Centrifugation Calculator

Centrifugation is one of the most fundamental separation techniques in molecular biology, biochemistry, and clinical laboratories. By applying centrifugal force, researchers can separate particles based on size, density, and shape. Understanding the relationship between rotational speed (RPM), rotor radius, and the resulting force (RCF) is critical for reproducible protocols and successful experiments.

Why Centrifugation Matters in Research

• Cell Harvesting: Pelleting cells from culture media or biological samples for downstream analysis, lysis, or storage.
• Protein Purification: Separating cellular debris, precipitated proteins, or clearing lysates before chromatography.
• DNA/RNA Extraction: Pelleting nucleic acids after precipitation or separating phases in extraction protocols.
• Subcellular Fractionation: Differential centrifugation to isolate organelles, nuclei, mitochondria, or membrane fractions.
• Clinical Diagnostics: Separating blood components, preparing serum/plasma, or concentrating samples for testing.

The Physics of Centrifugation

When a centrifuge spins, it creates a centrifugal force that pushes particles outward from the center of rotation. This force is described by the equation:

RCF = 1.118 × 10⁻⁵ × r × RPM²

Where RCF is measured in units of gravity (× g), r is the rotor radius in centimeters, and RPM is revolutions per minute. This equation shows that RCF increases with the square of RPM, meaning doubling the speed quadruples the force.

The centrifugal force acts on particles according to their mass, causing denser and larger particles to sediment faster than smaller, less dense particles. This differential sedimentation is the basis for all centrifugation-based separations.

Fixed-Angle vs. Swing-Bucket Rotors

Fixed-angle rotors hold tubes at a fixed angle (typically 25-45°) during centrifugation. Particles travel along the tube at an angle and collect against the side of the tube. These rotors are ideal for quick pelleting and are commonly used for routine applications like cell harvesting and clearing lysates. The shorter sedimentation path makes them faster for most applications.

Swing-bucket rotors hold tubes vertically at rest but swing out to horizontal during centrifugation. Particles sediment straight down to the bottom of the tube, creating tight pellets. These rotors are preferred for density gradient centrifugation and when pellet integrity is critical, such as for enzyme assays or when you need to carefully resuspend the pellet.

Time-Speed Equivalents

The K-factor method allows you to calculate equivalent centrifugation conditions when changing rotors or speeds. The principle is that the product of time, the square of angular velocity, and radius remains constant for equivalent separations:

K = t × ω² × r (constant)

This is particularly useful when scaling up or down, switching to a different centrifuge, or when published protocols use different equipment than you have available. However, remember that fixed-angle and swing-bucket rotors have different sedimentation paths, so exact equivalence between rotor types is approximate.

Frequently Asked Questions

What is the difference between RPM and RCF?

RPM (revolutions per minute) measures rotational speed, while RCF (relative centrifugal force, measured in × g) measures the actual force applied to samples. RCF depends on both RPM and rotor radius, making it the more accurate way to report centrifugation conditions for reproducibility across different centrifuges.

Why should I report centrifugation in RCF instead of RPM?

Reporting in RCF ensures reproducibility because different centrifuges have different rotor radii. The same RPM on two different rotors produces different centrifugal forces. RCF accounts for this variation, allowing researchers worldwide to reproduce your protocol regardless of their centrifuge model.

How do I find my rotor radius?

The rotor radius (r) is measured from the center of rotation to the bottom of the tube at maximum speed. This information is found in your rotor's manual or specification sheet. For fixed-angle rotors, measure to the tube bottom; for swing-bucket rotors, measure to the tube bottom when horizontal at full speed.

What causes sample heating during centrifugation?

Friction between the spinning rotor and air generates heat. Long centrifugation runs or high speeds can significantly warm samples, potentially denaturing proteins or affecting enzyme activity. Use refrigerated centrifuges for heat-sensitive samples, and allow the rotor to cool between runs. Pre-cooling tubes and samples also helps.

Why is balancing the centrifuge important?

An unbalanced rotor causes vibration and uneven forces that can damage the centrifuge, shear samples, or cause tube breakage. Always balance tubes by weight (not just volume) to within 0.1 grams for microcentrifuges and 1 gram for larger centrifuges. Place balanced tubes opposite each other in the rotor.

What's the difference between microcentrifuges and standard centrifuges?

Microcentrifuges (microfuges) handle 0.2-2.0 mL tubes at high speeds (typically up to 15,000-20,000 × g) and are ideal for quick spins and small-volume work. Standard benchtop or floor centrifuges handle larger tubes (15-50 mL) and volumes but typically reach lower maximum RCF values. Ultracentrifuges can reach 100,000-1,000,000 × g for specialized applications.

Common Centrifugation Applications and Conditions

• Quick spin (brief centrifugation): 1,000-2,000 × g for 30-60 seconds to collect liquid at tube bottom
• Bacterial cell harvest: 3,000-5,000 × g for 10-15 minutes at 4°C
• Mammalian cell harvest: 200-500 × g for 5-10 minutes (gentler to avoid cell damage)
• Clearing cell lysates: 12,000-15,000 × g for 10-20 minutes at 4°C
• DNA precipitation (ethanol/isopropanol): 12,000-15,000 × g for 10-30 minutes at 4°C
• Protein precipitation: 10,000-15,000 × g for 10-20 minutes at 4°C
• PCR cleanup/plasmid prep spin columns: 12,000-16,000 × g for 1-2 minutes
• Blood separation (serum/plasma): 1,000-2,000 × g for 10 minutes at room temperature

Safety and Maintenance

Never open a centrifuge while it's spinning. All modern centrifuges have lid locks that prevent opening during operation. If you hear unusual noises or vibrations, stop the centrifuge immediately and check for proper balancing or damaged tubes.

Inspect tubes for cracks before use, as tubes can fail under high centrifugal force. Use only tubes rated for your centrifuge's maximum speed. Check rotor O-rings regularly and replace them if they show wear. Clean spills immediately, as unbalanced dried residue can cause vibration in subsequent runs.

Keep maintenance logs for each centrifuge, documenting rotor usage hours. Rotors have finite lifespans and must be retired after reaching manufacturer-specified limits to prevent catastrophic failure. Annual professional servicing is recommended for critical equipment.