Every cut you make destroys material. Not much — a few millimetres — but it is gone, and it is gone at every single pass. Ignore it across a whole cut list and you will finish the job one part short. This is the most common way a cutting plan fails, and it is entirely avoidable.
What kerf actually is
The kerf is the width of the slot a blade leaves behind. It is wider than the steel plate of the blade itself, because the teeth are set — bent alternately left and right, or tipped with carbide wider than the body — so the blade does not bind in its own cut.
Typical widths, for the tools most shops actually own:
- Table saw, standard blade: 1/8" (3.2 mm)
- Table saw, thin-kerf blade: 3/32" (2.4 mm)
- Track saw / circular saw: roughly 2.0–2.2 mm
- Bandsaw: 0.9–1.5 mm, depending on the blade
- Panel saw (industrial): 3.2–4.8 mm — the scoring blade widens it further
- Glass cutter: effectively zero; you score and snap
Measure your own — do not trust the box
Blade specifications are nominal, and a blade that has been sharpened a few times is not what it was. Measure it once:
- Take a scrap wider than 12" and measure its exact length.
- Crosscut it roughly in the middle.
- Measure both pieces and add them together.
- Original length minus the sum of the two pieces is your kerf.
A single cut gives you a number accurate to whatever your tape can read. If you want better, make ten cuts in a long strip and divide the total loss by ten — the measurement error spreads across all ten.
The arithmetic that catches people out
Here is the classic failure, and it is worth doing the numbers slowly.
You need four shelves at 24" and you have a 96" board. Four times twenty-four is ninety-six. It fits exactly — on paper.
It does not fit. Cutting four pieces out of one board takes three cuts between them, and with a 1/8" blade that is 3/8" of material gone. You need 96.375". You are three-eighths of an inch short, which means your fourth shelf comes up short, not the offcut.
The general form is simple. For n parts in a row out of one length:
total needed = (sum of part lengths) + (n − 1) × kerf
Scale that up and it gets expensive. Eight parts across a 96" sheet means seven cuts — 7/8" gone, nearly a full inch of your 96. On a full sheet broken into a grid of twenty parts you can easily lose two to three inches in each direction, purely to dust.
Edge trim compounds it
Most sheet goods do not arrive square, and the factory edge is often damaged in transit or bowed from storage. Most people square up all four edges before cutting parts — commonly 1/4" per edge.
That is another 1/2" off each dimension before a single part is cut. A nominal 96" × 48" sheet with a 1/4" trim is really 95.5" × 47.5" of usable material. Combine that with kerf loss and the sheet you are actually cutting from is meaningfully smaller than the one you paid for.
Why this is worth automating
None of this is hard arithmetic. It is just arithmetic you have to do correctly at every boundary, in both directions, across every part in the list — and get wrong once to ruin a sheet.
That is exactly the kind of work to hand to software. Set your blade's kerf once, set your edge trim, and let the layout account for it everywhere. Kesim subtracts a full kerf between every pair of neighbouring parts and squares off the edge trim before placing anything, so the plan it gives you is the plan you can actually cut.
Measure your blade. Write the number on the fence. It is the single most useful number in your shop.