Laser Cutter Kerf Compensation Calculator

Laser cutting removes a narrow strip of material along the toolpath. That removed width is the kerf (K). If you design a part at the “finished” size but do not compensate for kerf, the cut part will come out slightly too small on external profiles, and holes/slots will come out slightly too large on internal cuts. This calculator converts your target (original) dimension into a compensated design dimension based on the cut type.

What this calculator does

• Inputs: kerf width K (mm), target dimension D (mm), and cut type (external or internal).
• Output: compensated design dimension D_c (mm) that you should draw/offset so the cut result matches your target D.

Core idea (why it’s D ± K)

Kerf is effectively removed on both sides of the cut centerline. On a straight edge, the cut boundary shifts by roughly K/2 relative to the centerline. For a complete external profile (outside of a part), that shift happens on opposite sides, so the overall finished size changes by about one full kerf width K compared to your drawn dimension. The same logic applies to internal features like holes and slots, but in the opposite direction.

Formulas

Let:

• K = kerf width (mm)
• D = desired/target finished dimension (mm)
• D_c = compensated dimension to draw (mm)

External cut (outside profile): increase the design so the cut part matches the target size.

Internal cut (holes/slots): decrease the design so the cut opening matches the target size.

These equations assume the kerf is approximately symmetric about the laser path and that you’re entering the kerf as the total measured cut width.

Interpreting the result

• If you choose External Cut and the calculator returns D_c > D, that means you should draw the part slightly larger so the finished part shrinks back to the target after kerf removal.
• If you choose Internal Cut and the calculator returns D_c < D, that means you should draw the hole/slot slightly smaller so the finished opening grows to the target after cutting.

Worked example

Suppose you want a tab that is D = 50.00 mm wide and you measured your machine’s kerf on this material as K = 0.15 mm.

• External profile (cutting the outside of the tab):
  D_c = 50.00 + 0.15 = 50.15 mm. Draw the tab at 50.15 mm so the cut tab ends up near 50.00 mm.
• Internal slot (cutting a slot that should be 50.00 mm):
  D_c = 50.00 − 0.15 = 49.85 mm. Draw the slot at 49.85 mm so the cut slot ends up near 50.00 mm.

Quick comparison

How to measure kerf (practical method)

1. Cut a simple test shape (commonly a square or a rectangle) in the same material, thickness, and settings you’ll use for the real job.
2. Measure the resulting part with calipers and compare it to the design size.
3. The difference between the drawn dimension and the cut dimension is an estimate of K for that setup.
4. Record kerf values by material + thickness + settings; kerf can change with focus, power, speed, lens, and assist gas.

Assumptions & limitations

• Kerf varies: K is not a universal constant; it changes with material, thickness, optics, focus height, power/speed, and condition of the lens/nozzle.
• Symmetry assumption: The calculator assumes kerf is symmetric around the cut path. Some situations (tilted beam, poor focus, heavy taper) can break this.
• Single-pass, typical profiles: Multi-pass cuts, heavy melt, or significant taper can change the effective kerf at different depths.
• Geometry effects: Tight corners and small holes can deviate due to acceleration limits and heat buildup; effective kerf may differ from straight-line tests.
• Fit tuning still needed: For press-fits, finger joints, and precision assemblies, expect to cut small test coupons and adjust beyond pure kerf compensation to account for material compliance and heat-affected edges.
• Units: Inputs are in millimeters. If you measure in inches, convert to mm before entering values.

If your laser/CAM software supports kerf compensation (toolpath offset), you can use this calculator as a cross-check: the concept is the same—offset external contours outward and internal contours inward by K/2 per side, resulting in a net dimension change of about K across the full feature.