K Factor Centrifuge Calculator

Calculator

Choose a mode and enter values

Mode

Switch modes to reveal the right inputs.

Radius unit

Use the same unit for rmin and rmax.

Example Data Table

Scenario	rmin	rmax	RPM	Computed K	RCF max
Microcentrifuge rotor	9.6 cm	12.2 cm	15,000	~216	~30,700 × g
Swinging-bucket rotor	8.0 cm	16.0 cm	10,000	~1,755	~17,900 × g
Reduced speed adjustment	Rated K = 133 at 45,000 rpm		35,000	Adjusted K ≈ 220

Example values are illustrative. Always verify rotor specifications and safe operating limits.

Formula Used

K factor from radii and speed

The clearing factor is commonly estimated using:

K = (2.53 × 10¹¹) × ln(r_max/r_min) / RPM²

You may also see the equivalent form K = (2.53 × 10⁵) × ln(r_max/r_min) / (RPM/1000)².

RCF reference

RCF = 1.118 × 10⁻⁵ × RPM² × r (with r in cm)

Time scaling

For similar conditions, run time often scales with K. When a rated K is known, the adjusted value is:

K_adj = K_rated × (RPM_rated/RPM_actual)²

How to Use This Calculator

Select a mode that matches your task.
Enter radii using the same unit (cm or mm).
Provide speed in rpm, or a target K value.
Press Calculate to view results above.
Download CSV or PDF for records and sharing.

Practical Notes

K is a rotor efficiency indicator. Smaller values usually pellet faster.
Temperature, viscosity, density, and sample volume can change outcomes.
Always follow the manufacturer’s maximum speed and load guidance.

K Factor Centrifuge Guide

1) What the K factor represents

The K factor (clearing factor) is a rotor performance number that summarizes how quickly a rotor can clear particles through a liquid. Lower K values mean faster separations. Manufacturers publish K at a rated speed, often the maximum permitted RPM for that rotor.

2) Inputs that drive the calculation

This calculator uses minimum radius (r_min), maximum radius (r_max), and speed (RPM). Radii determine the path length and the centrifugal field gradient. The term ln(r_max/r_min) typically falls around 0.1–0.6 for many swinging-bucket and fixed-angle designs.

3) Typical K ranges you may see

As a rough guide, high-speed micro rotors may publish K values near 10–50, while mid-size fixed-angle rotors may sit around 100–400. Large swinging-bucket rotors designed for high capacity can exceed 1,000, especially at lower rated speeds. Always rely on the rotor’s manual for its official rating.

4) Why speed changes K so strongly

K scales approximately with 1/RPM². If you reduce speed from 45,000 RPM to 35,000 RPM, the adjusted K increases by about (45/35)² ≈ 1.65. That means a run that cleared at 20 minutes may need about 33 minutes, assuming the same sample and conditions.

5) Linking K to spin time and S value

For many planning workflows, spin time can be estimated from t = K / s, where s is the sedimentation coefficient in Svedbergs (1 S = 10⁻¹³ s). For example, a rotor with K = 200 clearing a 10 S particle suggests t ≈ 20 minutes under comparable assumptions.

6) Practical factors that shift real outcomes

Viscosity, temperature, density gradients, fill volume, and tube geometry can change clearance time even when K is unchanged. A colder sample can be noticeably more viscous, slowing sedimentation. If you change buffer composition or run at a different temperature, treat K-based times as starting estimates, not guarantees.

7) Good reporting and record keeping

When documenting a run, record rotor model, r_min/r_max, RPM, duration, and temperature. Exporting results to CSV or PDF helps keep protocols consistent across technicians and instruments, and makes it easier to reproduce successful separations later for audits, training, and method transfer.

FAQs

What is a “good” K factor?

Lower is better for clearing at the same conditions. Compare only within the same rotor family and speed rating. Use published K at rated RPM as the reference, then adjust when you run slower.

Does K factor depend on sample volume?

K itself is a rotor metric from geometry and speed, so it does not change with volume. However, volume and fill height can change actual clearing time because the sedimentation path and gradients can differ.

Why do I need both rmin and rmax?

Particles travel from the inner radius toward the outer radius. The ln(rmax/rmin) term captures that path length and field change across the tube, which affects clearance performance.

Can I use K factor to replace RCF?

They answer different questions. RCF reports the instantaneous centrifugal field at a radius, while K summarizes overall rotor clearing efficiency. Many protocols specify RCF and time; K helps translate between rotors.

How accurate is t = K/s?

It is a planning estimate. It assumes similar solvent properties, temperature, and particle behavior. Use it to choose a starting time, then refine using experimental results for your specific sample.

What should I export in a report?

Include rotor name, rmin, rmax, RPM, run time, unit choices, and any notes. Adding temperature and tube type improves repeatability. Attach the CSV/PDF to your lab notebook or SOP.