Centrifuge speed shows up in a lab protocol two different ways: as revolutions per minute (RPM) or as relative centrifugal force (RCF, usually written as ×g). The two are related but not interchangeable, and using the wrong one — or assuming a setting from one centrifuge will produce the same result on another — is a quiet, common source of failed pelleting, inconsistent yields, and protocols that mysteriously stop working when a sample moves between benches or labs.
RPM vs. RCF: What Each One Actually Measures
RPM (revolutions per minute) describes how fast the centrifuge rotor is spinning — a property of the machine, not of the force experienced by your sample. Two centrifuges set to the identical RPM can spin your sample at very different forces if their rotors are different sizes.
RCF (relative centrifugal force), expressed in multiples of standard gravity (×g), describes the actual force acting on the sample. It accounts for both rotational speed and the radius of the rotor — the distance from the center of rotation out to the sample. RCF is the value that determines whether cells lyse, whether a pellet actually forms, and whether a separation is reproducible. Most peer-reviewed protocols, kit inserts, and published methods specify speed in ×g (RCF) for exactly this reason: it is portable across different centrifuges, while an RPM value is only meaningful for the specific rotor it was measured on.
Why the Two Aren’t Interchangeable
Centrifugal force is proportional to the rotor radius and to the square of the angular velocity. That squared relationship means small differences in RPM translate into large differences in force, and it means a fixed RPM value produces a different RCF on every rotor with a different radius. A benchtop microcentrifuge with a short rotor radius (roughly 6–8 cm) spinning at 13,000 RPM produces meaningfully less force than a large floor-model rotor with a 15+ cm radius spinning at the same 13,000 RPM. Set both to “13,000 RPM” and you have run two different experiments.
This is the single most common way a protocol written for one lab’s centrifuge under-performs (or over-performs, risking cell lysis or sample damage) when followed literally on a different instrument. Converting through RCF, not copying the RPM number, is what keeps a protocol portable.
The Conversion Formula
The standard conversion between RPM and RCF is:
RCF = 1.118 × 10-5 × r × RPM2
- RCF — relative centrifugal force, in multiples of g
- r — rotor radius in centimeters, measured from the center of the rotation axis to the bottom of the tube/sample at maximum radius
- RPM — rotor speed in revolutions per minute
- 1.118 × 10-5 — a fixed unit-conversion constant derived from (2π/60)2 ÷ 980.665 (standard gravity in cm/s2), which converts angular velocity in RPM and a radius in cm into a dimensionless g-force ratio
Rearranged to solve for RPM when you know the target RCF:
RPM = √(RCF ÷ (1.118 × 10-5 × r))
Worked Example: RPM to RCF
A microcentrifuge rotor has a radius of 8.5 cm and is set to 14,000 RPM. What is the RCF?
RCF = 1.118 × 10-5 × 8.5 × (14,000)2
RCF = 1.118 × 10-5 × 8.5 × 196,000,000
RCF ≈ 18,626 × g
So “14,000 RPM” on this specific rotor is roughly 18,600 × g — the number you’d report in a methods section, and the number that stays valid if someone repeats the spin on a different centrifuge set to whatever RPM produces the same RCF on their rotor.
Worked Example: RCF to RPM
A protocol calls for pelleting bacterial cells at 4,000 ×g. Your rotor’s radius is 10.5 cm. What RPM should you set?
RPM = √(4,000 ÷ (1.118 × 10-5 × 10.5))
RPM = √(4,000 ÷ 0.0001174)
RPM = √34,072,000
RPM ≈ 5,837
You’d set the centrifuge to approximately 5,800–5,850 RPM to achieve 4,000 ×g on this rotor.
