How to Convert RPM to × g (RCF): Formula and Speed Table
7 min read · Updated September 24, 2026
A protocol gives a spin in × g because force is what pellets cells and precipitates, and a centrifuge shows RPM because rotation is what its motor controls. The two are joined by one number that belongs to the rotor rather than to the machine: its radius. The same 13,000 rpm is about 13,200 × g in a rotor with a 7 cm radius and about 17,000 × g in one with a 9 cm radius.
This guide gives the conversion in both directions, explains which radius a protocol's × g refers to, and tabulates common microcentrifuge speeds by rotor radius, so you can see how much the radius matters before trusting any converted number.
The formula, in both directions
RCF = 1.118 × 10⁻⁵ × r × RPM², where r is the rotor radius in centimetres and RCF is expressed as a multiple of g. Rearranged for the other direction: RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)).
The constant is not an empirical fudge factor. Relative centrifugal force is the centripetal acceleration ω²r divided by standard gravity (9.80665 m/s²), with the angular speed ω = 2π × RPM ÷ 60. Collecting the constants for a radius in centimetres gives (2π/60)² × 0.01 ÷ 9.80665 = 1.1182 × 10⁻⁵, which protocols round to 1.118 × 10⁻⁵ or 1.12 × 10⁻⁵.
Because the speed is squared, force climbs steeply with it: doubling the RPM quadruples the × g, a 10% error in speed is a 21% error in force, and turning a rotor down by 30% roughly halves the force (0.7² = 0.49).
A worked conversion: a protocol asks for 16,000 × g and your rotor's maximum radius is 8.4 cm. RPM = √(16,000 ÷ (1.118 × 10⁻⁵ × 8.4)) ≈ 13,050 rpm. Run the other way, the same rotor at 15,000 rpm delivers about 21,100 × g.
Which radius: r max, r min or r av
A tube in a rotor spans a range of distances from the axis of rotation: the meniscus is closest (r min), the bottom of the tube is farthest (r max), and r av sits between them. Force is proportional to radius, so it is not the same along the tube. If the top of the liquid sits at 5 cm from the axis and the bottom of the tube at 8.4 cm, the liquid at the top feels 5 ÷ 8.4 = 60% of the force at the bottom.
Unless it says otherwise, a protocol's × g means the force at r max, because that is where the pellet forms, and rotor manuals quote maximum RCF at r max as well. Use the r max figure from your rotor's manual or the rotor itself — not a ruler held across the rotor's outer rim, which overstates it.
Swing-bucket rotors are the case that catches people out: the tube swings out to horizontal during the run, so its radius while spinning is larger than it looks at rest. Use the manufacturer's r max for the swung-out position.
Microcentrifuge speeds in × g, by rotor radius
Microcentrifuge rotors have maximum radii of roughly 6 to 10 cm, so 'full speed' is a different force on every bench. Each line gives the RCF at r max for the speed on the left, calculated from the formula above:
- 5,000 rpm: 1,677 × g at 6 cm, 1,957 at 7 cm, 2,236 at 8 cm, 2,516 at 9 cm, 2,796 at 10 cm.
- 8,000 rpm: 4,294 × g at 6 cm, 5,010 at 7 cm, 5,725 at 8 cm, 6,441 at 9 cm, 7,157 at 10 cm.
- 10,000 rpm: 6,709 × g at 6 cm, 7,828 at 7 cm, 8,946 at 8 cm, 10,064 at 9 cm, 11,182 at 10 cm.
- 12,000 rpm: 9,662 × g at 6 cm, 11,272 at 7 cm, 12,882 at 8 cm, 14,492 at 9 cm, 16,103 at 10 cm.
- 13,000 rpm: 11,339 × g at 6 cm, 13,229 at 7 cm, 15,119 at 8 cm, 17,008 at 9 cm, 18,898 at 10 cm.
- 13,300 rpm: 11,868 × g at 6 cm, 13,846 at 7 cm, 15,824 at 8 cm, 17,803 at 9 cm, 19,781 at 10 cm.
