ToolnozaMINING TOOL
Mill Critical Speed Calculator
Estimate theoretical mill critical speed and operating RPM from internal diameter, grinding-ball size and selected critical-speed percentage.
Mill Critical Speed Calculator
FREE ONLINE TOOLSTEP BY STEP
How to use mill critical speed calculator
- Check the input units and assumptions.
- Review the result and example.
IN PRACTICE
Example
D = 4 m; d = 50 mm → Nc = 21.28 RPM; 75% → 15.96 RPM.
GOOD TO KNOW
Can I use the outside mill diameter?
No. Use effective inside diameter at the liner surface. Outside shell dimensions overstate the available radius.
Formula and worked example
Nc (RPM) = (60/2π) × √[2g/(D − d)]; g = 9.80665 m/s²; d (m) = d (mm)/1000; Ntarget = Nc × fraction/100.
D = 4 m; d = 50 mm → Nc = 21.28 RPM; 75% → 15.96 RPM.
How to interpret the result
Critical speed is the ideal speed at which a ball at the shell can remain held against it by centrifugal acceleration. Grinding normally requires relative charge movement, so critical speed is a reference rather than a target.
A liner replacement changes effective diameter. Use internal dimensions rather than the outside shell diameter, and compare using the same ball-size convention.
Frequently asked questions
Can I use the outside mill diameter?
No. Use effective inside diameter at the liner surface. Outside shell dimensions overstate the available radius.
What assumptions limit this calculation?
Ideal single-ball gravity/centrifugal balance. D must exceed ball diameter. Charge shape, lifters, slurry and wear are excluded. The selected percentage is an input, not an operating recommendation.
How to Calculate Mill Critical Speed: Formula & Example