Knowledge Base
RCF vs RPM
Why your protocol says ×g and your centrifuge says RPM — and how to move between them without guessing. The formula, a worked example, a conversion table, and how to choose the right centrifuge.
The Short Answer
RPM is how fast the rotor turns. RCF (×g) is the force your sample actually feels. Your protocol is written in ×g because that number is reproducible in any lab, on any centrifuge. Your instrument may show RPM because that is what its motor measures. To translate between them you need one more piece of information: the rotor radius.
Never compare two centrifuges by RPM alone. A bigger rotor spinning slower can hit your sample harder than a small rotor spinning faster. Always convert to ×g first — and if you just want the number now, use the RCF ⇄ RPM Calculator.
What RCF and RPM Actually Mean
RPM — revolutions per minute — is a measure of speed. It tells you how many full turns the rotor completes each minute. It says nothing, by itself, about how hard your sample is being pushed, because a point near the centre of the rotor travels a much shorter circle than a point out at the edge.
RCF — relative centrifugal force, written in units of ×g— is the force on your sample expressed as a multiple of Earth's gravity. A setting of 3,000 ×g means the sample experiences 3,000 times its normal weight. Because it is a force, not a speed, it is what determines whether your pellet forms, your gradient separates, or your cells stay intact.
The link between them is radius: the distance from the spin axis to the sample. Two tubes at the same RPM but different radii feel different forces. That is the entire reason protocols standardise on ×g and never on RPM.
The One Formula You Need
RCF = 1.118 × 10⁻⁵ × r × N²
- •RCF — relative centrifugal force, in ×g (what you want).
- •r — rotor radius in centimetres, measured from the centre of the rotor to the bottom of the tube (the manufacturer lists this as rₘₐₓ).
- •N — rotor speed in RPM.
- •The constant 1.118 × 10⁻⁵ bundles the unit conversions so you can work in cm and RPM directly.
To go the other way — you know the ×g your protocol wants and need the dial setting — rearrange to RPM = √( RCF ÷ (1.118 × 10⁻⁵ × r) ). This is generic centrifuge physics; it holds for every rotor and every brand. The only variable you supply is your rotor radius.
A Worked Example
Say your rotor radius is 10 cm and you set the centrifuge to 4,000 RPM. Plug in:
RCF = 1.118 × 10⁻⁵ × 10 × (4,000)²
RCF = 1.118 × 10⁻⁵ × 10 × 16,000,000
RCF ≈ 1,789 ×g
Now the reverse. Your protocol says spin at 3,000 ×g and your rotor radius is 9.5 cm. What speed do you dial in?
RPM = √( 3,000 ÷ (1.118 × 10⁻⁵ × 9.5) )
RPM ≈ 5,315 RPM
Takeaway: the exact RPM depends on yourrotor. If a colleague hands you a protocol at "5,315 RPM," it is only correct on the rotor it was written for. The ×g value is the one that travels safely between labs.
Conversion Table for Common Rotor Radii
RCF (×g) at a given speed for typical rotor radii. Read down to your radius, across to your speed — the cell is the force your sample sees. Values are rounded whole numbers from the formula above.
| Rotor radius | 3,000 RPM | 5,000 RPM | 8,000 RPM | 10,000 RPM | 15,000 RPM |
|---|---|---|---|---|---|
| 5 cm | 503 ×g | 1,397 ×g | 3,578 ×g | 5,590 ×g | 12,577 ×g |
| 7.5 cm | 755 ×g | 2,096 ×g | 5,366 ×g | 8,385 ×g | 18,866 ×g |
| 10 cm | 1,006 ×g | 2,795 ×g | 7,155 ×g | 11,180 ×g | 25,155 ×g |
| 12.5 cm | 1,258 ×g | 3,494 ×g | 8,944 ×g | 13,975 ×g | 31,444 ×g |
| 15 cm | 1,509 ×g | 4,192 ×g | 10,733 ×g | 16,770 ×g | 37,732 ×g |
Look along any single row: the force climbs with the square of speed. Look down any single column: at the same RPM, a larger radius means a stronger spin. Both effects are why RPM alone never tells the whole story.
What Buyers Get Wrong
- •Comparing spec sheets by max RPM. A headline "15,000 RPM" is meaningless without the rotor it applies to. Compare maximum RCF instead — and the RCF with the rotor you will actually run.
- •Copying an RPM value between machines. A protocol handed over as "X RPM" only reproduces the original force if the rotor radius matches. Re-derive the RPM from the ×g for your own rotor.
- •Ignoring the rotor type. Fixed-angle and swing-out rotors on the same centrifuge list different max ×g. Swing-out is right for cell pellets and layered gradients; fixed-angle typically reaches higher force.
- •Forgetting radius units. The formula needs centimetres. Drop in millimetres and your answer is off by a factor of ten.
- •Over-spinning "to be safe." Too much force lyses cells, smears gradients, and packs pellets so hard they will not resuspend. The protocol's ×g is a target, not a floor.
- •Assuming the display is in ×g. Confirm the mode. Many units toggle between RPM and RCF, and it is easy to run a protocol in the wrong one.