Finding Your Rotor’s Radius
The radius used in this formula (often called rmax or ravg depending on the manufacturer’s convention) is specific to each rotor, not each centrifuge — the same centrifuge base can often run several interchangeable rotors, each with its own radius. Sources, in order of reliability:
- The rotor’s own manual or the specification sheet/label often printed directly on the rotor
- The centrifuge manufacturer’s published rotor specifications (most manufacturers publish an RPM/RCF nomograph or lookup table specific to each rotor model)
- Many modern digital centrifuges accept a stored rotor radius and will display/toggle between RPM and RCF automatically once the correct rotor is selected in the instrument software — verify the stored radius matches the physical rotor in use, since a wrong stored value silently produces a wrong RCF display
If a rotor’s exact radius isn’t available, manufacturers commonly publish rmax (radius to the bottom of the tube) and rmin (radius to the top of the sample); protocols typically intend rmax or ravg unless stated otherwise — check the source protocol’s methods section for which convention it used before converting.
Why This Matters Beyond Getting the Number Right
Reproducibility. A methods section that reports “13,000 RPM” without a stated rotor radius is not reproducible by a lab using different equipment — it only fully specifies the experiment if the rotor is also specified. Reporting RCF (×g) instead removes that ambiguity, which is why most journals and protocol repositories now expect g-force values in methods sections rather than raw RPM.
Protocol portability. A kit insert or published protocol was validated at a specific RCF on the manufacturer’s reference rotor. Translating that RCF to the correct RPM for your own rotor — rather than copying the RPM number verbatim — is what actually reproduces the intended force on your equipment.
Sample and equipment safety. Because force scales with the square of RPM, a rotor spun beyond its rated maximum RCF (not just its rated maximum RPM) is a real over-speed risk. Rotor speed ratings are ultimately RCF-based; always confirm a target RPM doesn’t exceed the rotor’s rated maximum RCF for its radius, and see Centrifuge Rotor Balancing: Safety Best Practices for the mechanical-safety side of centrifuge operation (load balancing, rotor inspection, and imbalance failure modes) — this guide focuses specifically on the RCF/RPM unit conversion and force calculation, a distinct topic from rotor balancing.
Common Mistakes
- Copying an RPM value between different rotors or centrifuges without converting through RCF first — the single most frequent error, since RPM alone says nothing about force without the radius.
- Confusing rmax and ravg when a manufacturer’s chart specifies which one it used — using the wrong radius convention produces a systematically off RCF.
- Assuming a digital display’s RCF value is correct without confirming the correct rotor is selected in the instrument’s stored settings, especially on shared equipment where the last user may have run a different rotor.
- Reporting RPM instead of RCF in a methods section without also stating the rotor model/radius, which leaves the spin condition unreproducible by another lab.
Frequently Asked Questions
Is RCF the same as g-force?
Yes. RCF is expressed as a multiple of standard gravity and is commonly written as ×g (for example, “spin at 500 ×g”). RCF and g-force are the same measurement, just two common names for it.
Why do published protocols use ×g instead of RPM?
Because RCF is independent of any specific centrifuge or rotor, while RPM only means something once you also know the rotor radius. Reporting ×g lets another lab reproduce the exact force regardless of which centrifuge or rotor they own.
Can I just use an online RPM-to-RCF calculator instead of the formula?
Yes — the formula above is exactly what those calculators implement, and a calculator is a reasonable shortcut once you know your rotor’s radius. Understanding the underlying relationship still matters for catching an obviously wrong result (for example, a value that seems far too high or low), and for cases like protocol troubleshooting where you need to reason about why a spin condition failed.
Does rotor radius change between the top and bottom of the tube?
Yes. The radius (and therefore the RCF) is greater at the bottom of the tube than at the top of the sample, because the bottom is farther from the center of rotation. Manufacturers typically publish rmax (at the tube bottom), rmin (at the sample’s top surface), and sometimes ravg; most published protocols intend rmax or ravg unless they specify otherwise.
What happens if I exceed a rotor’s rated maximum speed?
Rotors are rated for a maximum RCF (and a corresponding maximum RPM for their specific radius). Exceeding it is a genuine mechanical safety risk — rotor stress scales with the square of speed, so even a modest overspeed increases stress substantially. See Centrifuge Rotor Balancing: Safety Best Practices for rotor inspection, load balancing, and safe operating practices.
