Your Z-offset is the gap between the nozzle and the bed on that all-important first layer, and it’s the single setting that decides whether a print sticks or peels. Get it right and the first layer goes down as a smooth, fused sheet. Get it wrong and you’re either grinding the nozzle into the plate or laying down loose spaghetti that won’t bond.
Z-offset isn’t one of those “no wrong answers” sort of things. You either have it correct or it’s off. You don’t have to have it perfect, but deviating from the ideal in either direction will hurt the quality of your print, so it’s worth a few minutes to dial in.
Print a single-layer test first
Don’t try to judge Z-offset from a whole model. Print a single-layer test patch (a wide, one-layer-tall square) so you’ve got a big, flat area to read. Most printers and slicers have a first-layer or bed-level test built in; if yours doesn’t, slice a thin square and set it to one layer.
Watch it go down, and read the result while it’s still on the plate. Here’s what you’re looking for.
Signs it’s too low (nozzle too close)
The nozzle is dragging through plastic it already laid down, so it pushes material up and out of the way.
If you have “lawnmower lines” where the filament squished up on either side of the nozzle, it’s too low.
If you have wavy, bubbly areas, it’s too low.
Too low is the one that can actually damage things (a nozzle scraping bare plate will gouge a PEI sheet or glass), so if you’re unsure which way you’re off, back the nozzle away first and sneak up on it.
Signs it’s too high (nozzle too far)
The nozzle is too far off the bed, so the lines sit as loose round strands instead of squishing flat and bonding.
Take a flashlight, hold your head directly over the build plate, and shine the light down at it. If you can see the bed at all between the lines, it’s too high.
If you can pull the strands of the test apart after it prints, it’s too high. The lines never fused.
What correct looks like
Ideally you’re looking for a smooth, flat top on the first layer, with the lines fused into each other and no gaps between them. Not shiny-flattened, not round and separated. Just a solid, even sheet.
How to adjust it
The quickest way is to tune it live, on the first layer of that test print, while you can see what each change does. This is usually called baby-stepping:
- Start the print and let it lay down a pass or two.
- Nudge the Z-offset in small steps (0.01 to 0.02 mm at a time) and watch the line change. Lowering the offset moves the nozzle closer to the bed; raising it pulls the nozzle away.
- Keep nudging until the lines fuse into that smooth flat top.
- Save the value so it sticks for the next print. If you don’t save it, you’re doing this again tomorrow.
Where that live-adjust control lives depends on your firmware (Marlin’s tune menu, Klipper’s SET_GCODE_OFFSET or a screen macro, the tuning screen on a Bambu), and so does how you save it, so check your printer’s specifics. The reading-the-first-layer part above is the same on every machine.
When it’s good in some spots and bad in others
If you’re dealing with too high in some spots and too low in others, that’s not a Z-offset problem, it’s a leveling problem underneath it. If the high spots are all off to one side, that’s poor tramming (the bed physically isn’t level). If they’re scattered around randomly, your mesh bed leveling isn’t compensating the way it should. Fix that before you keep chasing the offset, because no single Z number can cover a bed that isn’t flat.
Don’t over-squish it
Some people like to squish more than is necessary because they’re trying to eliminate the little gaps at the edges where the infill makes a u-turn. This is wrong and will push you into printing with the nozzle too low. Those tiny gaps are a first-layer line-width and flow issue, not a reason to bury the nozzle in the plate. Set the offset for a clean, flat, fused layer and leave it there.
Once your first layer goes down flat and sticks, most of the “why won’t my print adhere” problems disappear on their own.