- 14,000 rpm: 13,151 × g at 6 cm, 15,342 at 7 cm, 17,534 at 8 cm, 19,726 at 9 cm, 21,918 at 10 cm.
- 15,000 rpm: 15,096 × g at 6 cm, 17,612 at 7 cm, 20,128 at 8 cm, 22,644 at 9 cm, 25,160 at 10 cm.
Converting a protocol written for someone else's centrifuge
Kit protocols often give both numbers, and the pair tells you which rotor they assumed. QIAGEN's QIAprep miniprep handbook runs its spins at 13,000 rpm (about 17,900 × g) in a conventional table-top microcentrifuge; solving the formula for r gives a radius of about 9.5 cm. In a rotor with a 7.5 cm radius, 13,000 rpm is only about 14,200 × g, and matching the handbook's force would take about 14,600 rpm — which may be beyond that centrifuge's top speed.
When a protocol gives only RPM, it is describing a force in someone else's rotor, and an honest conversion needs their radius. If the protocol names the centrifuge or rotor, look up its r max; if it names nothing, treat the RPM as approximate and use the × g of a comparable, better-specified protocol instead.
Benchtop and swing-bucket centrifuges have larger radii and lower speeds, and the same logic applies. A gentle cell spin at 300 × g is about 1,340 rpm in a rotor with a 15 cm radius and about 1,220 rpm at 18 cm; going the other way, 1,500 rpm is 377 × g at 15 cm and 453 × g at 18 cm.
Mistakes that change the force without anyone noticing
- Using the rotor's outer radius, or r min, in place of r max.
- Copying an RPM from a protocol written for a different rotor. It is the most common way a 'full-speed' spin quietly runs at a fraction of the force the authors used — 13,000 rpm in a 6 cm rotor is under two-thirds of what it is in a 9.5 cm one.
- Reusing a × g-to-RPM conversion worked out for a fixed-angle rotor on a swing-bucket rotor, or the other way round. Different radius, different answer.
- Forgetting the square. Force scales with RPM², so a small change in speed is a large change in force.
- Treating × g as the whole story. How well a spin pellets something also depends on time and on how far particles have to travel through the tube, which differs between rotors — the reason ultracentrifuge rotors are compared by k-factor rather than by force alone.
- Spinning a tube past its rated force. Microcentrifuge tubes, conical tubes and plates carry maximum RCF ratings from their manufacturers, and a number that is right for the rotor can still be wrong for the plastic.
Frequently asked questions
What is 13,000 rpm in × g?
It depends on the rotor's radius: about 13,200 × g at 7 cm, 15,100 × g at 8 cm, 17,000 × g at 9 cm and 18,900 × g at 10 cm, from RCF = 1.118 × 10⁻⁵ × r × RPM². Look up your own rotor's maximum radius rather than relying on a single conversion figure.
What is the formula to convert RPM to × g?
RCF = 1.118 × 10⁻⁵ × r × RPM², with r the rotor radius in centimetres. To go from × g to RPM, rearrange it: RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)). For 16,000 × g in a rotor with an 8.4 cm radius that is about 13,050 rpm.
Is RCF the same as × g?
Yes. Relative centrifugal force is expressed as a multiple of standard gravity, so an RCF of 10,000 is written 10,000 × g. RPM is not a force at all — it is how fast the rotor turns, and the force it produces depends on the radius.
Which rotor radius should I use for the conversion?
The maximum radius, r max — the distance from the axis of rotation to the bottom of the tube — unless the protocol says otherwise. A protocol's × g conventionally refers to r max, and rotor manuals list it. For a swing-bucket rotor, use the figure for the swung-out position.
How do I convert a protocol's × g into RPM for my centrifuge?
Find your rotor's r max in its manual, then compute RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)). Check the result against the centrifuge's maximum speed and the tube's rated force before you spin. A converter that takes the radius does the arithmetic in either direction.
Why do protocols give × g instead of RPM?
Because × g is the same instruction on every centrifuge and RPM is not. The force at a given speed scales with the rotor's radius, so an RPM written for one rotor is a different spin in another — 13,000 rpm is about 13,200 × g at a 7 cm radius and about 17,000 × g at 9 cm.
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