Centrifuge RCF (g) ↔ RPM Converter — Relative Centrifugal Force Calculator
A protocol says “centrifuge at 12,000 × g for 10 minutes,” but your centrifuge panel only shows rpm. You cannot simply copy one number to the other, because the conversion depends on the rotor radius.
Conversion formula:
RCF = 1.118 × 10⁻⁵ × r × N²
RCF: relative centrifugal force, expressed as multiples of gravitational acceleration (× g)r: rotor radius in cm — the distance from the axis of rotation to the bottom of the centrifuge tubeN: rotor speed in rpm
To solve for speed: N = √( RCF / (1.118×10⁻⁵ × r) )
The most common mistake is choosing the wrong value of r. For the same tube, the distance from the tube bottom to the rotational axis differs from that at the tube mouth, so manufacturers specify r_max (tube bottom) and r_min (tube mouth). For pelleting, the relevant force acts at the tube bottom — use r_max. For rough estimates, the rotor’s nominal maximum radius is adequate; to reproduce a published result, consult the official rotor specification sheet for your instrument.
Typical bench-top centrifuge rotors have radii of roughly 6–10 cm; ultracentrifuge rotors span 5–15 cm, but always defer to the instrument manual — the reference values on this page are for order-of-magnitude checks only.
A worked example
Rotor radius 8.5 cm, target 12,000 × g:
N = √( 12000 / (1.118×10⁻⁵ × 8.5) ) ≈ 11,237 rpm
At that same speed, a rotor with radius 6 cm produces only about 8,470 × g — a 30% difference. This is why a protocol that reports only rpm without specifying the rotor model is nearly useless for replication.
Why bench-top and floor-standing centrifuges differ so much
RCF is proportional to radius and to the square of rotor speed. Bench-top microcentrifuge rotors typically have radii of 6–9 cm; large floor-standing centrifuges can reach 15–20 cm. At the same 10,000 rpm:
- r = 7 cm → approximately 7,800 × g
- r = 18 cm → approximately 20,100 × g
Copying rpm across instruments can introduce a factor of two or more. Always convert using × g when switching centrifuges — never carry over the rpm directly.
FAQ
Should r be the radius at the tube bottom or the tube mouth?
For pelleting, use the tube-bottom radius (r_max), because the pellet forms where centrifugal force is greatest. Manufacturer spec sheets typically list both r_max and r_min.
If I switch centrifuges, will the same rpm give the same result?
No. Different rotor radii produce very different g values at the same rpm. To reproduce an experiment on a different instrument, always convert using × g.
Why does the RCF shown on my instrument panel differ from my calculation?
Most likely the radius values differ. Manufacturers typically calibrate against r_max (tube bottom), whereas you may have used r_average. On a bench-top centrifuge this can mean a 1–2 cm difference in r, translating to a difference of ten-plus percent in RCF. Use the rotor radius given in the instrument manual — that is the value the instrument itself uses for its own conversion.
Where does the constant 1.118×10⁻⁵ in the formula come from?
It is the combined coefficient after substituting angular velocity, radius units (cm), and gravitational acceleration. RCF = ω²r/g, where ω = 2πN/60. Substituting g = 980.665 cm/s² gives the coefficient (2π/60)²/980.665 ≈ 1.118×10⁻⁵. This is why r must be in cm and N must be in rpm.
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