User Rating 0.0 ★★★★★

Total Usage 0 times

Category Science & Math

Rotor Radius

Distance from axis to tube bottom (r_max)

Speed (RPM)

rev/min

Force (RCF)

× g

Enter values and press Calculate

Quick Presets

Sedimentation Estimator (Stokes' Law)

Particle Diameter

μm

Particle Density

kg/m³

Fluid Density

kg/m³

Fluid Viscosity

Pa·s

Sedimentation Path

Liquid column length from meniscus to tube bottom

Is this tool helpful?

Your feedback helps us improve.

★ ★ ★ ★ ★

About

Misconfigured centrifuge settings destroy samples and invalidate experiments. The relationship between rotor speed (n, in RPM) and relative centrifugal force (RCF, in multiples of g) is nonlinear and depends on rotor radius (r). A 10% error in radius input produces a 10% error in calculated force. This tool computes bidirectional RPM - RCF conversion using the standard coefficient 1.118 × 10⁻⁵ and estimates particle sedimentation velocity via Stokes' law. It assumes Newtonian fluid, spherical particles, and laminar flow. Results approximate ideal conditions. Real separations depend on sample viscosity, temperature, and rotor geometry (fixed-angle vs. swinging-bucket).

Formulas

The relative centrifugal force experienced at the rotor tip is:

RCF = 1.118 × 10⁻⁵ × r × n²

Solving for rotational speed:

n = √RCF1.118 × 10⁻⁵ × r

Stokes' law gives the terminal settling velocity of a spherical particle in a viscous medium:

v = 2 r_p² ⋅ (ρ_p − ρ_f) ⋅ RCF ⋅ g9 η

Where r = rotor radius in cm, n = rotational speed in RPM, RCF = relative centrifugal force in multiples of g, r_p = particle radius in m, ρ_p = particle density in kg/m³, ρ_f = fluid density in kg/m³, η = dynamic viscosity in Pa⋅s, g = 9.80665 m/s². The constant 1.118 × 10⁻⁵ derives from converting angular velocity (2πn/60)² divided by g with radius in centimetres.

Reference Data

Protocol / Sample	Typical RCF (g)	Time	Temp	Notes
Human blood - serum separation	1,500 - 2,000	10 - 15 min	20°C	Gel or plain tube; no brake
Platelet-rich plasma (PRP)	200 - 300	10 min	20°C	Soft spin first pass
Platelet-poor plasma (PPP)	2,000 - 3,000	15 min	20°C	Hard spin second pass
Cell culture pellet	300 - 500	5 - 10 min	4°C	Gentle; avoid lysis
Bacterial pellet (E. coli)	3,000 - 5,000	10 - 15 min	4°C	Standard harvest
Mitochondria isolation	10,000 - 12,000	10 min	4°C	Differential centrifugation
Microsome fraction	100,000	60 min	4°C	Ultracentrifuge required
DNA precipitation (ethanol)	12,000 - 16,000	15 - 30 min	4°C	Pellet may be invisible
RNA extraction (column spin)	12,000	1 min	20°C	Per manufacturer protocol
Protein lysate clarification	14,000 - 16,000	10 - 20 min	4°C	Remove debris before SDS-PAGE
Exosome isolation	100,000 - 120,000	70 - 120 min	4°C	Ultracentrifuge; multiple washes
Virus concentration	80,000 - 100,000	90 min	4°C	Sucrose cushion optional
Urine sediment	400 - 500	5 min	20°C	Clinical urinalysis
Saliva processing	2,600	15 min	4°C	Remove mucins and debris
Yeast cell harvest	3,000 - 4,000	5 min	4°C	Thick cell wall; tolerates higher g
Red blood cell wash	500 - 1,000	5 min	4°C	Saline wash × 3
Sucrose gradient (density)	100,000	120 - 240 min	4°C	Rate-zonal or isopycnic
PCR cleanup (spin column)	17,900	1 min	20°C	Max speed on benchtop micro

Frequently Asked Questions

RCF depends on both RPM and rotor radius. A larger rotor at the same RPM generates higher g-force because RCF = 1.118 × 10⁻⁵ × r × n². Always report protocols in RCF (× g), not RPM, to ensure reproducibility across labs with different equipment.

For fixed-angle rotors, use r_max (distance from axis to the bottom of the tube) to report maximum force experienced by the pellet. For swinging-bucket rotors, r_max is also standard. Some protocols specify r_avg (midpoint of the liquid column) for sedimentation time estimates. Check your rotor manual; manufacturers print r_min, r_avg, and r_max in the specifications.

Temperature changes fluid viscosity (η). Water viscosity at 4 °C is approximately 1.55 × 10⁻³ Pa·s versus 1.00 × 10⁻³ Pa·s at 20 °C - a 55% increase. Higher viscosity slows sedimentation proportionally. Biological protocols specify 4 °C to inhibit proteases, not to optimise pelleting speed. Compensate by increasing time, not RCF, to avoid damaging heat-sensitive samples.

Every rotor has a rated maximum RPM printed by the manufacturer. Exceeding it risks catastrophic mechanical failure. Derate by 10-20% when using adapters, partially filled tubes, or solutions denser than water (e.g., CsCl gradients at ρ = 1.7 g/cm³). Never run an unbalanced rotor; mass difference between opposing tubes must be within 0.1-0.5 g depending on rotor class.

Within limits, yes. Sedimentation distance equals velocity × time, and velocity is proportional to RCF. Halving RCF roughly doubles the required time. However, extended spin times increase thermal exposure and may activate enzymatic degradation. For mammalian cells, 300 × g for 5-10 minutes is the accepted gentle standard. Going below 200 × g often fails to pellet cells of diameter < 10 µm in reasonable time.

Common causes: (1) particle density is too close to fluid density (Δρ ≈ 0), producing near-zero sedimentation velocity per Stokes' law; (2) particles are too small - sedimentation velocity scales with r² of the particle; (3) viscosity is too high (glycerol stocks, dense sucrose); (4) tube angle or rotor type mismatches the protocol. Try increasing time, raising RCF, reducing sample viscosity by dilution, or switching to an ultracentrifuge for sub-micron particles.