Layer Count from Build Height

Build height divided by layer thickness. There is no physics here at all, and it is on this page because it is the multiplier that turns every per-layer cost into a build cost, and because two things about it are consistently underestimated.

The first is quantisation. A dimension in the build direction is not continuous — it is a whole number of layers. On 40 µm layers a feature specified at 0.55 mm can be built at 0.52 mm or at 0.56 mm and nothing in between, and which one you get depends on where the slicer put its plane. This is not a rounding nuisance; on thin walls, small holes and shallow steps it is a real tolerance, and it is why the build direction always has a looser achievable tolerance than the two in-plane directions. It is also why moving a part 20 µm up or down on the plate can change which features come out cleanly.

The second is what the height is measured from. $H$ runs from the build plate, not from the bottom of the part, so every millimetre of support structure underneath is layers the machine has to scan and pay for. And the layer count of a build is set by the tallest object on the plate. Every other part sits there being recoated, layer after layer, for all the layers above its own top — earning nothing and consuming recoat time. This is why nesting parts of similar height is worth real money, why one tall bracket can double the effective cost of the twenty short ones sharing its plate, and why the first question a good build planner asks is not "will it fit?" but "how tall is the tallest thing?"

Choosing the layer thickness is a genuine three-way trade and this page is only one leg of it. Thinner layers mean more of them and so more machine hours; they also mean a better staircase finish on every sloped face and finer feature resolution. Thicker layers mean fewer of them and a faster build, at the cost of surface finish and of needing more energy per layer to melt through — which pushes toward higher power and, through the normalised enthalpy, toward keyholing. There is a floor as well: a layer much thinner than the largest particles in the powder cannot be spread at all, since the recoater simply drags them, which puts a practical minimum near 20 µm for a conventional 15–45 µm powder cut.