CNC Scallop Height Calculator

Scallop height is the small ridge left between adjacent ball end mill toolpaths. As the ball end mill's rounded profile passes back and forth across a workpiece at a given stepover, it leaves a wave-shaped ridge on the surface; the height of that ridge is the scallop height.

Ra is the "arithmetic mean roughness" — the average absolute deviation of the profile from the centerline, and the most commonly used roughness metric. Rz is the "ten-point mean roughness" — the average height difference between the five highest peaks and five lowest valleys, making it more sensitive to sharp peaks and deep valleys. As a rule of thumb, Rz ≈ 4×Ra, though the actual ratio varies by machining method.

General guidance: use 30-50% of tool diameter for roughing, 10-20% for finishing, and 5-10% or smaller for precision finishing. Smaller stepover gives a smoother surface but longer machining time. Estimate the roughness you need with this calculator first, then work backward to the right stepover.

The calculated value is theoretical. Actual roughness is affected by many factors: tool wear, machine rigidity and accuracy, material properties, cutting speed and feed rate, coolant use, and vibration. Real-world Ra is typically 20-50% higher than the theoretical value, so plan for a safety margin.

CAM software (such as Fusion 360 or Mastercam) may factor in additional variables like tool runout and surface curvature changes. This calculator gives the theoretical value for flat-plane milling, which is useful for quick estimates and understanding the underlying math. For production work, use your CAM software's recommendation combined with shop experience.