Centrifuge Calculator
Calculate Relative Centrifugal Force (RCF), RPM, or rotor radius for your centrifuge. Enter any two values to find the third.
Select the value you want to calculate, then enter the other two parameters to get an instant result.
Centrifuge Calculator
Calculate Relative Centrifugal Force (RCF), RPM, or rotor radius for your centrifuge. Enter any two values to find the third.
About the Centrifuge Calculator
A centrifuge is one of the most widely used instruments in biology, biochemistry, medicine, and chemistry laboratories. It works by spinning samples at high speeds to generate centrifugal forces that accelerate the sedimentation of particles based on their size, shape, density, and the viscosity of the surrounding medium. By carefully controlling the rotational speed and the geometry of the rotor, scientists can separate complex mixtures into distinct fractions for analysis, purification, or processing.
Relative Centrifugal Force (RCF), also referred to as g-force, is the standardized measure of centrifugation intensity. It expresses the centrifugal force acting on the sample contents as a multiple of Earth's standard gravitational acceleration (g = 9.81 m/s²). For example, an RCF of 1000 × g means the sample is experiencing a force 1000 times greater than gravity alone. Using RCF rather than RPM ensures that protocols are reproducible across different centrifuge models and rotor types, because the actual force depends on both speed and rotor radius.
The relationship between RCF, rotor speed (RPM), and rotor radius (r) is defined by the formula: RCF = 1.118 × 10⁻⁵ × r (cm) × RPM². This formula is derived from the fundamental centripetal acceleration equation a = ω²r, where ω is the angular velocity in rad/s. Converting RPM to rad/s and radius to centimetres, and dividing by g = 980 cm/s², yields the constant 1.118 × 10⁻⁵. The rotor radius used in this formula should be measured from the centre of the rotor spindle to the bottom of the sample tube, not to the top of the tube or to the outer edge of the rotor.
This calculator can solve for any one of the three variables when the other two are known. To find RCF from RPM and radius, enter the RPM and radius and select 'RCF.' To find the RPM needed to achieve a specific g-force, enter the target RCF and the rotor radius and select 'RPM.' To find the minimum rotor radius required to achieve a specific RCF at a given RPM, enter the RCF and RPM and select 'Radius.'
Common centrifugation protocols use a wide range of RCF values depending on the application. Low-speed centrifugation (100–600 × g) is used to pellet whole cells, yeast, and large cellular debris. Medium-speed centrifugation (1,000–15,000 × g) is used for bacterial cell pellets, mitochondria, and membrane fragments. High-speed centrifugation (15,000–100,000 × g) separates microsomes, ribosomes, and viral particles. Ultracentrifugation (100,000–500,000 × g) is used for separating subcellular organelles, large macromolecular complexes, and analytical density-gradient separations.
Accuracy is critical in centrifugation. Using an incorrect RCF — for example, confusing the maximum radius with the average or minimum radius — can result in incomplete separation, loss of sample, or damage to fragile biological structures. Always measure the radius from the rotor centre to the bottom of the tube in its loaded position, as this is where the sample pellets. Tubes should always be balanced in opposing positions within the rotor to prevent vibration and potential rotor failure at high speeds.
Centrifuge Calculator Examples
Common laboratory centrifugation protocols and their calculated parameters.
| Known Parameters | Result | Protocol / Application |
|---|---|---|
| RPM = 3000, Radius = 85 mm → RCF | RCF ≈ 855 × g | Cell pelleting protocol. Low-speed spin to sediment whole mammalian cells from culture medium while leaving debris in suspension. |
| RCF = 12000 × g, Radius = 85 mm → RPM | RPM ≈ 11,241 | High-speed spin for bacterial cell pellet. Protocol specifies 12,000 × g; the calculator gives the exact RPM to set on the centrifuge. |
| RCF = 500 × g, RPM = 1500 → Radius | Radius ≈ 198.8 mm | Find minimum rotor radius needed to achieve 500 × g at 1500 RPM. r = RCF / (1.118×10⁻⁵ × RPM²) × 10 mm — useful when selecting a rotor for a protocol. |
| RPM = 50000, Radius = 50 mm → RCF | RCF ≈ 139,750 × g | Ultracentrifuge run at 50,000 RPM with a 50 mm rotor. Forces of this magnitude separate ribosomes and large protein complexes efficiently. |
How to Use the Centrifuge Calculator
- Choose what to calculate: RCF (g-force), RPM (speed), or Radius. Click the corresponding button at the top of the calculator.
