We’ve all been there – you start a woodworking project and measure all your boards, cut them all, start to assemble it, and for some reason everything is ¼” too short. “How did I screw up measuring everything by a quarter-inch? I must not be good at this!” Nope, you just didn’t account for kerf.

Kerf is the material that a cutting tool like a saw or a router bit removes from your material. We all grew up using scissors or shears to cut things — these shear the material, resulting in no material lost. But many cutting tools remove material. Think about it: saws make sawdust but scissors don’t make paper dust.

So how can you ensure you leave enough room for kerf?

SAW — With saws it’s easy. For starters, don’t measure all your pieces at one time. Measure, then cut, then measure, and so on. Don’t cut on your line; cut to the far side of it. The kerf will be removed from the unmeasured section and your board will be the correct length. Once you get good at it, you can measure the width of your saw blade’s teeth (some teeth angle outward so they’re wider than the body of the blade) and add that width into your board lengths, knowing it will be removed.

With digital fabrication tools, the best way to handle kerf is to plan for it in your design. In general, make holes smaller and joint spacing tighter to take up room for your kerf. But how much?

CNC ROUTER — Also easy. What’s the width of your end mill: ⅛”, ¼”, ½”? The width of your bit will be the kerf removed from your part — on a good tool. If you really need tight tolerances, it’s worth measuring your kerf in your chosen material to check for runout. Runout is when a spinning tool wobbles and doesn’t stay centered; this can cause your kerf to be wider than expected. A bent end mill, bad collet or bearing, or loose spindle could all cause runout.

LASER CUTTER — Probably the hardest kerf to account for. You’ve seen those press-fit wooden boxes made on laser cutters; they make great project enclosures. But if the finger joints are too loose, they need glue to stay together; looser than that and it’s hard to even square them up for gluing. Too tight and you’ll never be able to press them together. You need the right kerf settings to get them to fit well.

With lasers, two major factors will decide how wide your kerf is:

  •  Beam width — This is established by the focal length set by the lens, but it’s tricky. Beam width can change depending on the thickness of your material, so you might have more kerf on a ¼” sheet than an ⅛” sheet.
  •  Material — Wood gets burnt away by the laser, leaving behind nothing but the cut edge. But plastics may shrink away as they are not only cut but melted. This will make your kerf wider than expected.

Measuring the actual kerf cut by your laser is the way to know for sure. But this is tricky too — it’s so narrow, it’s hard to get a tool in to measure.

My trick to is to use a jig that I cut on a spare piece of material. I cut a key that’s a given dimension (knowing that when it’s cut, it will be smaller because of the kerf) and a series of slots to fit it into, each one 0.1mm smaller than the last. For instance, a 20mm key with 20, 19.9, 19.8, 19.7, 19.6, 19.5, and 19.4mm slots. Then I press the key into each slot and decide which fits best.The actual kerf of my laser is half the difference between the key and the slot (because the kerf was removed from both sides). Download my jig and try it yourself!