Choosing a Centrifuge by RCF, Not RPM
Nobody in a lab thinks "I need 14,000 RPM." They think "I need to pellet at 16,000 ×g," or "I need to spin blood tubes at 3,000 ×g, refrigerated." Buy from that intent. Three questions settle almost every centrifuge decision:
- •What ×g do your protocols call for? Confirm the unit reaches that force with a rotor that fits your tubes — not just its headline RPM.
- •What are you spinning? 1.5 / 2.0 mL microtubes, 15 / 50 mL conical tubes, PCR strips, microplates, or blood-collection tubes. The rotor must accept the vessel — this is a hard constraint, not a preference.
- •Do you need refrigeration, and how much throughput? Heat-sensitive samples need a refrigerated unit; high sample counts push you from a benchtop toward a floor-standing model.
Match those to a class of instrument below. Every model links to its full spec page, where the manufacturer-published max RCF and rotor options are listed. Browse the full range on the Centrifuges category page.
Centrifuges by the Job They Do
Quick spins & microtubes — mini centrifuges
For collecting droplets, quick spin-downs, and PCR strips at the bench. Fixed low-to-moderate ×g, tiny footprint.
High-speed microtube work — microcentrifuges
When the protocol calls for thousands to >20,000 ×g on 1.5 / 2.0 mL tubes: molecular biology, spin columns, DNA/RNA prep.
Blood tubes & clinical — clinical centrifuges
Swing-out and fixed-angle rotors sized for 10–15 mL blood-collection tubes; serum/plasma separation and hematocrit.
Mixed-format lab workhorses — benchtop / universal
Accept multiple rotor types so one instrument covers 15/50 mL tubes, microplates, and more as your work changes.
Cold chain & high volume — refrigerated / floor-standing
Refrigerated for temperature-sensitive protocols; floor-standing when you need large volumes at high ×g.
Hermle Z216-MK Refrigerated Microcentrifuge
Refrigerated high-speed microtube spins
Hermle Z366-K Refrigerated Universal Centrifuge
Refrigerated universal bench unit
Hermle Z446-K Refrigerated Centrifuge
Up to 26,328 ×g, temperature-controlled
Hermle Z496-K High-Volume Floor Centrifuge
Floor-standing, large-batch throughput
Frequently Asked Questions
What is the difference between RCF and RPM?
RPM (revolutions per minute) is how fast the rotor spins. RCF (relative centrifugal force), written in units of ×g, is the actual force applied to your sample, expressed as a multiple of Earth's gravity. RPM is a property of the motor; RCF is what your sample feels. Two centrifuges running at the same RPM can apply very different forces because RCF also depends on the rotor radius.
How do I convert RPM to RCF (×g)?
Use RCF = 1.118 × 10⁻⁵ × r × N², where r is the rotor radius in centimetres (measured from the centre of the rotor to the bottom of the tube) and N is the speed in RPM. To go the other way, RPM = √(RCF ÷ (1.118 × 10⁻⁵ × r)). Our free RCF ⇄ RPM Calculator does both instantly.
Why does my protocol list ×g but my centrifuge only shows RPM?
Protocols specify ×g because it is reproducible across labs — the same force gives the same result no matter whose centrifuge or rotor you use. Older or simpler centrifuges display RPM because that is what the motor controller measures directly. You convert using your rotor radius. Most modern instruments let you switch the display to ×g so you can dial in the protocol value directly.
Do I really need to know my rotor radius?
Yes — it is the whole reason RPM alone is not enough. The same 4,000 RPM produces about 1,006 ×g on a small 10 cm rotor but roughly 1,509 ×g on a 15 cm rotor. Without the radius, an RPM figure is not reproducible. Manufacturers publish the radius (often labelled rₘₐₓ) for each rotor; it is on the rotor spec sheet.
Is a higher RPM always a stronger spin?
No. Because RCF scales with the rotor radius, a larger rotor at a lower RPM can out-pull a small rotor spinning faster. Always compare in ×g, not RPM. This is the single most common mistake when people shop for or compare centrifuges.
Which rotor radius do I use in the formula — rₘₐₓ or rₘᵢₙ?
Use rₘₐₓ, the distance from the axis to the bottom of the tube, when you want the maximum force your sample sees (the number manufacturers quote for max RCF). Some applications reference rₐᵥ (the average radius) for the force at the middle of the tube. Match whichever your protocol specifies; if it just says "×g," rₘₐₓ is the safe default.
Should I choose a centrifuge by its maximum RPM or its maximum RCF?
By maximum RCF, and specifically the RCF it reaches with the rotor you will actually run. A spec sheet headline RPM means little on its own. Check that the unit hits the ×g your protocols call for with a rotor that accepts your tube format (15/50 mL, microtubes, PCR strips, microplates, or blood bags).
Why do fixed-angle and swing-out rotors list different max RCF on the same centrifuge?
Because they have different effective radii and speed ratings. A fixed-angle rotor holds tubes closer to the axis and is usually rated for higher RCF; a swing-out (bucket) rotor extends the tube outward at speed and pellets flat against the tube bottom, which is better for cell work and layered separations. The same machine can list two different max ×g figures — one per rotor.
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