- Enter the two known values. For RCF, enter RPM and Radius (mm). For RPM, enter RCF and Radius (mm). For Radius, enter RCF and RPM.
- Measure the rotor radius as the distance from the centre of the rotor spindle to the bottom of the sample tube in its loaded position for greatest accuracy.
- Click Calculate. The result appears in the appropriate unit: × g for RCF, RPM for speed, or mm for radius.
- Use the calculated value to set your centrifuge, document your protocol, or verify that an existing protocol meets the specified g-force requirements.
Centrifuge Calculator FAQ
What is RCF and why use it instead of RPM?
RCF (Relative Centrifugal Force) is the force experienced by the sample contents, expressed as a multiple of Earth's gravitational acceleration. RPM (Revolutions Per Minute) measures only the rotational speed, not the actual force applied to the sample. Because centrifugal force depends on both RPM and rotor radius, the same RPM in two different centrifuges or rotors will generate different RCF values. Scientific protocols specify RCF to ensure reproducibility across instruments — a protocol specifying 3000 × g will produce the same separation regardless of which centrifuge or rotor is used.
How do I measure rotor radius accurately?
The rotor radius should be measured from the centre of the rotor spindle (the rotational axis) to the bottom of the sample tube when the tube is seated in the rotor bucket at its operating position. This is the distance at which the sample will actually pellet. Using the radius to the tube mouth or the outer edge of the rotor will overestimate the RCF. For swinging-bucket rotors, the radius changes during acceleration as the buckets swing outward, so use the radius at full horizontal position for the maximum RCF calculation.
What RPM should I use for cell culture pelleting?
Most mammalian cell culture protocols pellet cells at 200–400 × g for 5–10 minutes. This gently sediments cells without lysing them or compacting the pellet too tightly. For a rotor with 85 mm radius, 200 × g corresponds to about 1456 RPM, and 400 × g corresponds to about 2060 RPM. Use this calculator to find the exact RPM for your rotor. Bacterial cells, being smaller and denser, typically require 3000–5000 × g to pellet efficiently.
What is the difference between a fixed-angle and swinging-bucket rotor?
In a fixed-angle rotor, tubes are held at a constant angle (typically 25–45°) to the rotor axis during centrifugation. The sedimentation path is short because particles travel to the tube wall quickly, making fixed-angle rotors fast and efficient for pelleting. In a swinging-bucket rotor, buckets pivot to a horizontal position during spinning, so particles travel the full length of the tube to the bottom. Swinging-bucket rotors are preferred for density-gradient separations because banding is sharper. The effective radius differs between the two designs, affecting RCF calculations.
Can I convert g-force to RPM for any centrifuge?
Yes, but you need the rotor radius of your specific centrifuge model. The formula RPM = √(RCF / (1.118 × 10⁻⁵ × r_cm)) gives the exact speed. Enter the target RCF and the radius of your rotor into this calculator, select 'RPM,' and get the answer instantly. If you do not know your rotor radius, consult the centrifuge manufacturer's documentation or measure it directly with a ruler from the rotor spindle centre to the tube bottom.
Why does ultracentrifugation require special equipment?
Ultracentrifuges operate at 50,000–150,000 RPM, generating forces of 100,000–1,000,000 × g. At these extremes, the rotor experiences enormous stress from centrifugal forces acting on its own mass, requiring precision-engineered titanium or carbon-composite rotors. The rotor spins in a vacuum chamber to eliminate air friction and heat, and sophisticated imbalance detection systems shut the instrument down if any tube is slightly unbalanced. Routine laboratory centrifuges are not designed for these speeds and would fail catastrophically if run beyond their rated maximum RCF.