Centrifuge Calculator: Convert RCF ↔ RPM Accurately
Need a fast way to jump between RCF (×g) and RPM for your rotor and tubes? This practical guide shows you the formula, the caveats that matter in the lab, and handy reference tables so you can dial in the right setting with confidence.
RCF vs RPM in plain English
RPM is speed. It tells you how many revolutions the rotor makes in one minute.
RCF is force. It expresses acceleration on the sample as multiples of Earth’s gravity (×g). RCF travels well across instruments because it includes the radius.
Two rotors can run at the same RPM yet deliver different ×g because their radii differ. That is why many protocols specify RCF instead of RPM.
The conversion formula you can trust
Use centimeters for the radius. Then apply:
RCF = 1.118 × 10⁻⁵ × rcm × RPM²
RPM = √( RCF / (1.118 × 10⁻⁵ × rcm) )
This comes from a = ω²r with ω = 2π·RPM/60 and g = 9.80665 m/s² baked into the constant.
- Convert inches or millimeters to centimeters before you calculate.
- RCF scales with the square of RPM. Small speed bumps can multiply force fast.
- Round final numbers sensibly after you compute. Keep one extra significant figure internally.
Visual: where radius comes from
How to use a centrifuge calculator
If you know RPM
- Measure radius in centimeters.
- Enter RPM and radius.
- Read RCF in ×g. Match the protocol target.
RPM alone does not guarantee the same force on a different rotor.
If you know RCF
- Enter radius in centimeters.
- Enter desired ×g.
- Read the required RPM. Check rotor speed limits.
Always verify that the calculated RPM sits below the rotor maximum.
Worked examples
Example 1 — RPM → RCF
Spin at 10,000 rpm with a radius of 10 cm.
RCF = 1.118×10⁻⁵ × 10 × 10,000² = 11,180 ×g
Double the speed and the force quadruples. That leap can change pellet texture.
Example 2 — RCF → RPM
Target 12,000 ×g at 8 cm.
RPM = √(12,000 ÷ (1.118×10⁻⁵ × 8)) ≈ 11,588 rpm
Round to the nearest step on the panel. Many instruments step by 10–100 rpm.
Example 3 — Same RPM, different radii
Two rotors run at 14,000 rpm. One holds microtubes at 5 cm. Another holds 50 mL tubes at 12 cm.
- 5 cm → RCF ≈ 1.118×10⁻⁵ × 5 × 14,000² ≈ 10,958 ×g
- 12 cm → RCF ≈ 1.118×10⁻⁵ × 12 × 14,000² ≈ 26,299 ×g
Same speed yet very different force. Radius rules the outcome.
Example 4 — Inch to centimeter conversion
Your manual lists radius as 4 inches. Convert first.
4 in × 2.54 = 10.16 cm. Use 10.16 in the formula.
Quick reference tables
Numbers below use the standard relation with r in centimeters. Values are rounded for readability. Tables are responsive on mobile.
| Radius | 3,000 rpm | 5,000 rpm | 10,000 rpm | 15,000 rpm | 20,000 rpm |
|---|---|---|---|---|---|
| 2 in (5.08 cm) | ~510 ×g | ~1,420 ×g | ~5,680 ×g | ~12,780 ×g | ~22,720 ×g |
| 4 in (10.16 cm) | ~1,020 ×g | ~2,840 ×g | ~11,350 ×g | ~25,540 ×g | ~45,400 ×g |
| 6 in (15.24 cm) | ~1,530 ×g | ~4,260 ×g | ~17,040 ×g | ~38,340 ×g | ~68,160 ×g |
| Target RCF (×g) | Required RPM (r = 10 cm) |
|---|---|
| 1,000 | ~2,990 rpm |
| 5,000 | ~6,683 rpm |
| 10,000 | ~9,460 rpm |
| 20,000 | ~13,379 rpm |
| 30,000 | ~16,384 rpm |
Measure rotor radius the right way
- Axis to sample. Measure from the center of rotation to the sample location at run position.
- Swinging buckets. Measure along the arc to the bottom of the tube when swung out.
- Fixed angle rotors. Measure to the tube bottom centerline at the installed angle.
- Use the manual. Many manuals list “radius to max fill line.” That value aligns with filled tubes.
- Be consistent. Enter the same radius every time when you repeat a protocol.
Common mistakes to avoid
Mixing units
The constant expects centimeters. Convert inches or millimeters first. That single habit kills most errors.
Copying RPM across rotors
Same speed does not mean same ×g. Radius changes the force. Always convert.
Ignoring rotor limits
Check the maximum speed rating for the exact rotor and buckets in use. Stay under that limit.
Rounding too early
Keep one extra significant figure inside the calculator. Round only the displayed result.
Using diameter
If you measured diameter halve it to get radius. Then calculate.
Wrong sample position
Measure to the sample location not to the rotor rim. The difference matters.
FAQ
What does ×g mean in practice?
It means how many times stronger the acceleration is compared to standard gravity. 10,000 ×g means the sample feels ten thousand times g.
Why do protocols prefer RCF?
RCF lets you reproduce force on any rotor. RPM alone changes the force when the radius changes.
Can I enter radius in inches or millimeters?
Yes. Convert to centimeters before you apply the formula. Many calculators convert behind the scenes.
Do I need to adjust for temperature?
Not for the conversion. The formula links kinematics and units. Temperature affects density and viscosity which affects separation behavior not the RCF calculation.
What about angular velocity?
Angular velocity is ω = 2π·RPM/60. If you want acceleration in m/s² use a = ω²·r with r in meters. RCF is then a/g.
Use these links to cross-check numbers if you need a second opinion. They use the same equations.
