Which layer height is strongest? What the tensile tests show

In published strength tests of PLA (polylactic acid) on a 0.4 mm nozzle, 0.15 to 0.20 mm is the strongest layer height. Thinner layers add print time without bonding better, and CNC Kitchen’s hooks lost half their strength at 0.30 mm. Within that range, which way the load runs through the layers matters more than the layer height.

Which layer height is strongest?

On a 0.4 mm nozzle, 0.15 to 0.20 mm is strongest. In CNC Kitchen’s layer adhesion test, 0.15 mm PLA hooks held the most and 0.30 mm about half. A 2025 Bambu Lab P1S study found 0.20 mm strongest across the layers, with 0.08 mm weakest. Along the layers, 0.20 mm or thinner did best.

Load direction	Strongest, 0.4 mm nozzle	What the tests found
Across the layers (part printed standing)	0.15 to 0.20 mm	CNC Kitchen: 0.15 mm strongest, 0.30 mm about half. Bambu Lab P1S study: 0.20 mm strongest, 0.08 mm weakest
Along the layers (part printed lying flat)	0.20 mm or thinner	CNC Kitchen: 0.05 to 0.20 mm equal, 0.30 mm weaker. Materials 2018 and Polymers 2023: thinner stronger, by 11% and up to 23%. P1S study: no clear trend

Across the layers: the layer adhesion tests

CNC Kitchen’s Stefan Hermann printed test hooks in Prusament PLA on an Original Prusa i3 MK2.5 with a 0.4 mm nozzle. He used Slic3r PE’s 0.15 mm preset with 3 perimeters and changed only the layer height. With the hooks printed standing (three per height), the load pulled the layers apart:

• 0.05 to 0.20 mm all held well, and 0.15 mm was strongest.
• 0.30 mm held only half the load.
• 0.40 mm held “basically nothing”.

Thicker layers made lighter hooks, but dividing each failure load by the hook’s weight did not rescue the thick layers.

A 2025 New Mexico State University study (Chavez et al.) pulled ASTM D638 tensile bars printed on a Bambu Lab P1S: 0.4 mm nozzle, white Bambu Lab PLA Basic at 220 °C, 100% infill, one wall loop, five bars per setting. Tensile strength of bars printed upright:

Layer height	0.31 mm lines	0.42 mm lines (Bambu default)	0.62 mm lines
0.08 mm	21.36 MPa	25.02 MPa	26.50 MPa
0.20 mm	25.85 MPa	29.83 MPa	31.92 MPa
0.28 mm	23.26 MPa	29.12 MPa	31.17 MPa

0.20 mm was strongest at every line width. With the default 0.42 mm lines it beat 0.08 mm by (29.83 − 25.02) / 25.02 = 19%, and 0.28 mm came within 2.4%.

Above 0.20 mm the tests disagree. CNC Kitchen’s 0.30 mm hooks lost half their strength, while the P1S bars at 0.28 mm nearly matched 0.20 mm. The two used different printers, parts and line widths, and both put the peak at 0.15 to 0.20 mm.

Along the layers: parts printed lying flat

All four tests used a 0.4 mm nozzle and PLA.

Test	Printer and setup	Layers	Result
CNC Kitchen hooks, 2019	Prusa i3 MK2.5, Prusament PLA	0.05 to 0.40 mm	0.05 to 0.20 mm all failed around 80 kg; 0.30 mm noticeably earlier, 0.40 mm far worse
Frunzaverde et al., Polymers 2023	UltiMaker 2+ Connect, Verbatim PLA in 4 colors, 210 °C, 100% infill, 5 bars each	0.05 to 0.20 mm	Strength fell as layers got thicker, by 4.77% (grey) to 23.41% (black) between the extremes
Rodríguez-Panes et al., Materials 2018	Prusa i3, 0.4 mm lines, 200 °C, 20% infill	0.10 and 0.20 mm	38.47 MPa at 0.10 mm, 34.37 MPa at 0.20 mm: 11% lower
Chavez et al., 2025	Bambu Lab P1S, as above	0.08 to 0.28 mm	“No clear trend”: 29.01, 34.05 and 32.84 MPa yield strength with 0.42 mm lines

In the Polymers study, filament color moved strength more than layer height did, and natural and red PLA rose slightly at 0.10 mm. CNC Kitchen’s advice from the lying hooks: even when the layers aren’t the weak point, don’t go much above 0.20 mm.

Are thinner layers stronger than thicker layers?

Not when the load pulls the layers apart. CNC Kitchen’s hooks gained nothing below 0.15 mm, and 0.08 mm was weakest in a 2025 Bambu Lab P1S test. Along the layers, thinner was stronger in two of four tests: by 11% (0.10 against 0.20 mm, Materials 2018) and up to 23% (0.05 against 0.20 mm, Polymers 2023).

“Thin layers did not bond better, which surprised me,” CNC Kitchen wrote. The P1S study’s authors put the brittleness of thin upright bars down to their large number of weak layer interfaces.

Thicker layers help only up to a point. The P1S authors cite earlier publications that found thicker layers best for upright prints, but CNC Kitchen found a limit at half the nozzle: “past a ratio of 2, so layers thicker than half the nozzle, strength starts to suffer.” That test used only a 0.4 mm nozzle.

Thicker layers also stretched further before breaking. In the P1S study, flat bars with 0.31 mm lines went from 6.68% elongation at break at 0.08 mm to 26.15% at 0.28 mm.

Thin layers cost time: print time is inversely proportional to layer height (CNC Kitchen), so 0.10 mm layers take about twice as long as 0.20 mm.

What is the best layer height for strength?

Set 0.20 mm on a 0.4 mm nozzle, or 0.15 mm, where CNC Kitchen’s standing hooks peaked. The P1S study’s authors back 0.20 mm layers with 0.42 mm lines, the Bambu Studio default, as a balance of strength and ductility in both load directions.

1. Set the layer height: Process > Quality > Layer height in Bambu Studio, Print Settings > Layers and perimeters in PrusaSlicer.
2. On another nozzle, stay at or under CNC Kitchen’s half-the-nozzle limit: 0.6 × 0.5 = 0.30 mm on a 0.6 mm nozzle. The layer height calculator gives the full usable range for any nozzle.
3. For more strength, change the settings below instead of going thinner than 0.15 mm.

Settings that matter more than layer height

Setting	Change tested	Effect on strength
Print orientation	Layers across the load instead of along it	At least 50% lower (CNC Kitchen hooks). 28% lower, against 11% for 0.10 to 0.20 mm layers (Rodríguez-Panes et al.)
Walls	More perimeters	“The strength of a model is mostly defined by the number of perimeters (not infill)” (Prusa Knowledge Base)
Infill	20% to 50%	27% higher (Rodríguez-Panes et al.)
Line width	0.31 to 0.62 mm, averaged over layer heights	Upright bars 23.49 to 29.86 MPa; flat bars 34.34 to 30.37 MPa yield (Chavez et al.)
Nozzle temperature, PLA	200 °C to 230 °C, then 270 °C	Standing samples 37 to 40 MPa, then 32 MPa; lying about 60 MPa (CNC Kitchen)

Orient the part so the load runs along its layers, then add walls. Bambu Studio sets them under Strength > Walls > Wall loops, and the wall thickness guide gives the count for a target thickness. In the P1S study, wider lines helped upright bars but hurt flat ones, so check what a width does to your walls with the line width calculator.