Centrifuge speed: why RCF and RPM aren't interchangeable across rotors
I followed a published protocol calling for a specific RCF, set my centrifuge to the RPM listed in a similar older paper, and got inconsistent pellet results — it turned out their rotor had a different radius, making the
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I followed a published protocol calling for a specific RCF, set my centrifuge to the RPM listed in a similar older paper, and got inconsistent pellet results — it turned out their rotor had a different radius, making the RPM figure meaningless without it.
I still keep a screenshot of this relationship because memory is where the wrong cousin of a unit sneaks in.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
RCF = 1.118 × 10⁻⁵ × r(mm) × RPM² — depends on rotor radius, not RPM alone
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Copying an RPM setting from a published protocol without checking that your rotor has the same radius — RCF depends on both RPM squared and radius, so the same RPM gives different RCF on different rotors.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in RPM (fixed rotor radius, r = 100 mm).
- Confirm the target unit is really RCF (× g), not a similar-looking one.
- Apply the factor: RCF = 1.118 × 10⁻⁵ × r(mm) × RPM² — depends on rotor radius, not RPM alone.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A protocol specifies a target RCF, and you're calculating the RPM needed for your specific rotor's 100 mm radius.
At 100 mm radius: RCF = 1.118e-5 × 100 × 4000² ≈ 17,888 × g, using the squared-RPM relationship, not a linear one.
That's about 17,888 × g at 4,000 RPM on this rotor — swap rotors and the same RPM gives a completely different RCF.
If you want a second check, invert the conversion and see whether you recover the original quantity within normal rounding.
Quick reference table
| RPM (fixed rotor radius, r = 100 mm) | Calculation | RCF (× g) |
|---|---|---|
| 2,000 RPM | 1.118e-5×100×2000² | ~4,472 × g |
| 4,000 RPM | 1.118e-5×100×4000² | ~17,888 × g |
| 8,000 RPM | 1.118e-5×100×8000² | ~71,552 × g |
| 12,000 RPM | 1.118e-5×100×12000² | ~160,992 × g |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
RCF, not RPM, is what actually determines pelleting and separation efficiency in a centrifuge — using an RPM number without accounting for rotor radius can under- or over-spin a sample.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Always calculate RCF using your specific rotor's radius, since RPM alone means nothing without it — the relationship is squared in RPM and linear in radius.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact RPM (fixed rotor radius, r = 100 mm) to RCF (× g) factor here?+
RCF = 1.118 × 10⁻⁵ × r(mm) × RPM² — depends on rotor radius, not RPM alone
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Copying an RPM setting from a published protocol without checking that your rotor has the same radius — RCF depends on both RPM squared and radius, so the same RPM gives different RCF on different rotors.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-26 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.