RPM to RCF: Conversion Formula and Table for PRP Centrifuges
RPM to RCF: Conversion Formula and Table for PRP Centrifuges
Last updated: 2026-09-20 · VivaPRP technical reference
Short answer: RCF (× g) = 1.118 × 10⁻⁵ × r(cm) × rpm². Because the rotor radius r is part of the formula, rpm on its own never tells you the force a sample actually experiences. At 1,500 rpm, a rotor of 8 cm radius delivers about 201 × g, while a rotor of 16 cm delivers about 402 × g — double the force at the same displayed speed.
This page gives the RCF formula, a pre-calculated rpm to g conversion table covering the speed range used in PRP and PRF protocols, and the reverse calculation for protocol sheets that specify g rather than rpm.
What is the difference between RPM and RCF?
RPM (revolutions per minute) is how fast the rotor turns. RCF (relative centrifugal force), usually written as × g, is the force applied to the sample as a result. RPM is a property of the machine setting; RCF is a property of the machine setting and the rotor geometry — specifically the distance from the centre of rotation to the point where the sample sits.
This distinction matters clinically because separation depends on force, not on speed. Two centrifuges set to the same rpm, with rotors of different radii, will not produce the same plasma layer. Protocol values quoted in rpm are therefore only reproducible on the machine they were written for; values quoted in × g travel between machines.
What is the RPM to RCF conversion formula?
The standard conversion used across laboratory practice is:
RCF = 1.118 × 10⁻⁵ × r × rpm²
Where:
- RCF — relative centrifugal force, expressed as a multiple of gravity (× g).
- r — the radius in centimetres, measured from the centre of rotation to the sample. This is the quantity most often got wrong: the number you need is the radius to the sample position, not the rotor's outer dimension.
- rpm — revolutions per minute as displayed on the machine.
- 1.118 × 10⁻⁵ — the constant that absorbs the unit conversions and the 4π²/3600 factor.
The same RCF formula is published by reagent and equipment suppliers, for example Sigma-Aldrich's G-Force Calculator, which states RCF = (RPM)² × 1.118 × 10⁻⁵ × r.
Centrifuge RPM to RCF conversion table
The table below covers 1,000–4,000 rpm at six rotor radii, from a compact 8 cm rotor to a large 16 cm swing-out. Read down a single radius column to see how force scales with speed, or read across a single speed row to see how much the radius alone changes the answer.
| Speed (rpm) | r = 8 cm | r = 10 cm | r = 12 cm | r = 12.3 cm | r = 14 cm | r = 16 cm |
|---|---|---|---|---|---|---|
| 1,000 rpm | 89 × g | 112 × g | 134 × g | 138 × g | 157 × g | 179 × g |
| 1,500 rpm | 201 × g | 252 × g | 302 × g | 309 × g | 352 × g | 402 × g |
| 2,000 rpm | 358 × g | 447 × g | 537 × g | 550 × g | 626 × g | 716 × g |
| 2,500 rpm | 559 × g | 699 × g | 838 × g | 859 × g | 978 × g | 1,118 × g |
| 3,000 rpm | 805 × g | 1,006 × g | 1,207 × g | 1,238 × g | 1,409 × g | 1,610 × g |
| 3,500 rpm | 1,096 × g | 1,370 × g | 1,643 × g | 1,685 × g | 1,917 × g | 2,191 × g |
| 4,000 rpm | 1,431 × g | 1,789 × g | 2,147 × g | 2,200 × g | 2,504 × g | 2,862 × g |
Values are rounded to the nearest whole × g. The r = 12.3 cm column is the implied effective radius of our TD4C desktop centrifuge, derived by inserting its published maximum of 4,000 rpm and 2,200 × g into the formula and solving for r.
How do I convert RCF back to RPM?
