamped on the rotor body, the tube rating or the high-density fluid derating limit specified by the manufacturer.
Equations and variables
Relative centrifugal force from RPM and radius
RCF = 1.118 × 10⁻⁵ × rcm × RPM²
- RCF
- Relative centrifugal force expressed as a dimensionless multiple of standard gravity (× g)
- rcm
- Radial distance from the rotor axis to the stated tube position (cm)
- RPM
- Rotational speed in revolutions per minute (min⁻¹)
Inverse rotor speed from target RCF
RPM = √[RCF / (1.118 × 10⁻⁵ × rcm)]
- RPM
- Required rotor speed to achieve the specified relative centrifugal force (min⁻¹)
- RCF
- Target centrifugal force specified by the protocol (× g)
- rcm
- Rotor radius on the declared rmin, ravg or rmax basis (cm)
Scope and limitations
- The standard laboratory coefficient 1.118 × 10⁻⁵ is a rounded representation of (2π / 60)² / 980.665 cm s⁻², which differs from the exact physical constant by about 0.022 percent.
- Entering a rotor radius in millimetres while leaving the unit selector on centimetres multiplies the calculated RCF by ten or under-estimates required RPM by √10.
- The conversion checks only an optional user-supplied maximum RPM limit; it does not certify rotor mechanical rating, bucket balance, tube chemical compatibility or derating for dense caesium chloride solutions.
Sources and provenance
- Calculator inputs and independent arithmetic verificationOriginal synthetic inputs; see individual fixture notices
- Independent centrifuge RCF/RPM verification casesPython fractions and decimal exact reference; coefficient check documented
Technical questions
Why should protocols state centrifugation conditions in × g rather than RPM?
Two centrifuges running at 5,000 RPM exert very different sedimentation forces if one uses a microcentrifuge rotor with a 6 cm radius and the other uses a swinging-bucket benchtop rotor with an 18 cm radius. Quoting RCF in multiples of gravity allows any laboratory to reproduce the same acceleration using their own rotor geometry.
What is the difference between rmin, ravg and rmax on a rotor diagram?
Maximum radius (rmax) measures from the centre spindle to the bottom tip of the tube where the pellet collects; minimum radius (rmin) measures to the top meniscus of the liquid column; average radius (ravg) is their midpoint. Fixed-angle and swinging-bucket rotors have different radial spans, so changing the convention changes the calculated speed.
Why does doubling the RPM quadruple the relative centrifugal force?
Centripetal acceleration equals angular velocity squared multiplied by radius. Because RPM is squared in the governing equation, a two-fold increase in rotational speed produces a four-fold increase in RCF and mechanical stress on the rotor and tubes.
Connected workflows
- Viable-cell seeding volumes
Use accurate pelleting conditions before resuspending and counting viable cells.
- GenBank and FASTA plasmid mapping
Review downstream plasmid construct inspection after alkaline-lysis clearing spins.
- Protein A280 concentration calculation
Clarify protein preparations by centrifugation to remove light-scattering aggregates before UV reading.