The centrifuge RCF calculator above converts rotor speed in revolutions per minute into relative centrifugal force, expressed as a multiple of standard gravity, and converts back the other way. It needs one piece of information that speed alone cannot supply: the radius from the axis of rotation to the sample. That single number is the reason a protocol written for one centrifuge cannot be run on another simply by copying the RPM setting.
Arb Digital builds free calculators that make the missing variable visible instead of hiding it. This page reports the force at the bottom of the tube and at the top of the liquid column, because those differ substantially, and it answers the transfer question directly: given a force you want to reproduce, what speed does a second rotor of a different radius need? The underlying physics is ordinary circular motion, which our circular motion calculator covers in the general case.
What This Centrifuge RCF Calculator Does
In its default mode you enter the rotor radius and the speed, and it returns the relative centrifugal force. Switch modes and you enter the force a protocol specifies, and it returns the speed your rotor must run at to produce it. Both directions use the same equation; only the rearrangement differs.
Two radius fields are provided because a centrifuge tube is not a point. The end of the tube furthest from the axis experiences the greatest force, and the surface of the liquid nearest the axis experiences the least. In a typical fixed-angle rotor the ratio between them can approach two to one, which means the pellet at the bottom is being pressed considerably harder than the material still suspended near the top. Reporting only one figure hides that gradient, so the tool shows both.
The second rotor field exists for the situation that actually sends people looking for this calculation. You have a published method that says fifteen minutes at 4,000 RPM, your machine has a different rotor, and copying the number would give you the wrong experiment. Enter the second rotor's radius and the tool reports the speed it needs to match the force you calculated for the first.
How to Use It
- Choose the direction. RCF from speed is the default; switch to speed from RCF when you are implementing a published protocol.
- Enter the rotor radii in millimetres. Both are measured from the axis of rotation: one to the outermost point of the tube, one to the top of the sample.
- Enter the speed or the target force. Whichever your mode requires; the other appears in the results.
- Add the second rotor radius. The equivalent speed for that rotor is reported so a method transfers correctly between machines.
- Set the spin time. The g-minute product lets you compare two runs of different duration at the same force, which is the crude but common way of judging equivalence.
The Formula: How RCF Is Calculated
Relative centrifugal force is the centripetal acceleration of the sample divided by standard gravity. The acceleration of an object moving in a circle at angular velocity ω and radius r is rω2, a result set out in OpenStax University Physics Volume 1, section 6.3 on centripetal force, which gives the centripetal force as both mv2/r and mrω2 and discusses centrifuges directly as an application. Angular velocity in radians per second is 2πN/60 when N is the speed in revolutions per minute.
Dividing by standard gravity turns the acceleration into a dimensionless multiple. That constant is fixed by convention at 9.806 65 m s−2, listed as exact in the NIST CODATA value for standard acceleration of gravity. It is a defined reference value, not a measurement of local gravity, which is why an RCF figure means the same thing in every laboratory.
Work the defaults. A radius of 100 mm is 0.1 m, and 4,000 RPM is 2π × 4000 ÷ 60 = 418.88 rad/s. Squaring gives 175,460 s−2, and multiplying by 0.1 m gives 17,546 m/s2. Dividing by 9.806 65 gives 1,789.2, so the sample at the bottom of the tube experiences about 1,789 × g. At the 60 mm radius the same speed gives 1,073.5 × g, six-tenths of the value, because force scales linearly with radius.
Why Rotor Radius Makes Protocols Non-Transferable
This is the practical heart of the matter. Force depends on radius to the first power and on speed to the second, so two machines running the identical RPM produce different forces whenever their rotors differ in size — and rotors differ constantly. A compact microcentrifuge rotor might have a radius of 45 mm while a floor-standing clinical rotor extends past 180 mm. At the same 4,000 RPM, the large rotor delivers four times the force of the small one.
That is why any method worth reproducing states its condition in RCF rather than RPM, and why a method that states only RPM is incomplete unless it also names the exact rotor. If you inherit such a protocol, find the rotor it was written for, look up its radius, compute the force here, then convert that force back into the speed your own rotor needs. Skipping that step is one of the most common quiet sources of irreproducible sample preparation.
The same reasoning applies within a single machine when you change rotors. Swapping a fixed-angle rotor for a swing-out rotor of different geometry changes both the radius and the effective path length through the sample, so the speed that produced a clean pellet last week may under- or over-spin the sample today.
Fixed-Angle Versus Swing-Out Geometry
A swing-out rotor lets the buckets pivot outward until the tube lies along the radius, so particles travel the full length of the tube toward the bottom and the pellet forms as a flat layer at the end. A fixed-angle rotor holds the tube at a set angle, typically somewhere between twenty and forty-five degrees, so particles travel a much shorter distance before hitting the tube wall and then slide down it. The shorter path is why fixed-angle rotors pellet faster at comparable force.
