This centrifuge calculator converts between rotor speed (RPM) and relative centrifugal force (RCF, in × g) for your rotor radius — the translation every protocol needs when it specifies one unit and your centrifuge displays the other.

–

Why Protocols Should Speak in × g

RCF = 1.118 × 10⁻⁵ × r(cm) × RPM² — force grows with the square of speed and linearly with radius, so 10,000 RPM is a gentle spin in a microfuge and a serious force in a floor centrifuge. RPM alone is meaningless across rotors; × g is the transferable physical quantity, which is why methods written in RPM don’t reproduce between labs. Sample prep’s other great translator is our Dilution Calculator. Part of our Biology Calculators collection.

Worked Examples (Verified)

  • 10,000 RPM, r = 10 cm: RCF = 1.118×10⁻⁵ × 10 × 10⁸ = 11,180 × g — the widget’s default, a typical cell-pelleting regime
  • The same force on a bigger rotor (r = 15 cm): needs only √(11,180 ÷ (1.118×10⁻⁵ × 15)) ≈ 8,165 RPM — radius is leverage
  • Target 10,000 × g at r = 15 cm: 7,722 RPM (reverse mode)
  • Microfuge reality: 13,000 RPM at r = 7.3 cm ≈ 13,800 × g — the “full speed” of a benchtop legend, two multiplications demystified

Practical Rotor Sense

  • Measure radius to the pellet position (tube bottom at full swing for swinging buckets; mid-tube conventions vary — manufacturers publish rmin/rmax per rotor).
  • Fixed-angle vs swinging-bucket: the effective radius and the pellet’s location differ; RCF quoted for one geometry misleads in the other.
  • Speed derating is real: rotors lose rated maximums with age, corrosion and dense loads — the manual’s derating table outranks any calculator.
  • k-factor (clearing time) builds on the same physics for ultracentrifugation run planning.

Related Tools

Frequently Asked Questions

How do I convert RPM to RCF?

RCF (× g) = 1.118 × 10⁻⁵ × radius (cm) × RPM². At 10,000 RPM with a 10 cm rotor: 11,180 × g. You must know your radius — it’s printed in the rotor manual and often on the rotor itself.

Why can’t protocols just use RPM?

Because the same RPM produces different forces in different rotors (radius enters the formula, and speed is squared). A protocol at “3,000 RPM” delivers ~1,000 × g in one common rotor and ~1,800 × g in another — cells notice the difference even when write-ups don’t.

Which radius do I measure?

From the center of the rotor axis to the point of interest in the tube — conventionally the bottom (rmax, where the pellet sits) for the headline RCF. For gradients, rmin (top of the column) bounds the gentlest force.

Is RCF the same as g-force?

Yes — RCF expressed in × g is exactly “how many Earth gravities of acceleration the sample experiences” at that radius. 11,180 × g means sedimentation behaves as if gravity were 11,180 times stronger.

What RCF pellets mammalian cells?

Typically 200–500 × g for 5 minutes — gentle, because cells are fragile and nearly buoyant; bacteria need ~3,000–5,000 × g, subcellular fractions tens of thousands, and viruses ultracentrifuge territory (100,000+ × g). Err low and longer for viability.

Is this centrifuge calculator free?

Yes — free, instant, no sign-up.