Three slicer settings have an outsized effect on how your part turns out: layer height, wall count, and line width. They control print time, surface finish, and structural strength all at once, and they interact with each other in ways most beginners don't realise. This is the working operator's guide to picking the right numbers for the job.
0.2 mm layers are the right default with a 0.4 mm nozzle (PLA, PETG). 3 walls for everyday prints; 5+ walls for mechanical strength — walls add more strength than infill does, weight for weight. Set line width = nozzle diameter (0.4 mm with a 0.4 mm nozzle) unless you specifically want stronger or smoother prints, in which case bump line width to 0.45–0.5 mm.
Layer height is the thickness of each printed slice (Z direction). It directly controls vertical resolution, surface finish on curved/sloped surfaces, and print time. Print time scales nearly linearly with the inverse of layer height: a 0.1 mm print takes about twice as long as a 0.2 mm print.
The usable range is roughly 25% to 75% of nozzle diameter :
Go below 25% and you risk under-extrusion as the molten plastic struggles to flow through the gap. Go above 75% and layer adhesion suffers because the new layer can't fully fuse with the previous one's flat top.
On most printers (8mm leadscrew, 1.8° stepper, 16x microstepping) the Z-axis moves in increments of 0.04 mm. Layer heights that are exact multiples of 0.04 mm (0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32) avoid Z-stepping rounding artifacts. On 12mm leadscrews the magic increment is 0.0125 mm (so 0.10, 0.15, 0.20, 0.25 all work). Modern printers with quality leadscrews don't show visible artifacts at non-magic heights, but it remains a useful rule for tall, smooth-surfaced prints.
Walls are the perimeters that make up the outside (and inside, on hollow features) of your part. They're typically the strongest part of an FDM print — significantly stronger than infill at the same weight.
Walls > infill for strength per gram. If you're tempted to crank infill to 60% to make a part stronger, increase walls to 5 first — you'll get a stronger part that prints faster.
Treat these as horizontal walls. Defaults of 4–5 top layers and 3–4 bottom layers are good. Too few top layers = pillowing (the top surface sinking into the infill). Multiply the layer count by your layer height to know how thick your top/bottom shell is — aim for 1.0–1.5 mm minimum.
Line width is how wide each printed line is laid down. It's set independently from nozzle size; the slicer adjusts flow to make the line the width you ask for.
Most slicers (PrusaSlicer, OrcaSlicer, Cura, Bambu Studio) let you set different line widths for different features. A common setup:
Get the picture by looking at wall thickness :
For most parts, designing wall thicknesses that are multiples of your line width avoids "gap fill" passes (which are slow and weak). A 1.6 mm wall in CAD will perfectly fit 4 passes of 0.4 line width; a 1.5 mm wall forces the slicer to choose between under-filling or adding a thin "gap fill" pass.