First layer line width: 0.5 mm on a 0.4 mm nozzle, and why it is wider

In 3D printing, the first layer line width is usually 125% of the nozzle diameter, because a wider first line grips the bed better.

What first layer line width should you use?

Use about 125% of the nozzle diameter: 0.5 mm on a 0.4 mm nozzle. That is the value Bambu Studio and Prusa’s MK4S and CORE One profiles ship, and Prusa’s guide suggests 120 to 140%. On 0.6 and 0.8 mm nozzles, Bambu Lab’s presets stay near 100% of the diameter: 0.62 and 0.82 mm.

First layer line width in each slicer’s shipped profiles:

Slicer and profile	0.2 mm nozzle	0.4 mm nozzle	0.6 mm nozzle	0.8 mm nozzle
Bambu Studio, Bambu Lab presets	0.25 mm	0.5 mm	0.62 mm	0.82 mm
PrusaSlicer, Prusa MK4S and CORE One	no profile	0.5 mm	0.68 mm	1.0 mm
PrusaSlicer, Original Prusa i3 MK3S and MK3S+	no profile	0.42 mm	0.65 mm	0.9 mm
OrcaSlicer, generic Klipper printer (120%)	0.24 mm	0.48 mm	0.72 mm	0.96 mm
UltiMaker Cura, built-in default (100%)	0.2 mm	0.4 mm	0.6 mm	0.8 mm

• Prusa ships 0.25 mm nozzle profiles instead of 0.2 mm ones: 0.32 mm first lines on the MK4S and CORE One, 0.3 mm on the MK3S.
• OrcaSlicer’s generic Marlin printer uses a fixed 0.5 mm, and its built-in default of 0 prints each first layer feature at its own width.
• PrusaSlicer’s built-in default is 200% of the first layer height: 0.4 mm at a 0.2 mm first layer.
• Bambu Studio 02.08.04.57 also has an “Initial layer infill” width for the bottom infill. Bambu Lab’s presets set it to the same values.

The slicers also disagree on first layer height. Bambu Lab’s presets use half the nozzle diameter (0.1 to 0.4 mm). Prusa’s MK4S and CORE One profiles print a 0.2 mm first layer on every nozzle, so their 0.8 mm nozzle lays a 1.0 mm line only 0.2 mm high. The layer height calculator gives the first layer height for your nozzle, and the line width calculator gives the widths for every layer after the first.

Why is the first layer line width wider than the other layers?

Slicer makers widen the first layer for bed adhesion: Prusa, Bambu Lab, UltiMaker and the OrcaSlicer team all give that reason in their slicer tooltips or documentation. OrcaSlicer adds that the wider line compensates for uneven build surfaces. Prusa names the cost: lower resolution in the XY plane on that layer.

Source	What it says about a wider first layer
PrusaSlicer 2.9.6 tooltip	“You can use this to force fatter extrudates for better adhesion.”
Prusa Knowledge Base	Makes the first layer “stick better to the print bed (e.g. 120-140% of nozzle diameter) at the cost of lower resolution in the XY plane”
Bambu Lab Wiki	A higher width-to-layer-height ratio “can improve First Layer adhesion and help ensure print success”
OrcaSlicer wiki	“improves bed adhesion and compensates for uneven build surfaces”
UltiMaker Cura 5.13 tooltip	“Increasing this could improve bed adhesion.”

Bambu Lab ties the gain to the width-to-height ratio. Its 0.4 mm preset lays a 0.5 mm line on a 0.2 mm first layer, a ratio of 0.5 ÷ 0.2 = 2.5. The 0.2 mm layers above it use 0.42 mm outer and 0.45 mm inner walls, a ratio of 2.1 to 2.25. Bambu Lab also says a wider bottom infill line packs the lines more tightly and increases “the contact area between the First Layer and the build plate”.

Does a wider first layer need more volumetric flow?

