Centrifuge RCF Calculator: RPM ↔ g Using Rotor Radius

A protocol says “pellet cells at 300 × g for 5 minutes.” You walk to the centrifuge, and the dial shows RPM — not g. You punch in 300, hit start, and wonder why the pellet is barely there. That is because 300 RPM on most benchtop rotors produces roughly 10 × g, not 300. A centrifuge RCF calculator converts between RPM (revolutions per minute) and RCF (relative centrifugal force, the “× g” number) using the rotor radius, so the force your sample actually sees matches what the protocol intended.

The most common mistake: ignoring rotor radius entirely and treating RPM and × g as interchangeable. They are not. Two centrifuges spinning at the same RPM produce different forces if the rotors have different radii. A microcentrifuge rotor with a 7 cm radius at 10,000 RPM gives ~7,800 × g. A large floor centrifuge with a 20 cm radius at the same 10,000 RPM gives ~22,400 × g. Same speed, nearly 3-fold difference in force. Always convert.

Once you have the correct RPM for your target RCF, label it on the centrifuge or in your protocol with both values and the rotor model. This way anyone repeating the protocol on a different centrifuge can recalculate for their rotor instead of blindly copying RPM.

RCF depends on two things: how fast the rotor spins (RPM) and how far the sample sits from the axis of rotation (radius in cm). The relationship is quadratic in RPM — doubling the speed quadruples the force. That is why small errors in RPM matter more than you might expect. Setting 4,000 RPM instead of 3,000 RPM does not increase force by 33%; it increases it by 78%.

Protocols written as “× g” are rotor-independent. Protocols written as “RPM” are not, and should always include the rotor model or radius so others can reproduce the run. If you inherit a protocol that says only “spin at 4,000 RPM,” ask which centrifuge was used. Without the radius, you cannot know the actual force applied.

One more subtlety: the radius that matters is the distance from the center of the rotor to the bottom of the tube (r_max), not the center of the liquid column. Most manufacturers specify r_max in the rotor manual. If you measure it yourself, measure from the spindle axis to the bottom of the tube holder with a ruler.

When you move a protocol between centrifuges, build a quick comparison table: write down the target RCF, then calculate the RPM needed for each rotor you might use. Keep this taped to the centrifuge lid or saved in your lab notebook. Here is what that looks like for a few common scenarios:

• 300 × g on a rotor with r = 15 cm → ~1,340 RPM
• 300 × g on a rotor with r = 10 cm → ~1,640 RPM
• 300 × g on a rotor with r = 7 cm → ~1,960 RPM
• 16,000 × g on a microcentrifuge (r = 7 cm) → ~14,300 RPM

Notice how the RPM changes by nearly 50% across rotors for the same 300 × g. If you just copy the RPM from one centrifuge to another, your pellet quality will vary — loose and resuspendable one day, packed and hard to resuspend the next, depending on which machine you grabbed.

Fixed-angle rotors hold tubes at a constant angle (typically 25–45°) relative to the spin axis. The effective radius is measured from the axis to the bottom of the tube at that angle. Swinging-bucket rotors let the tubes swing outward to horizontal during the run, so the effective radius is measured from the axis to the bottom of the bucket when fully extended.

For the same rotor body, a swinging-bucket configuration usually has a larger r_max than a fixed-angle insert. That means at the same RPM, the swinging-bucket version produces higher RCF. This catches people who swap between the two on the same centrifuge without recalculating — the pellet comes out tighter or looser than expected.

Fixed-angle rotors smear the pellet along the tube wall because the sedimentation path is angled. Swinging-bucket rotors produce a flat pellet at the very bottom. For cell pelleting, most protocols prefer swinging-bucket because the pellet is easier to aspirate supernatant from without disturbing it. For DNA/protein precipitations where pellet shape does not matter, fixed-angle is fine and often faster.

The protocol says × g but my centrifuge only displays RPM. How do I set it?
Look up the rotor radius (r_max in cm) from the rotor manual or the label on the rotor itself. Plug it into the RCF formula with your target × g and solve for RPM. Or use this calculator — that is exactly what it does. Write both the RPM and the target × g in your notebook.

My centrifuge has an RCF mode. Can I just enter the × g value directly?
Yes, but only if the centrifuge knows which rotor is installed. Many modern centrifuges auto-detect the rotor or let you select the rotor model from a menu. If the centrifuge does not know the radius, the RCF mode is meaningless — it might assume a default radius that does not match your actual rotor.

Does temperature affect the conversion?
Temperature does not change the RPM-to-RCF math, but it affects sample viscosity. A cold spin (4°C) makes aqueous solutions slightly more viscous, so particles sediment a bit slower at the same RCF compared to room temperature. For most cell pelleting protocols, this difference is negligible. For gradient ultracentrifugation, temperature control matters much more.

I accidentally spun my cells at 3,000 × g instead of 300 × g. Are they ruined?
Possibly. Many mammalian cells lyse or suffer membrane damage above ~500 × g. Check viability by trypan blue after resuspending. If viability dropped significantly, discard and start fresh. Bacteria and yeast tolerate much higher forces — 5,000–10,000 × g is routine for bacterial pellets.

The full derivation starts from centripetal acceleration and converts units to lab-friendly form:

Units note: r must be in centimeters, RPM in revolutions per minute, and g = 980.665 cm/s². The constant 1.118 × 10⁻⁵ absorbs the unit conversions. If you accidentally enter radius in millimeters, the RCF will be 10-fold too high.

Scenario: You need to pellet PBMCs at 300 × g for 10 minutes. Your lab has two centrifuges: a benchtop with a swinging-bucket rotor (r_max = 17.5 cm) and a compact clinical centrifuge with a fixed-angle rotor (r_max = 8.5 cm). What RPM do you set on each?

Centrifuge A (r = 17.5 cm):
RPM = √(300 / (1.118 × 10⁻⁵ × 17.5))
RPM = √(300 / 0.00019565) = √1,533,248 ≈ 1,238 RPM.

Centrifuge B (r = 8.5 cm):
RPM = √(300 / (1.118 × 10⁻⁵ × 8.5))
RPM = √(300 / 0.00009503) = √3,156,898 ≈ 1,777 RPM.

Result: Same 300 × g force, but centrifuge A needs ~1,240 RPM while centrifuge B needs ~1,780 RPM. If you set 1,240 RPM on centrifuge B, you would only get ~146 × g — roughly half the intended force. The pellet would be loose and you would lose cells when aspirating the supernatant.