Keep layer height between about a quarter and three quarters of your nozzle diameter. For a 0.4 mm nozzle that is 0.10 to 0.30 mm. The exact ends of that range depend on whose documentation you read, and above about 0.2 mm the limit you actually hit is usually not the nozzle at all. It is how fast your hotend can melt plastic.
Who says what
The “25 to 80 percent” rule gets repeated without a source. Here are the sources, and they do not agree.
| Source | Stated range | On a 0.4 mm nozzle | On a 0.6 mm nozzle |
|---|---|---|---|
| Prusa’s layers and perimeters guide | below 80% of nozzle diameter | up to about 0.32 mm | up to about 0.48 mm |
| OrcaSlicer’s layer height guide | 20% to 80% | 0.08 to 0.32 mm | 0.12 to 0.48 mm |
| Bambu Lab’s nozzle guide | 25% to 75% | 0.10 to 0.30 mm | 0.15 to 0.45 mm |
| Bambu Studio’s layer height page | 20% to 70% | 0.08 to 0.28 mm | 0.12 to 0.42 mm |
Prusa states only a ceiling. It adds a hard stop: PrusaSlicer shows an error if you enter a layer height above the nozzle diameter. OrcaSlicer’s wiki gives reasons for both ends. Below 20 percent you risk flow inconsistencies and a “fish scale” pattern, especially at speed. Above 80 percent you risk weak layer adhesion and worse quality. Bambu publishes two different ranges on two of its own pages, which tells you how soft these edges are. Its nozzle guide also calls 0.2 mm the standard for a 0.4 mm nozzle and 0.3 to 0.4 mm common for a 0.6 mm one. For the 0.2 mm nozzle it narrows the range to 25 to 60 percent, 0.05 to 0.12 mm.
My reading: treat 25 percent as the floor and 75 percent as the ceiling. That is Bambu’s nozzle guide range, it sits inside both Prusa’s and OrcaSlicer’s, and it overshoots the strictest figure (Bambu Studio’s 70 percent) by 0.02 mm on a 0.4 mm nozzle. The gap between 0.28, 0.30 and 0.32 mm is not worth an argument.
The first layer is its own case. OrcaSlicer recommends 0.25 mm on a 0.4 mm nozzle (62.5 percent) with a maximum of 65 percent, because a thicker first layer absorbs small errors in the bed surface.
What each setting controls
Prusa’s article on nozzle diameters has the clearest explanation I have read. Nozzle diameter sets detail in the horizontal plane, parallel to the bed. Layer height sets detail on vertical and sloped surfaces. Embossed text on the top face looks the same at any layer height, and a box with flat vertical walls gains almost nothing from thin layers except a longer print.
Thin layers pay off on curves and shallow slopes, where the stair-steps show. Prusa’s own verdict on rectangular parts is “little to no difference” in the result, and 0.28 mm finishes in under half the layers of 0.12 mm.
The ceiling that actually bites: flow
Every line the printer lays down is a volume of plastic per second:
flow (mm³/s) = layer height x line width x speed
Take a 0.4 mm nozzle with a 0.45 mm line width, moving at 200 mm/s.
- At 0.10 mm layers: 0.10 x 0.45 x 200 = 9 mm³/s
- At 0.20 mm layers: 0.20 x 0.45 x 200 = 18 mm³/s
- At 0.28 mm layers: 0.28 x 0.45 x 200 = 25.2 mm³/s
Now compare that with what a hotend can melt. Prusa’s volumetric speed guidance gives typical maximums of 8 to 12 mm³/s for a standard all-metal hotend such as an E3D V6, 15 to 20 mm³/s for its Nextruder, and 12 to 20 mm³/s as the range PLA itself tolerates. PETG is lower at 8 to 15 mm³/s.
Say your limit is 15 mm³/s. Turn the formula around and ask what speed each layer height allows:
- 0.10 mm: 15 / (0.10 x 0.45) = 333 mm/s
- 0.20 mm: 15 / (0.20 x 0.45) = 167 mm/s
- 0.28 mm: 15 / (0.28 x 0.45) = 119 mm/s
That is the whole story in three lines. Going from 0.20 to 0.28 mm cuts the number of layers by 29 percent. It also cuts the fastest speed the hotend can feed by the same 29 percent. On every move that was already at the flow limit, the two cancel and you save nothing. The slicer’s maximum volumetric speed setting enforces this automatically: it never raises your speed, it only slows moves that would ask for more plastic than the hotend can deliver.
You still save some time with thicker layers, because not every move is flow-limited. Outer walls, small perimeters, overhangs and anything slowed for cooling run well below the limit, and fewer layers means fewer of those slow passes and fewer travel moves. But the saving is smaller than the layer count suggests. Bambu’s layer height page says the same thing from the other direction: at higher layer heights the extrusion flow is large and a slower speed is needed for the hotend to fully melt the filament.
Prusa draws the practical conclusion. If the slicer preview shows most of the model capped by volumetric speed, lower the layer height. You get more detail and the print takes about the same time, because the printer can move faster. It is the rare free upgrade.
One note on the arithmetic. Our volumetric flow calculator uses the simple rectangle (height x width) shown above. PrusaSlicer models the line as a stadium shape with rounded ends, which gives a slightly smaller area. The rectangle errs on the safe side: it predicts a little more flow than the slicer will actually command.
Picking a number
For a 0.4 mm nozzle, 0.20 mm is the default for a reason: it is half the nozzle, comfortably inside every published range, and a 15 mm³/s hotend can still feed it at 167 mm/s.
Go down to 0.12 or 0.10 mm for organic shapes, domes and shallow slopes. Do not go below 0.08 mm, which is the lowest floor any of these sources gives.
Go up to 0.28 or 0.30 mm for draft parts and blocky functional pieces, then check the flow preview. If it is mostly capped, you have picked a rougher surface for no gain.
If you want thick layers and the time saving that should come with them, the honest route is a bigger nozzle and enough hotend to feed it. A 0.6 mm nozzle takes you to 0.45 mm layers inside Bambu’s range. Our guide to 0.6 vs 0.4 mm nozzles covers what that changes.
