Introduction
This centrifugation calculator helps researchers, lab technicians, and graduate students convert between RPM (revolutions per minute) and RCF (relative centrifugal force, ×g) using the rotor radius. It is essential whenever a published protocol specifies a force in ×g but your centrifuge display only shows RPM, or when moving a protocol between machines with different rotors.
Select a conversion direction with the tabs above, enter your rotor radius (or use a quick preset for a common rotor), enter the known speed value, and click Calculate to get an instant, rotor-specific result.
About the Tool
RPM describes how fast a rotor spins but does not by itself tell you the force applied to a sample — that depends entirely on the rotor radius. Two centrifuges running at the same RPM with different rotors apply completely different forces. RCF, expressed as ×g, is the universally comparable unit because it is independent of rotor size, which is why published protocols specify RCF rather than RPM.
This tool converts between the two in either direction, using your rotor's actual radius, so you can reproduce a published protocol accurately on your own instrument.
Input Explanation
The calculator takes two inputs, both of which must match your actual rotor and protocol:
Formula Explanation
RPM = √(RCF / (1.118 × 10⁻⁵ × r))
RCF = Relative Centrifugal Force, in ×g | r = rotor radius, in centimetres | RPM = revolutions per minute
The 1.118 × 10⁻⁵ coefficient comes from converting angular velocity (RPM) into centripetal acceleration and expressing that acceleration as a multiple of standard gravity. Because RCF scales with the square of RPM, doubling the rotor speed quadruples the force on the sample, while RCF scales only linearly with radius.
Worked Example
Result Interpretation
When converting RPM → RCF, the result tells you the actual gravitational force (×g) your sample will experience — compare this against the force specified in your protocol or against typical ranges for your application (see Practical Applications below). When converting RCF → RPM, the result is the speed setting to enter on your specific centrifuge to reproduce a published force.
A result flagged above 21,000 ×g exceeds the typical range of a benchtop microcentrifuge and likely requires a high-speed or ultracentrifuge rotor. A rotor radius outside roughly 40–350 mm is unusual for common lab centrifuges and is worth double-checking against your rotor's datasheet before relying on the result.
Practical Applications
Typical RCF ranges used for common laboratory separations, with approximate RPM equivalents on a mid-sized microcentrifuge rotor:
| Application | Typical RCF (×g) | Typical RPM* |
|---|---|---|
| Cell pelleting (mammalian cells) | 300 – 500 ×g | ~1,200 – 1,500 |
| Bacterial cell pelleting | 3,000 – 5,000 ×g | ~4,000 – 6,000 |
| DNA/RNA precipitation | 10,000 – 16,000 ×g | ~10,000 – 13,000 |
| Protein precipitation | 10,000 – 20,000 ×g | ~10,000 – 15,000 |
| Microcentrifuge max speed | 16,000 – 21,000 ×g | ~13,000 – 15,000 |
| Ultracentrifugation (virus pellet) | 100,000 – 200,000 ×g | ~35,000 – 50,000 |
* RPM values are approximate at r ≈ 107 mm (Eppendorf 5424 rotor)
Scientific Notes & Limitations
Some rotor manufacturers publish both an average radius (midpoint of the sample column) and a maximum radius (tube tip). Using the maximum radius gives the highest RCF a sample experiences; using the average radius gives a value closer to the typical force across the whole sample column. Confirm which figure a given protocol assumes before comparing results across instruments.
Temperature is not part of this calculation but matters for the underlying protocol — most biological samples are spun at 4°C, since heat generated during centrifugation can degrade sensitive proteins, RNA, or cell viability. This tool assumes ideal rigid-body rotation and a fixed radius; it does not account for rotor deceleration profiles, swinging-bucket geometry changes during a run, or manufacturer-specific rotor corrections, so always cross-check against your rotor's official specification sheet for critical work.
Practical Tips
- Use the quick preset buttons for common rotors when you don't have the exact radius on hand, then double-check against your rotor's datasheet for critical work.
- Double-check whether the radius you've entered is in mm or cm using the unit selector — a 10× unit error is a common source of wildly incorrect results.
- When moving a protocol to a new instrument, always re-convert from the published RCF value rather than reusing the old RPM number.
- Record both the RPM and RCF (×g) values for your protocol, since future users of your notes may have a different rotor.
Common Mistakes
- Confusing RPM with RCF: treating a rotor speed as if it were a force. The same RPM setting produces different forces on different rotors.
- Mixing up mm and cm: entering a radius value in the wrong unit produces a result that is off by a factor of 10.
- Ignoring rotor radius when switching instruments: copying an RPM number directly to a new machine without re-converting for that rotor's radius.
- Mixing average and maximum radius: using a different radius reference point than the one a protocol assumed, leading to a systematically high or low result.
Frequently Asked Questions
What is the difference between RPM and RCF?
RPM (revolutions per minute) is simply how fast the rotor spins, while RCF (relative centrifugal force, expressed as ×g) is the actual force applied to the sample. RCF depends on both RPM and the rotor radius, so the same RPM on two different rotors produces two different forces. Published protocols almost always specify RCF because it is comparable across machines, whereas RPM alone is meaningless without knowing the radius of the rotor being used.
Why do I need the rotor radius to convert RPM to RCF?
Centrifugal force increases with the distance from the axis of rotation, so a sample sitting farther from the centre experiences more force at the same rotational speed than one sitting closer to it. The radius term in the RCF formula accounts for this geometry. Without it, an RPM value alone cannot be translated into a force, which is why every rotor's datasheet lists its radius alongside its maximum speed.
What is the difference between average and maximum rotor radius?
Maximum radius is measured from the centre of rotation to the bottom (tip) of the tube, while average radius is measured to the midpoint of the sample column. Using the maximum radius gives the highest RCF the sample experiences, whereas the average radius gives a value closer to the typical force across the whole sample. Some rotor manufacturers publish both figures, so it's worth checking which one a protocol assumes before comparing results between instruments.
Can I use the same RPM setting on a different centrifuge or rotor?
Not reliably, because two rotors of different radii spinning at the same RPM apply different forces to the sample. To reproduce a protocol on a new machine, convert the published RCF value to the correct RPM for your specific rotor's radius, rather than copying the RPM number directly. This is the main reason RCF, not RPM, is considered the standard unit for centrifugation protocols.
What RCF values are typically used for common lab applications?
Gentle pelleting of mammalian cells is usually done around 300–500 ×g, bacterial cell pelleting around 3,000–5,000 ×g, and DNA, RNA, or protein precipitation around 10,000–20,000 ×g, which is near the maximum for most benchtop microcentrifuges. Isolating smaller structures such as mitochondria or membrane fractions typically requires higher forces, and pelleting viruses or ribosomes requires ultracentrifugation in the range of 100,000–200,000 ×g. Always confirm the exact force specified in your own protocol, since acceptable ranges vary by sample type and downstream use.