Waterjet Kerf Calculator
Entry and exit kerf, taper and the tool offset to program, for any material and thickness.
https://calculators.nirajiitr.com/hobbies/waterjet-kerf-calculator
How it works
The stream cuts slightly wider than the mixing tube that formed it, so kerf starts from the tube bore rather than the orifice. It then widens on the way down, because the jet loses energy as it goes and spreads: that difference between entry and exit is taper, and it is why a waterjet cut is very slightly V-shaped. Taper grows with thickness, grows in softer materials that let the stream spread, and shrinks when you slow the traverse down for a finer edge — which is most of what edge-quality settings actually change.
entry kerf ≈ nozzle bore × 1.05 offset = entry kerf ÷ 2- nozzle bore
- Internal diameter of the mixing tube
- entry kerf
- Slot width at the top face
- taper
- How much wider the exit is than the entry
- offset
- Half the kerf — how far the tool path sits off the line
Worked example
12 mm mild steel on a 0.76 mm mixing tube at standard Q3 edge quality.
- 1Entry kerf: 0.76 × 1.05 = 0.80 mm
- 2Thickness in inches: 12 ÷ 25.4 = 0.472
- 3Taper: 0.472 × 0.025 × 1.2 (steel) × 1.04 (Q3) = 0.015 mm
- 4Exit kerf: 0.80 + 0.015 = 0.81 mm
Program a 0.40 mm offset. Taper is negligible at this thickness in steel — it only becomes a real problem past about 25 mm, or in softer materials.
Frequently asked questions
Why does kerf change as the mixing tube wears?
Abrasive erodes the bore from the inside, so the tube gets wider with every hour of cutting, and the kerf widens with it. A tube that started at 0.76 mm may be cutting 0.85 mm near the end of its life, which is enough to put parts out of tolerance if you are still using the original offset. The practical answer is to cut a test coupon periodically, measure it, and re-enter the offset rather than trusting a number for a whole shift.
How do I get rid of taper?
Three ways, in increasing order of cost. Slow the traverse down, which gives the stream more time and evens out the cut — this is what higher edge-quality settings do. Cut a sacrificial lead-in so the unstable pierce sits outside the part. Or use a tilting head, which physically angles the nozzle to compensate and can very nearly eliminate taper. For thin material taper is often small enough to ignore entirely.
Does taper ever go the other way?
Yes. On very thin material cut quickly, the exit kerf can be narrower than the entry, because the jet has not had time to spread but the top face gets extra erosion from the abrasive at the point of entry. This reverse taper is one reason thin sheet often needs its own tested offset rather than a value extrapolated from thicker work.
Why is the offset half the kerf rather than the whole kerf?
Because the slot is centred on the tool path, so the cut removes material equally from both sides of the programmed line. To leave a part at its nominal size, the path has to sit half a kerf outside the profile. Getting this wrong by a factor of two is a common and expensive first mistake — parts come out a full kerf undersized.
How accurate are these numbers?
Treat them as a starting point that gets you close on the first test cut, not as production values. Real kerf depends on pump pressure, abrasive type and flow rate, tube wear, standoff distance and the specific alloy — none of which this calculator knows. Every shop ends up with its own measured offsets per material and thickness, and those beat any formula.