Rearrange the same formula and take the square root:
RPM = √ ( RCF ÷ (1.118 × 10⁻⁵ × r) )
Worked against the same 12.3 cm radius, the target forces commonly quoted in protocol sheets convert as follows:
| Target RCF | r = 12.3 cm | r = 12 cm |
|---|---|---|
| 300 × g | ≈ 1,477 rpm | ≈ 1,495 rpm |
| 500 × g | ≈ 1,907 rpm | ≈ 1,931 rpm |
| 1,000 × g | ≈ 2,697 rpm | ≈ 2,730 rpm |
| 2,200 × g | 4,000 rpm | ≈ 4,049 rpm |
Why do two centrifuges at the same RPM give different g?
Because the radius term sits in the formula, and it is fixed by the rotor rather than by the speed setting. A fixed-angle rotor that holds tubes close to the spindle and a swing-out rotor that presents them further out are physically different machines with respect to force, even when both are set to 1,500 rpm.
The gap is not marginal. Reading across the 1,500 rpm row of the table above, an 8 cm rotor delivers 201 × g while a 16 cm rotor delivers 402 × g — the same displayed speed, twice the force. If a protocol says "1,500 rpm for 8 minutes" with no radius given, that instruction is under-specified, and copying it onto a different machine is the most common reason a PRP preparation separates differently than expected.
How do I find the rotor radius?
Take the value from the centrifuge's own documentation, not from the tube or the rotor's outside diameter. Three places to look, in order:
- The rotor specification sheet or manual — it should state the working radius or the maximum RCF at maximum speed. Where it gives both a maximum rpm and a maximum × g, you can derive the radius from the two.
- The machine's RCF display — if the centrifuge can be programmed in × g directly, the internal radius is already applied for you, and you can set the protocol in g and skip the conversion entirely.
- Measurement, as a last resort — distance from the centre of the spindle to the middle of the sample column when the rotor is in its running position. Note that for swing-out rotors the tube swings level during operation, so the running geometry is not what you see when the machine is stopped.
A 12.3 cm figure derived from a published rpm and g pair is a useful cross-check on any of these, but it belongs in your notes as an implied value rather than as the manufacturer's declared radius.
What RCF do PRP and PRF protocols use?
PRP and PRF protocols are low-speed by laboratory standards, which is exactly why the rpm-to-g conversion matters more here than in a high-speed application: at low speeds, small radius differences translate into proportionally large force differences. A protocol stated as 1,500 rpm corresponds to roughly 309 × g at a 12.3 cm effective radius — the low-force end of the range, appropriate for separating plasma from the red cell layer without disturbing it.
Our PRP centrifuge range is built for this band, including the TD4C desktop low-speed centrifuge whose specifications are used for the worked example above, and the 10–15 ml capacity model for higher-throughput clinics. For protocol selection — single spin versus double spin, and at what speed each is run — see the comparison in our single spin vs double spin guide.
Where a device is being purchased for a regulated setting, note that declared performance figures belong in the technical documentation drawn up for the CE mark. Our CE marking guide explains which documents to request and how to check them.
Frequently asked questions
What is 1,500 rpm in g?
It depends on the rotor radius: 201 × g at 8 cm, 309 × g at 12.3 cm, and 402 × g at 16 cm. Without a radius, "1,500 rpm" does not correspond to a single g value.
Is RCF the same as g-force?
Yes in practice — RCF is expressed in multiples of gravity, which is why it is written as × g. The term "g-force" in centrifuge documentation means relative centrifugal force, not the gravitational constant alone.
Why do protocol sheets quote rpm instead of RCF?
Usually because the protocol was written on one specific machine and the author recorded the setting they used. That makes the protocol reproducible on that machine and under-specified on any other.
Can I use an online rpm to RCF calculator?
Yes, provided you supply the correct radius to the sample position. A calculator is only a faster route to the same formula, and it will reproduce the same error if the radius is wrong.
VivaPRP manufactures CE-marked PRP and PRF tubes in 8–15 ml, plus low-speed desktop centrifuges sized for 10–15 ml tubes. For specifications, compatibility advice or wholesale and OEM enquiries, contact our team.