Centrifuge RCF Calculator
Convert between RPM and relative centrifugal force (×g), calculate k-factor and estimate sedimentation time.
About this calculator
This calculator converts rotor speed (RPM) to relative centrifugal force (RCF, or ×g) using the standard formula RCF = 1.118×10⁻⁵ × r(cm) × RPM², and reports several downstream figures protocols commonly specify. Because RPM enters the formula squared while rotor radius enters only linearly, rotor speed is by far the dominant lever on RCF -- doubling RPM quadruples RCF, while doubling radius only doubles it. The calculator also computes the k-factor, a rotor-specific figure that estimates relative pelleting time (lower k means faster pelleting for a given particle), and derives it from the ratio of maximum to minimum rotor radius, so a smaller minimum radius relative to maximum radius (a longer path for particles to travel) increases k.
Particle diameter is used only for the optional sedimentation-time estimate via a simplified Stokes' law calculation -- it has no effect on RCF itself, since RCF depends purely on rotor geometry and speed, not on what is being spun. The sedimentation-time estimate assumes fixed water viscosity at 20°C and a fixed 0.1 g/cm³ particle-to-fluid density difference, both illustrative defaults rather than values measured for a specific sample -- an actual protocol's real sedimentation behavior depends on the specific particle and buffer properties, and this figure should be read as an approximate guide, not a validated protocol parameter.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
RCF at rMax (×g)
1,006.2
How to Use This Calculator
- Enter your rotor speed in RPM.
- Enter the max rotor radius in centimeters (center of rotation to the bottom of the tube) and the min rotor radius in centimeters (center of rotation to the top of the liquid).
- Optionally enter the particle diameter in micrometers to estimate sedimentation time.
- Review the calculated RCF at rMax and rMin (in ×g), plus the RCF ratio, k-factor, and RPM needed for 1,000 ×g.
- Check the angular velocity, tip speed, and estimated sedimentation time before running your protocol.
How the result changes with Rotor speed (RPM)
| Rotor speed (RPM) | RCF at rMax (×g) |
|---|---|
| 1,500 | 251.6 |
| 2,250 | 566 |
| 4,500 | 2,264 |
| 7,500 | 6,288.7 |
What each input means
- Rotor speed (RPM)
- Rotational speed in revolutions per minute.
- Max rotor radius (cm)
- Distance from center of rotation to the bottom of the tube (rMax).
- Min rotor radius (cm)
- Distance from center of rotation to the top of the liquid (rMin).
- Particle diameter (μm)
- Diameter of particle to sediment, for time estimate (Stokes' law).
What each result means
- RCF at rMax (×g)
- Relative centrifugal force at maximum radius. RCF = 1.118×10⁻⁵ × r(cm) × RPM².
- RCF at rMin (×g)
- Relative centrifugal force at minimum radius (top of liquid).
- RCF ratio (max/min)
- Gradient of g-force across the tube. Closer to 1 is more uniform.
- k-factor
- Rotor clearing factor. Lower k = faster pelleting. k = 2.53×10¹¹ × ln(rMax/rMin) / RPM².
- RPM for 1,000 ×g
- RPM needed to achieve 1,000 ×g at the given max radius.
- Angular velocity (rad/s)
- Angular velocity: ω = 2π × RPM / 60.
- Tip speed (m/s)
- Linear speed at rotor tip. Most rotors are rated to ~100 m/s max.
- Est. sedimentation time (min)
- Estimated time to pellet particles (Stokes' law, assuming Δρ=0.1 g/cm³ in water at 20 °C).
- Centripetal acceleration (m/s²)
- Centripetal acceleration at the rotor's maximum radius, in meters per second squared.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRotor speed (RPM) = 3000, Max rotor radius (cm) = 10, Min rotor radius (cm) = 5, Particle diameter (μm) = 1 = 4 input(s) provided
- Calculate RCF at rMaxRCF at rMax = 1.118e-5 * rotorRadiusCm * rpm * rpm1006.2 = 1006.2
- Calculate RCF at rMinRCF at rMin = 1.118e-5 * rotorRadiusMinCm * rpm * rpm503.1 = 503.1
- Calculate RCF ratio2 = 2
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does rotor speed affect RCF so much more than rotor radius does?
RCF is proportional to RPM squared but only to rotor radius to the first power, so a given percentage bump in RPM roughly doubles the resulting percentage change in RCF compared with an identical percentage bump in radius. Doubling RPM quadruples RCF, while doubling radius only doubles it -- which is why rotor speed is the input this calculator's RCF output responds to most.
Does the particle diameter you enter change the calculated RCF value?
No -- RCF depends only on rotor radius and rotational speed, since it describes the centrifugal force field the rotor generates, independent of what is being spun in it. Particle diameter is used exclusively for the separate, optional sedimentation-time estimate, which predicts how long a given particle takes to pellet under that force field.
Why does increasing the maximum rotor radius raise the calculated RCF?
Rotor radius is a direct positive linear term in the RCF formula (RCF = 1.118×10⁻⁵ × r × RPM²), so raising it raises RCF proportionally throughout the input's range, holding RPM constant. A rotor with a longer arm to the bottom of the tube generates more centrifugal force at the same rotational speed.
Why does a smaller minimum rotor radius increase the k-factor?
The k-factor formula includes the natural log of maximum radius divided by minimum radius, so shrinking the minimum radius (increasing that ratio) raises the k-factor throughout the input's valid range -- meaning particles have a longer relative path to travel from the top of the liquid to the bottom of the tube, which is associated with slower pelleting for a given particle.
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