It also means the pellet forms against the side wall rather than the bottom, which changes how you aspirate the supernatant without disturbing it. Neither geometry is better in general. What matters for this calculation is that the maximum radius differs between them for tubes of the same size, so the datasheet radius must match the rotor actually installed.
The angular velocity itself is identical in either case, and if you want that figure in radians per second rather than revolutions per minute, the angular velocity calculator converts between the two conventions. The force on a particle of known mass follows from the centripetal force calculator, which returns newtons rather than a dimensionless multiple of g.
Reading the Force Gradient Across the Tube
Because force falls linearly toward the axis, sedimentation is not uniform down the tube. A particle starting near the liquid surface begins its journey in the weakest part of the field and accelerates as it travels outward. In a density gradient separation this is a feature and the gradient medium is designed around it. In a simple pelleting step it mostly means the last material to arrive took disproportionately longer than a single average force figure would suggest.
This is also why the convention of quoting maximum RCF exists, and why some protocols specify average RCF instead. If a method does not say which it means, the maximum is the safer assumption, since it is what rotor manufacturers print and what most software reports. The tool gives both endpoints so you can see the spread and decide whether the difference is material for your application.
What the g-Minute Figure Does and Does Not Tell You
Multiplying force by time gives a rough equivalence between runs: 1,000 × g for twenty minutes and 2,000 × g for ten minutes are often treated as comparable. For simple pelleting of robust material that approximation is usually acceptable, and the tool reports the product for that purpose.
It breaks down quickly in other cases. Sedimentation rate depends on particle size, shape, density difference and the viscosity of the medium, so doubling the force does not halve the time for every component of a mixed sample. Fragile material can be damaged by high force in a way no amount of extra time at lower force reproduces. And separations that depend on differential rates — pelleting one population while leaving another suspended — depend on the force itself, not on the product. Treat g-minutes as a first approximation and nothing more.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Copying an RPM figure between machines — force depends on rotor radius as well as speed, so the same setting on a different rotor is a different experiment.
- Measuring radius from the rotor edge — it is measured from the axis of rotation to the sample, and the manufacturer's datasheet is the reliable source.
- Quoting one RCF for the whole tube — force falls linearly toward the axis, so the top of the sample sees considerably less than the bottom.
- Mixing millimetres and centimetres — the shortcut formulas in the literature use centimetres and this tool uses millimetres, and a factor of ten in radius is a factor of ten in force.
- Trusting g-minutes for delicate separations — force and time are not freely interchangeable when sedimentation rate or sample fragility is what limits the result.
Related Free Tools From Arb Digital
For the general rotational relationships behind this figure, use the circular motion calculator and the angular velocity calculator, or the centripetal force calculator if you need the force in newtons. On the sample-preparation side, the serial dilution calculator and the cell density dilution calculator handle the steps either side of a spin, and the DNA concentration calculator covers quantification afterwards. The speed converter rescales the rim velocity figure. Everything else is on the free online tools hub.
Frequently Asked Questions
RPM is how fast the rotor turns. RCF is the force the sample actually experiences, expressed as a multiple of standard gravity. Converting between them requires the rotor radius, because force depends on how far the sample sits from the axis.
Because the force also depends on rotor radius, and rotors vary widely. At the same speed a large rotor can deliver several times the force of a compact one, so copying the speed reproduces the setting rather than the experiment.
From the axis of rotation to the point of interest in the sample — usually the outermost point of the tube for maximum RCF. Rotor datasheets publish this figure, and using a measurement taken from the rotor edge will understate it.
Because force falls linearly as radius decreases, so the bottom of the tube and the top of the liquid column experience different forces. Showing both makes the gradient across the sample visible instead of hiding it behind one average.
Standard gravity, 9.806 65 metres per second squared, which is an exact defined constant rather than a local measurement. That is what makes an RCF figure mean the same thing in every laboratory.
Only roughly. For straightforward pelleting of robust material the product of force and time is a workable approximation, but sedimentation rate depends on particle size, density and viscosity, and fragile samples can be damaged by force in ways longer spinning cannot substitute for.
The equation is the same for fixed-angle and swing-out rotors. What changes is the radius, since the two geometries place a tube of the same size at different distances from the axis, and the path length a particle travels before reaching the wall.
This tool is provided for educational and estimating use. It converts between rotor speed and relative centrifugal force and does not account for sample properties, rotor derating, temperature limits or instrument safety requirements, so always follow the rotor manufacturer's rated limits.
