Wall thickness — what's actually printable
Concrete millimetre numbers for FDM. Save these and you'll never print a wall that snaps in half.
The single most important number
1.6 mm. That's the minimum sane wall thickness for FDM. It's four times the typical 0.4 mm nozzle width — enough for the slicer to lay down 2 perimeters on each side with infill in between. Walls thinner than this either fail to print or snap when you look at them sideways.
Below 1.6 mm? Possible — but you're in expert territory (single-perimeter "vase mode" tricks, specific filament tuning). Skip it until your fifth print.
By feature type
Visible walls (the shell of a box, the body of a phone stand): 1.6-2.4 mm. Thicker for things that get handled.
Standalone columns or fingers: 2.4 mm minimum diameter. A 1.6 mm finger snaps; a 3 mm finger is sturdy.
Embossed letters on a face: 0.6-1.2 mm raised. Higher than 1.2 mm and the letters look gloopy; lower than 0.4 mm and they vanish.
Engraved letters into a face: 0.6-1 mm deep. Deeper just wastes time.
Snap-fit clips: 1.2 mm at the thinnest flex point. Goes thinner only if you're tuning a specific filament.
Test before you commit
Every printer + filament combo has slightly different limits. Print a small "thickness test" — five walls of decreasing thickness (3, 2.4, 1.6, 1.2, 0.8 mm). Whichever ones survive being squeezed are your safe range. Costs 5 g of filament and answers the question forever.
1.6 mm minimum for any wall you'll ever touch. 2.4 mm for anything that takes load. 1.2 mm for snap-clips. Test once, trust forever.