g-force ↔ RPM converter
Relative centrifugal force depends on rotor speed and radius: RCF (×g) = 1.118 × 10⁻⁵ × r(cm) × RPM². At a 10 cm radius, 4,000 RPM is about 1,790 ×g. Protocols specify ×g because it is rotor-independent — the same RPM produces different forces in different rotors.
Result
RCF = 1.118×10⁻⁵ × r(cm) × RPM². Protocols specify ×g because it is rotor-independent — always convert through your rotor's maximum radius (printed on the rotor or its datasheet).
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Formula
RCF = 1.118 × 10⁻⁵ × r (cm) × RPM² · RPM = √(RCF / (1.118 × 10⁻⁵ × r))
Source: Standard centrifugation nomogram relation (Sigma-Aldrich / Eppendorf rotor documentation)
Worked example
Given: Microcentrifuge rotor, 10 cm maximum radius, spun at 4,000 RPM
- 1.RCF = 1.118 × 10⁻⁵ × 10 × 4,000² = 1.118 × 10⁻⁴ × 1.6 × 10⁷ = 1,789 ×g.
4,000 RPM at r = 10 cm ≈ 1,790 ×g
How the calculation flows
Units & constants
| Pellet bacteria | 3,000–5,000 ×g, 10–15 min |
|---|---|
| Pellet mammalian cells | 200–300 ×g, 5 min (harder spins damage them) |
| Plasmid miniprep lysate | ≥ 12,000 ×g |
| Ethanol-precipitated DNA | ≥ 14,000 ×g, 4 °C |
| Radius to use | r_max printed on the rotor (r_min/r_avg for gradients) |
| Price | Free |
Why do protocols say ×g instead of RPM?
Because RPM is meaningless without the rotor. 4,000 RPM is 1,790 ×g in a 10 cm rotor but 2,680 ×g in a 15 cm one — a 50% difference that decides whether mammalian cells survive the spin. Force in ×g transfers between machines; converting it to your rotor's RPM is the one step this calculator does.
Which radius do you plug in?
The maximum radius (r_max) — the distance from the axis to the bottom of the tube — for pelleting, which is where the pellet forms and where vendors define their rotor's rating. Density-gradient work sometimes quotes r_avg because the force varies along the tube; your rotor's datasheet lists all three.
Frequently asked questions
- What is 13,000 RPM in a standard microcentrifuge in ×g?
- For the common ~8.4 cm-radius fixed-angle rotor: 1.118 × 10⁻⁵ × 8.4 × 13,000² ≈ 15,900 ×g — which is why 'max speed' minipreps quote ~16,000 ×g. Your rotor's exact radius shifts this.
- Does tube angle matter?
- The formula uses radius only; the rotor's angle is already reflected in where r_max falls. Just read r_max from the rotor's spec rather than measuring the tube.
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