Centrifuge RCF Calculator (rpm ↔ × g)
Convert centrifuge speed to g-force and back. Solve RCF = 1.118 × 10⁻⁵ × r × rpm² for the relative centrifugal force, the speed or the rotor radius, and convert a protocol from one rotor to another so the sample sees the same g.
How to Use
- Pick what to solve for: RCF (× g) from a speed, the Speed (rpm) for a g-force, the Radius, or Convert a protocol between two rotors.
- Enter the rotor radius in cm, mm, m or inches. Use the maximum radius (centre of the rotor to the bottom of the tube) unless the protocol says otherwise.
- Enter the speed in rpm or the g-force the protocol asks for. Optionally add the inner radius (top of the liquid) to see how the force changes along the tube.
- The answer appears in the highlighted field and the first readout; the table under the rotor shows the same spin at other common radii.
- Press a preset to load a ready-made example, and check Show Work for both the exact ω²r/g₀ working and the 1.118 × 10⁻⁵ shortcut.
Worked Example
Speed to g-force. A rotor with a 10 cm radius spins at 3,000 rpm. ω = 2π × 3,000 ÷ 60 = 314.16 rad/s, so the acceleration at the tube bottom is ω²r = 314.16² × 0.1 m = 9,869.6 m/s². Dividing by standard gravity, 9,869.6 ÷ 9.80665 = 1,006 × g. The shortcut gives the same: 1.118 × 10⁻⁵ × 10 × 3,000² = 1,006 × g.
g-force to speed. A protocol says 300 × g and your rotor radius is 15 cm. rpm = √(300 ÷ (1.118 × 10⁻⁵ × 15)) = 1,337 rpm.
The common mistake: copying the rpm instead of the g-force. A method written for 4,000 rpm in a 15 cm rotor gives 2,684 × g. Run at 4,000 rpm in your 9 cm rotor, the sample only feels 1,610 × g, 40% less, and the pellet may not form. The right speed is 4,000 × √(15 ÷ 9) = 5,164 rpm, which gives the same 2,684 × g.
Show Work
Formulas
Why Labs Quote × g, Not rpm
Spinning things to separate them is old: in 1878 the Swedish engineer Gustaf de Laval patented a continuous centrifugal cream separator, which skimmed milk far faster than letting cream rise. The laboratory version became a precision instrument with Theodor Svedberg, who built the first ultracentrifuges in Uppsala in the 1920s to measure the size of protein molecules and received the 1926 Nobel Prize in Chemistry for his work on disperse systems. The sedimentation coefficient is still measured in svedbergs, which is where the 70S and 80S names of ribosomes come from.
In 1958 Matthew Meselson and Franklin Stahl spun DNA in a caesium chloride density gradient and showed that DNA copies itself semi-conservatively, one of the best-known experiments in biology and a centrifuge result.
Because every centrifuge has a different rotor radius, a speed in rpm only means something for one machine. The force on the sample, RCF in multiples of standard gravity (g₀ = 9.80665 m/s², fixed by the 3rd General Conference on Weights and Measures in 1901), means the same everywhere, so methods should quote × g and each lab works out its own rpm.
About This Tool
This calculator converts between centrifuge speed and relative centrifugal force for any rotor radius, finds the radius a target g-force needs, and translates a protocol from one rotor to another. It works from the exact centripetal acceleration ω²r divided by g₀, and shows the familiar 1.118 × 10⁻⁵ shortcut beside it so you can check either. Give the inner radius as well and it shows how the force changes from the top of the liquid to the bottom of the tube; the table under the drawing gives the same spin at other common radii.
Everything runs in your browser; nothing you enter is sent anywhere. Always stay within your rotor’s rated speed.
Related tools: Hemocytometer Calculator, Dilution Calculator, and Molarity Calculator.
Frequently Asked Questions
How do I convert rpm to × g?
Use RCF = 1.118 × 10⁻⁵ × r × rpm², with r in centimetres. At 3,000 rpm and a 10 cm radius: 1.118 × 10⁻⁵ × 10 × 3,000² = 1,006 × g. The constant is just (2π/60)² ÷ (100 × 9.80665), the exact centripetal acceleration ω²r divided by standard gravity.
How do I convert × g to rpm?
Turn the formula round: rpm = √(RCF ÷ (1.118 × 10⁻⁵ × r)). A protocol asking for 300 × g in a rotor with a 15 cm radius needs √(300 ÷ (1.118 × 10⁻⁵ × 15)) = 1,337 rpm; the same 300 × g in a 7 cm microcentrifuge rotor needs 1,958 rpm.
Which rotor radius should I use: minimum, average or maximum?
The force grows with the radius, so it varies along the tube. At 3,000 rpm with the liquid running from 4 cm to 10 cm from the axis, the top of the liquid feels 402.6 × g, the middle (7 cm) 704.5 × g and the bottom 1,006 × g. Protocols and rotor manuals usually quote RCF at the maximum radius, r_max, unless they say r_av.
Why can’t I use the same rpm in a different centrifuge?
Because g-force depends on the radius as well as the speed. 4,000 rpm in a 15 cm rotor is 2,684 × g, but the same 4,000 rpm in a 9 cm rotor is only 1,610 × g. To match the protocol you need 4,000 × √(15 ÷ 9) = 5,164 rpm on the smaller rotor.
If I double the speed, does the g-force double?
No, it goes up four times, because RCF depends on rpm squared. At a 10 cm radius, 1,000 rpm gives 111.8 × g and 2,000 rpm gives 447.3 × g. Doubling the radius at the same speed only doubles the force.
How do I use the Centrifuge RCF Calculator (rpm ↔ × g)?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Pelleting cells
A cell-culture protocol written as 300 × g becomes 1,337 rpm in a benchtop rotor with a 15 cm radius, or 1,958 rpm in a 7 cm rotor.
Microcentrifuge spins
A microcentrifuge at 13,000 rpm with a 7 cm rotor radius reaches 13,229 × g at the bottom of the tube.
Copying a method to your rotor
A method run at 4,000 rpm in a 15 cm rotor (2,684 × g) needs 5,164 rpm in a 9 cm rotor for the same force.
Ultracentrifugation
100,000 × g at an 8 cm radius needs 33,434 rpm, which is why ultracentrifuge rotors are built for tens of thousands of rpm.
Choosing a rotor
To reach 10,000 × g on a machine limited to 12,000 rpm, the tube bottom must sit at least 6.21 cm from the axis.
Last updated: