Centrifuge speed determines how efficiently particles separate, pellets form, or supernatants clarify. Two metrics describe that speed: revolutions per minute (RPM)—how fast the rotor spins—and relative centrifugal force (RCF, × g)—the actual acceleration the sample experiences. Understanding both ensures consistent protocols across different rotors and instruments.

1 | Key Terms

Term	Definition	Why It Matters
RPM	Rotor revolutions per minute	Direct read-out on the centrifuge display
RCF (× g)	Acceleration applied to the sample, expressed as a multiple of Earth’s gravity	Allows comparison of runs on rotors with different radii
Rotor radius (R)	Distance (cm) from the axis of rotation to the bottom of the sample tube	Larger radius → higher force at the same RPM

Sources from university biochemistry courses highlight that RCF is the only truly comparable metric across labs.

2 | The RPM-to-RCF Formula

RCF (× g)=1.118×10−5×Rcm​×(RPM)2 

• R = rotor radius in centimetres
• RPM = set speed on the instrument

Engineering manuals and academic appendices use this equation to size rotors for protocols ranging from cell pelleting to nanoparticle fractionation.

Quick example :

A fixed-angle rotor with a 10 cm radius at 12 000 RPM delivers :

RCF=1.118×10−5×10×(12000)2≈16100×g

3 | Why RCF Is the Safer Set-Point

Different rotors at the same RPM can exert dramatically different forces. One study showed a 14 000 RPM run producing 13 100 × g in a 6 cm mini-rotor yet 20 800 × g in a 9.5 cm swing-bucket—more than a 55 % increase. Always set the protocol by RCF when comparing between instruments or sharing SOPs.

4 | Choosing the Correct Speed

Application	Typical RCF Range	Typical Time	Notes
Platelet-rich plasma	100 × g – 300 × g (first spin) 400 × g – 750 × g (second spin)	5–17 min	Low force preserves platelet integrity.
Cell pelleting	300 × g – 1 500 × g	3–10 min	Adjust up for bacteria, down for fragile mammalian cells
Virus concentration	50 000 × g – 100 000 × g	1–2 h (ultra-centrifuge)	Requires sealed tubes and BSL-2 containment.
Protein precipitation	10 000 × g – 20 000 × g	15–30 min	Higher force compacts fine precipitates

5 | Safety and Biosafety Considerations

• Sealed buckets or safety cups minimize aerosol release with infectious or allergenic samples. CDC-endorsed practices load and unload rotors inside a biosafety cabinet whenever aerosols are possible.
• Balanced loads prevent vibration and premature bearing wear.
• Rotor inspection for hairline cracks must follow the manufacturer’s cycle-count or yearly schedule.

6 | Troubleshooting Speed-Related Issues

Symptom	Likely Cause	Corrective Action
Pellet too loose or incomplete	RCF too low or time too short	Increase force 10–20 % or extend run
Tube breakage	Excessive RCF or incompatible plastic	Verify tube rating exceeds planned × g
Sample overheating	High RPM without refrigeration	Use refrigerated rotor or shorten run
Rotor vibration	Load imbalance or worn spindle	Re-balance; inspect rotor and seals

7 | Calibration and Maintenance

• Verify displayed RPM with a handheld tachometer at least once a year or after major service.
• Record RCF calculations in the lab’s equipment log; many QA auditors request this documentation.
• Replace rubber seals and O-rings on sealed rotors per the manufacturer’s schedule to preserve vacuum integrity in ultra-centrifuges.

NIH standard operating procedures treat these steps as part of routine preventive maintenance.

8 | Quick-Reference Conversion Table

RPM	RCF (× g) at 8 cm	RCF (× g) at 10 cm	RCF (× g) at 12 cm
5 000	2 240	2 800	3 360
10 000	8 960	11 200	13 440
15 000	20 200	25 300	30 400
20 000	35 800	44 800	53 700

(Values calculated with the standard formula above; round to nearest 10 × g.)

• Setting centrifugation steps by RCF rather than RPM ensures reproducible results across different rotors.
• Always calculate force with the rotor’s actual radius and document it in the protocol.
• Adhering to biosafety guidance—sealed rotors, balanced loads, annual calibration—protects personnel and preserves sample integrity.
• Regular maintenance and accurate speed verification keep instruments reliable and extend rotor life.