Yes, but with PLA on a 0.4 or 0.6 mm nozzle it stays below the filament’s limit. Volumetric flow (mm³ of plastic melted per second) is line width × layer height × speed, the formula the flow calculator uses. Bambu Studio prints the first layer’s bottom infill faster than its walls. For the Bambu Lab X1 Carbon presets:

Nozzle	First layer line	Walls	Bottom infill
0.2 mm	0.25 mm wide, 0.1 mm high	40 mm/s: 1.0 mm³/s	70 mm/s: 1.75 mm³/s
0.4 mm	0.5 mm wide, 0.2 mm high	50 mm/s: 5.0 mm³/s	105 mm/s: 10.5 mm³/s
0.6 mm	0.62 mm wide, 0.3 mm high	35 mm/s: 6.51 mm³/s	55 mm/s: 10.23 mm³/s
0.8 mm	0.82 mm wide, 0.4 mm high	35 mm/s: 11.48 mm³/s	55 mm/s: 18.04 mm³/s

Worked example for the 0.4 mm walls: 0.5 × 0.2 × 50 = 5 mm³/s. Bambu Studio’s Generic PLA profile allows 12 mm³/s on the X1 Carbon’s 0.4, 0.6 and 0.8 mm nozzles, so the 0.4 and 0.6 mm first layers print at the preset speeds. Prusa’s MK4S 0.4 mm profiles land in the same range: 0.5 × 0.2 × 40 = 4 mm³/s for the first layer and 0.5 × 0.2 × 100 = 10 mm³/s for its solid infill.

Bambu Studio caps every line’s speed at the filament’s max volumetric speed divided by the line’s cross-section, and two first layers hit that limit:

• 0.8 mm nozzle: the bottom infill needs 18.04 mm³/s against Generic PLA’s 12 mm³/s, so it prints slower than 55 mm/s.
• TPU: Generic TPU allows 3.2 mm³/s, less than the 5 mm³/s of the 0.4 mm walls. A flexible first layer prints slower than the preset speed on the 0.4, 0.6 and 0.8 mm nozzles.

When should you change the first layer line width?

• The first layer doesn’t stick or shows gaps between lines. Widen it within Prusa’s suggested range: 0.48 to 0.56 mm on a 0.4 mm nozzle. In Bambu Studio you can widen only Initial layer infill, which Bambu Lab says improves first layer coverage. Stay under 150%, where Cura shows a warning and OrcaSlicer’s wiki says line widths can over-extrude.
• The bottom face carries fine detail, such as small text or holes. Narrow it toward 100% of the nozzle (Cura’s default) to win back the XY resolution Prusa says the wider line costs.
• The bottom edge of a tight-tolerance part measures oversize (elephant’s foot). Prusa puts this down to the first layer being squished against the bed. All four slicers have a setting that shrinks the first layer in XY to correct it, called Elephant foot compensation in PrusaSlicer, Bambu Studio and OrcaSlicer. Prusa suggests about 0.2 mm on a 0.4 mm nozzle, the value its MK4S and CORE One 0.4 mm profiles ship. Bambu Lab’s X1 Carbon presets use 0.15 mm and the A1’s use 0.075 mm. OrcaSlicer can taper the correction over several layers. In Cura, enter a negative Initial Layer Horizontal Expansion.

Where to set the initial layer line width

Slicer	First layer line width	First layer speed	Elephant foot correction
PrusaSlicer	Print Settings → Advanced → Extrusion width → First layer (mm, or % of the first layer height)	Print Settings → Speed → Modifiers → First layer speed, First layer solid infill speed	Print Settings → Advanced → Slicing → Elephant foot compensation
Bambu Studio	Process → Quality → Line width → Initial layer, Initial layer infill (mm)	Process → Speed → Initial layer speed	Process → Quality → Precision → Elephant foot compensation
OrcaSlicer	Process → Quality → Line width → First layer (mm, or % of the nozzle)	Process → Speed → First layer speed	Process → Quality → Precision → Elephant foot compensation
UltiMaker Cura	Print settings → Quality → Initial Layer Line Width (% of the line width)	Print settings → Speed → Initial Layer Speed	Print settings → Walls → Initial Layer Horizontal Expansion (negative value)

PrusaSlicer, Bambu Studio and OrcaSlicer show these fields only in Advanced mode (Advanced or Expert in PrusaSlicer). Watch the units: a PrusaSlicer percentage is taken of the first layer height, so 125% at a 0.2 mm first layer is a 0.25 mm line, not 0.5 mm; type 0.5 in millimeters instead. In Bambu Studio, the Initial layer width overrides every other line type on the first layer except Initial layer infill.