Centrifuge K-Factor & Run Time Calculator
Enter a successful source run and the target rotor data to estimate the equivalent run time. You can use manufacturer k-factors or calculate k from rotor radii and operating speed.
Equivalent target run time29.78
- Equivalent time as hours and minutes
- 30 minutes
- Source adjusted k-factor
- 787
- Target adjusted k-factor
- 781.25
- Target-to-source time ratio
- 0.99
- Run-time change
- -0.73%
- Target maximum RCF
- 31,482.88
- More efficient rotor
- Target rotor
Assumes the same particle, medium, temperature, fill condition, tube style, and sedimentation behavior in both runs.
Use either published full-tube k-factors or geometry measured for the actual fill condition; do not mix them for one rotor pair.
How to use this calculator
- Choose whether you are using published k-factors or rotor geometry for both rotors.
- Enter the source run time and the source rotor speed used by the original protocol.
- Enter the target rotor speed you plan to use and the matching k-factor or radii.
- Review the adjusted k-factors, time ratio, RCF, and notes before changing a protocol.
How the k-factor transfer is calculated
A rotor k-factor is a clearing factor: lower k means particles travel the required radial distance faster under the rotor's geometry and speed. To transfer a pelleting step from one rotor to another, the run time scales with the adjusted k-factor ratio rather than with RPM or maximum RCF alone.
When you enter manufacturer k-factors, the calculator first corrects each value from the rated speed to the actual speed:
kAdjusted = kRated × (NRated / NActual)^2
When you choose rotor geometry, the calculator derives k directly from the minimum and maximum sedimentation radii at the operating speed:
k = 2.533 × 10^11 × ln(rMax / rMin) / N^2
- rMax is the maximum sedimentation radius in centimeters.
- rMin is the minimum radius, usually the top of the filled sample column.
- N is rotor speed in RPM.
The protocol transfer then uses:
tTarget = tSource × (kTarget / kSource)
If the target adjusted k-factor is half the source value, the estimated run time is cut in half. If the target adjusted k-factor is twice the source value, the estimated run time doubles.
What moves the result most
Speed has strong leverage because k changes with the inverse square of RPM. Running a rotor at 80% of the speed used for its published k-factor raises the adjusted k-factor by 1 ÷ 0.8², or about 56%. The radii matter through the logarithm of rMax / rMin, so geometry changes are usually gentler than speed changes but still important when fill height or tube position changes.
The calculator also reports target maximum RCF using RCFMax = 1.118 × 10^-5 × rMax × N^2. RCF is useful for checking force exposure, but two rotors with the same maximum RCF can have different k-factors because the sample path length and radius profile are different.
What this calculator leaves out
This calculation assumes the same particle, medium, temperature, viscosity, density, and sedimentation behavior in both runs. It does not separately model acceleration, deceleration, braking, rotor angle, tube wall interception, sample concentration, aggregation, pellet firmness, purity, or recovery. Manufacturer k-factors may assume specific tubes, adapters, fill volumes, and rated speeds, so do not combine a full-tube published k-factor with radii measured from a partial-fill setup.
Worked example
Suppose a published protocol runs 30 minutes in a source rotor with k = 787 at 14,000 RPM, and you plan to use a target rotor listed as k = 500 at 20,000 RPM but run it at 16,000 RPM. The source adjusted k-factor stays 787 because actual and rated RPM match. The target adjusted k-factor is 500 × (20,000 / 16,000)^2 = 781.25.
The transfer ratio is 781.25 / 787 = 0.9927, so the equivalent target run is about 29.78 minutes, or about 30 minutes after practical rounding. With an 11 cm target maximum radius at 16,000 RPM, the target maximum RCF is about 31,483 ×g.
Common questions
What does a centrifuge k-factor measure?
A k-factor describes how quickly a rotor can clear or pellet material for a given radial path and speed. Lower k-factors are more efficient for the same sample behavior because the equivalent pelleting time is shorter.
How do I transfer a centrifugation protocol to another rotor?
Correct each rotor k-factor to the RPM actually used, then multiply the original time by the target adjusted k-factor divided by the source adjusted k-factor. Keep the same sample conditions when you make that comparison.
Why is matching RCF alone not always enough?
Maximum RCF only describes the force at one radius. K-factor also reflects the radial distance particles must travel, so two rotors can share a maximum RCF while requiring different pelleting times.
How do I adjust k-factor when running below maximum RPM?
Use kAdjusted = kRated × (rated RPM ÷ actual RPM)^2. Because speed is squared, a modest speed reduction can noticeably lengthen the equivalent run.
Can I use this calculation for partially filled tubes?
Yes only when the k-factor or radii match that fill condition. A published k-factor may assume full tubes and a specific adapter, so partial-fill geometry should not be mixed with that published value unless the manufacturer says it applies.