Centrifuge K-Factor: Converting Pelleting Time

A published pelleting protocol — “12,000×g for 15 minutes” — is only reliable on the exact rotor it was written for. Change rotors and the same RCF no longer guarantees the same result, because RCF alone does not capture how far a particle has to travel to reach the tube wall. Two rotors spinning at the same ×g can have very different path lengths from the liquid surface to the point of pelleting, and that difference changes how long a given particle actually takes to sediment out. The k-factor (also called the clearing factor) is the number that captures this, and it is what lets you convert a published pelleting time from someone else’s rotor to your own with a single ratio.

This guide covers what k-factor actually measures, the formula behind it, the conversion arithmetic worked through with real numbers, and — just as important — where that conversion stops being reliable. See CASRAI’s companion guide on RCF vs. RPM conversion first if you need the background on why ×g and RPM aren’t interchangeable across rotors; k-factor is the next layer on top of that same problem.

What K-Factor Actually Measures

K-factor expresses a rotor’s relative pelleting efficiency at its rated maximum speed: how long, in hours, it would take a particle with a sedimentation coefficient of 1 Svedberg (1S) to pellet completely in that rotor. A lower k-factor means a more efficient rotor — it clears a given particle faster — because the number is really describing path length and speed together, not just one or the other. Two rotors can share an identical rated RCF at the same RPM and still have meaningfully different k-factors, because k-factor also depends on r-max and r-min — the maximum and minimum radii from the axis of rotation to the sample — which set how far the particle actually has to travel.

This is precisely the gap RCF alone leaves open. RCF tells you the force applied to the sample; it says nothing about the distance that force has to act over before a particle reaches the tube wall. K-factor folds both into one number, which is why it is the correct basis for converting a pelleting time between rotors, where RCF by itself is not.

The K-Factor Formula

A rotor’s k-factor, with speed in RPM and radii in millimeters, is:

k = (2.53 × 10⁵ × ln(r_max / r_min)) / (RPM ÷ 1,000)²

In practice you will rarely need to compute this yourself: rotor manufacturers publish the k-factor for each rotor, at each speed setting, directly in the rotor’s spec sheet or manual, precisely because it depends on physical dimensions the manufacturer already knows precisely. Use the manufacturer’s published figure for your actual rotor rather than re-deriving it from measured radii, unless no published figure exists — the formula above is there so you understand what the published number represents, and so you can recompute it if you are running below the rotor’s rated maximum speed (see “Where This Breaks Down” below, since k-factor is speed-dependent and most published values are stated for max speed only).

Converting a Pelleting Time Between Rotors

Because k-factor and pelleting time are directly proportional for a given particle, converting a published time from one rotor to another is a single ratio:

T₂ = T₁ × (K₂ / K₁)

where T₁ and K₁ are the published run time and k-factor for the rotor the protocol was written for, and T₂ and K₂ are the run time you need to find and the k-factor of the rotor you are actually using. A rotor with a higher k-factor than the one the protocol specifies needs a longer run time to pellet the same particle — it is the less efficient of the two — and a lower-k-factor rotor needs less time.

Worked Example

The numbers below are a generic worked example, not a specific commercial rotor’s published specification — substitute your own protocol’s stated rotor and your own rotor’s manufacturer-published k-factor before using this arithmetic on a real sample.

A protocol specifies 45 minutes at maximum speed in Rotor A, which the protocol states has a k-factor of K₁ = 250 at that speed.
Your lab has Rotor B, whose manufacturer datasheet lists a k-factor of K₂ = 133 at its own maximum speed.

Applying the formula:

T₂ = 45 min × (133 / 250) = 45 × 0.532 = ~24 minutes

Rotor B is the more efficient rotor (lower k-factor), so the same separation completes in roughly half the published time. Running the original 45-minute protocol on Rotor B would over-pellet the sample — harmless for a simple crude pellet, but a real problem if the protocol depends on a specific pellet composition or a partial spin.

Where This Breaks Down

K-factor conversion is a reliable starting estimate, not an exact substitute for verifying a new protocol empirically. Specific limits worth knowing before you rely on it:

It only applies to pelleting, not gradient separations. Rate-zonal and isopycnic (density-gradient) separations depend on where a particle ends up within a gradient, not simply whether it has reached the tube wall — k-factor conversion does not apply to those run times.
It assumes the same particle under comparable conditions. The conversion holds a sedimentation coefficient constant across both rotors. Anything that changes the particle’s actual sedimentation coefficient — a different buffer viscosity or density, a different temperature, aggregation state — invalidates the comparison even if the arithmetic is done correctly.
Published k-factors are usually stated for the rotor’s maximum rated speed only. Because k-factor is itself a function of RPM (see the formula above), running a rotor below its rated maximum means its effective k-factor at that lower speed is higher than the published maximum-speed figure — using the datasheet number unmodified will understate the required time.
It does not account for acceleration and deceleration time. The formula describes time at speed, not the ramp up and ramp down every real run includes. For short pelleting runs, ramp time can be a meaningful fraction of the total and the conversion will underestimate real elapsed time.
It is an estimate, not a guarantee, when crossing rotor geometry types. Converting between a fixed-angle and a swinging-bucket rotor is mathematically the same ratio, but the two geometries clear particles somewhat differently in practice (particle travel and wall contact during the run differ), so treat a cross-geometry conversion as a closer estimate to verify, not a certainty.
Always verify a converted time on a real sample before treating it as validated, particularly the first time a protocol moves to unfamiliar equipment. Confirm the pellet looks and performs as expected before running production samples on the calculated time alone.

Frequently Asked Questions

Where do I find my rotor’s k-factor?

In the rotor’s manufacturer datasheet or instrument manual, typically listed per speed setting alongside the rotor’s r-max and r-min. If it genuinely is not published anywhere for your rotor, it can be computed from the formula above using the rotor’s r-max, r-min, and rated maximum RPM.

Is a lower or higher k-factor better?

Lower is more efficient: it means the rotor pellets a given particle faster. K-factor is inversely related to pelleting speed, which is easy to get backwards since most other centrifugation specs (RCF, RPM) run the opposite direction — higher is faster.

Can I use k-factor conversion instead of an RCF conversion?

They answer different questions. RCF conversion tells you what speed setting produces an equivalent force on a different rotor. K-factor conversion tells you how long a pelleting run needs to take on a different rotor to reach completion. A full protocol transfer between rotors typically needs both: match the RCF (or accept the rotor’s own rated maximum), then use k-factor to correct the run time.

Does k-factor conversion account for tube volume or sample concentration?

No. It is purely a function of rotor geometry and speed. A protocol that is sensitive to sample volume, concentration, or tube fill level needs those checked independently — k-factor conversion only corrects for the rotor’s physical clearing efficiency.

Centrifuge K-Factor and Pelleting Time Calculations: Converting a Published Protocol to Your Rotor

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