CNC Stepover Calculator
Stepover is the sideways distance a CNC cutter moves between adjacent passes, and it is the single biggest lever on surface finish and cycle time. Enter your tool diameter and a stepover percentage to see the stepover distance in your units, the scallop (cusp) height a ball-nose tool would leave on a flat surface, and how many passes it takes to cover one tool width. The scallop formula is pure geometry: the cusp is what remains between two circular tool paths, so the math is exact and does not depend on any material property.
Stepover and scallop formula
Radius R = tool diameter / 2
Stepover s = diameter * (percent / 100)
Scallop height h = R - sqrt(R^2 - (s / 2)^2)
Passes per tool width = diameter / s
The scallop height applies to a ball-nose cutter finishing a flat surface. It is the height of the cusp left midway between two adjacent passes. For a flat-bottomed end mill cutting a flat floor, scallop height is effectively zero and stepover instead governs cutting load.
Using stepover well
- Roughing with flat end mills commonly uses 40 to 50 percent stepover to clear material quickly.
- Finishing with ball-nose tools commonly uses 5 to 15 percent to keep scallops small.
- Scallop height falls roughly with the square of stepover, so small reductions sharply improve finish.
- Units are unitless in the ratio: enter diameter in mm or inches and outputs follow the same unit.
- Very large stepover (above tool diameter) leaves uncut material and is not valid here.
CNC stepover: frequently asked questions
What is stepover in CNC machining?
Stepover is the horizontal distance the cutting tool shifts sideways between adjacent passes. It is usually expressed as a percentage of the tool diameter. A smaller stepover leaves a smoother surface but takes more passes and more time; a larger stepover removes material faster but leaves a rougher finish with taller ridges (scallops).
How do I calculate scallop height for a ball-nose end mill?
For a flat surface machined with a ball-nose cutter, scallop height h = R - sqrt(R^2 - (stepover/2)^2), where R is the ball radius (half the tool diameter). The cusps left between adjacent passes form the scallop. Reducing stepover lowers scallop height nonlinearly, so halving stepover reduces scallop by roughly a factor of four.
What stepover percentage should I use?
A common roughing stepover is 40 to 50 percent of tool diameter for flat end mills. For finishing with ball-nose tools, machinists often use 5 to 15 percent to achieve a fine surface. The right value depends on material, finish requirements, tool deflection, and machine rigidity, so this calculator lets you enter any percentage.
Does stepover differ for flat versus ball-nose end mills?
Yes. With a flat end mill cutting a flat floor, there is effectively no scallop between passes when stepover is below tool diameter, so stepover mainly affects sidewall blending and load. With a ball-nose tool, every pass leaves a curved cusp, so stepover directly sets the scallop height of the finished surface.
How does stepover affect machining time?
Machining time is roughly inversely proportional to stepover for a given area. Cutting at 10 percent stepover instead of 20 percent doubles the number of passes and roughly doubles the time. The calculator reports passes per tool width so you can weigh finish quality against cycle time.
Official sources
- National Institute of Standards and Technology: NIST, machining and measurement standards.
- American Society of Mechanical Engineers: ASME, surface texture and tooling standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.