Lab-Tacular

RCF ↔ RPM Converter

Converts between RCF and RPM using rotor radius.Enter the rotor radius and either RCF or RPM.

Always enter the radius in mm. Enter only one of RCF or RPM.

Result

💡 Fill in the values and press Calculate to see the result here.

How to use

  1. Enter the rotor radius (mm) — the distance from the centre of rotation to the point in the tube you care about.
  2. Enter whichever you know, RCF (× g) or RPM.
  3. The other value is calculated. The conversion works in both directions.

💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.

Formula & notes

RPM is how many turns the rotor makes in a minute. Rotors differ in size from instrument to instrument, so the same RPM does not put the same force on the sample.

RCF, the relative centrifugal force, states that force as a multiple of gravity. It does not depend on the rotor, which is why protocols should be written in RCF.

The two are connected by this.

Relative centrifugal force
RCF = 1.118 × 10⁻⁶ × r(mm) × RPM²
Rotational speed
RPM = √( RCF ÷ (1.118 × 10⁻⁶ × r) )

The constant 1.118 × 10⁻⁶ is what is left after tidying up the angular velocity and the acceleration due to gravity into units of millimetres and minutes.

The square on RPM matters. Doubling the speed quadruples the force.

Practical notes

  • Where you measure the radius to changes the answer. From the axis to the bottom of the tube is the maximum radius (r_max); halfway along is the average (r_av). Check which one the protocol means — for pelleting it is normally r_max.
  • The radius is in the instrument manual. It differs between rotors, and changing the rotor on the same instrument changes it. Use the published figure rather than a ruler.
  • Record protocols in RCF. An RPM alone cannot be reproduced on another instrument. Where a paper's methods give only RPM, the rotor has to be tracked down alongside it.
  • Fixed-angle and swing-out rotors hold the sample in different places, so their effective radii differ.
  • This calculation covers speed only. Acceleration and braking time, temperature and brake settings are separate and affect the separation.

FAQs

References

The radii and rated speeds in the rotor list are transcribed from specification sheets published by seven manufacturers. The 124 entries that print an RCF beside a radius were put through this tool's equation; none disagreed.

  1. Thermo Fisher Scientific. Thermo Scientific LYNX 4000 and LYNX 6000 Superspeed Centrifuge Rotors (Superspeed-Centrifuge-Rotors-RTBA-EN.pdf). Read 2026-08-18.Radii and rated speeds of 18 rotors
  2. Beckman Coulter. Rotor specification tables on the manufacturer's store product pages. Read 2026-08-19.https://store.beckman.com/6 rotors — the only maker that also publishes the average radius (rAV), which is why that column exists
  3. CRYSTE. MULTI 6R rotor specification page. Read 2026-08-19.http://cryste.co.kr/centrifuge_multi_6r_rotor.html7 rotors — angle rotors are published as an In/Out radius pair, which is what gives a force range
  4. Hettich. MIKRO 2.0 / 2.0 R and ROTINA 380 / 380 RC Robotic data sheets (PDF). Read 2026-08-19.https://www.hettichlab.com/downloadcenter/Products/Datasheets/MIKRO_2.0-2.0-R_data-sheet_EN.pdf7 rotors
  5. OHAUS. Frontier 5000 rotor specification pages. Read 2026-08-19.https://us.ohaus.com/en-us/frontierrotors33 rotors — every page prints the radius next to the maximum RCF, so each row was checked against the other
  6. Sartorius. Centrisart A-14, D-16C and G-16C data sheets (PDF). Read 2026-08-19.https://www.sartorius.com/en/products/centrifuges13 rotors — one of the few sheets that prints both Rmax and Rmin
  7. Sigma Laborzentrifugen. 3-30KS / 3-30KHS rotor and accessory brochure (PDF). Read 2026-08-19.https://www.sigma-zentrifugen.de/en/products/high-speed-centrifuges/3-30ks/19 rotors

RCF = 1.118 × 10⁻⁶ × r × RPM² follows from angular velocity and g and is not cited. Until 2026-08-18 this tool used 10⁻⁵, the centimetre constant, and reported values ten times too large; the correction is recorded on the About page.

Often used together

Open →

Cell Counting Calculator

Turns haemocytometer counts into cell concentration and viability.

Open →

Cell Seeding & Split Ratio Calculator

Works out the seeding volume for each vessel, from your current density and viability.

Open →

DNA/RNA Concentration Calculator (A260)

Calculates nucleic acid concentration and purity ratios from A260.