Top and Bottom Shell Layers: How Many to Use in 3D Printing
For most FDM prints at 0.2 mm layer height, use 5 top shell layers (about 1 mm) and 3-4 bottom shell layers (0.6-0.8 mm). Add top layers when infill drops to 10% or less, when layers get thinner, or when the top surface has to look perfect; add bottom layers only for floors that must stay watertight or get sanded.
That's also what the common slicers ship with. Bambu Studio's 0.20 mm Standard profile uses 5 top and 3 bottom layers, Cura's default Top/Bottom Thickness is 0.8 mm (4 layers at 0.2 mm), and PrusaSlicer's stock profiles ask for at least 0.7 mm on top and 0.5 mm on the bottom. The rest of this guide explains why the top needs more than the bottom, how layers and thickness interact, and what extra layers cost.
What top and bottom shells are
A print has two kinds of shell. Walls (also called perimeters) are the vertical outlines around every layer. Top and bottom shells are the solid horizontal layers that close the part above and below the infill. Between them sits sparse infill, usually 15-20%.
The names change by slicer:
| Slicer | Setting | Unit |
|---|---|---|
| Bambu Studio, OrcaSlicer | Top shell layers, top shell thickness (and the same for bottom) | Layers and mm |
| PrusaSlicer | Solid layers top and bottom, minimum shell thickness | Layers and mm |
| Cura | Top/Bottom Thickness, Top Layers, Bottom Layers | mm or layers |
For how walls, infill and layers fit together as a whole, see layers, infill and shells explained. This guide is only about the top and bottom.
Layers or thickness: which one counts
What protects the part is the thickness of solid plastic, not the count. The count only gives a thickness once you multiply by the layer height:
| Layer height | 3 layers | 4 layers | 5 layers | 6 layers | 8 layers |
|---|---|---|---|---|---|
| 0.12 mm | 0.36 mm | 0.48 mm | 0.60 mm | 0.72 mm | 0.96 mm |
| 0.16 mm | 0.48 mm | 0.64 mm | 0.80 mm | 0.96 mm | 1.28 mm |
| 0.20 mm | 0.60 mm | 0.80 mm | 1.00 mm | 1.20 mm | 1.60 mm |
| 0.28 mm | 0.84 mm | 1.12 mm | 1.40 mm | 1.68 mm | 2.24 mm |
So 5 top layers is a solid 1 mm at 0.2 mm, but only 0.6 mm if you switch to a 0.12 mm fine profile and keep the count. That's why fine-detail prints often show a weak or bumpy top.
Bambu Studio, OrcaSlicer and PrusaSlicer let you set both a layer count and a minimum thickness, and use whichever gives more layers. Set the thickness (1 mm top, 0.6-0.8 mm bottom) and the count adjusts when you change layer height. Cura works from thickness and divides by the layer height.
How many top layers
The top needs more layers than the bottom because the first solid layer above the infill bridges across the gaps between infill lines. It sags a little, and each layer after it flattens the surface a bit more. With too few layers you get pillowing: bumps, pinholes or a visible grid on the top face.
| Situation | Top shell | At 0.2 mm |
|---|---|---|
| Most parts, 15-20% infill | 1.0 mm | 5 layers |
| Infill 10% or less, or lightning infill | 1.2-1.4 mm | 6-7 layers |
| Top surface that must look flawless | 1.2 mm | 6 layers |
| Part that gets sanded or filled on top | 1.4-1.6 mm | 7-8 layers |
| Draft or test print | 0.6-0.8 mm | 3-4 layers |
More top layers fix pillowing better than more infill does, and they cost less: they add plastic only on the top face, while infill adds it everywhere inside the part.
How many bottom layers
The first bottom layer prints straight on the bed, not over infill, so it starts flat. Three layers (0.6 mm) is enough for most parts, four (0.8 mm) is a safe default.
Add more only for a reason:
- Watertight floors. A planter tray or a bin that might see a spill: 4-5 bottom layers and at least 3 walls, because water finds the gap where floor meets wall.
- Sanded or flattened bottoms. Give yourself 1 mm or more so sanding doesn't break into the infill.
- Thin layers. At 0.12 mm, 3 layers is only 0.36 mm. Set a 0.6 mm minimum instead.
More bottom layers don't fix a bad first layer, warping or poor bed adhesion. Fix those at the bed.
What extra layers cost
Real numbers: we sliced our desk name plate (a 180 x 50 x 4 mm plate printed flat, plus its stand) at 0.2 mm, 2 walls and 20% infill, changing only the top and bottom layers.
| Top / bottom layers | Weight | Print time |
|---|---|---|
| 3 / 3 | 43.0 g | 62 min |
| 5 / 3 (the default) | 48.3 g | 67 min |
| 8 / 3 | 56.0 g | 76 min |
| 5 / 5 | 53.0 g | 71 min |
| 5 / 8 | 60.5 g | 78 min |
On a part this flat, each extra top or bottom layer adds about 2.5 g and 2-3 minutes. On parts with small tops it's close to nothing.
Then we did the same with our drawer organizer bin (100 x 75 x 40 mm, open top) at 3, 5 and 8 bottom layers. The weight stayed at 39.8 g and the time at 41-42 minutes. Its floor is 1.6 mm, 8 layers, and with 3 bottom layers under it and 5 top layers on its upper face it was already solid, so extra bottom layers had nothing left to fill. A floor or ceiling thinner than the top and bottom shells combined prints solid anyway.
Fixing common problems
| What you see | Likely cause | Fix |
|---|---|---|
| Bumps, pinholes or a grid on the top face | Too few top layers for the infill | Add 1-2 top layers, or raise the top shell thickness to 1.2 mm |
| Top looks fine at 0.2 mm, rough at 0.12 mm | Layer count fixed, thickness halved | Set a minimum top thickness instead of a count |
| Infill pattern visible from below | Very thin bottom or thin first layer | 4 bottom layers; check first-layer height and bed level |
| Bin or tray leaks at the floor | Floor-to-wall gap | 4-5 bottom layers and 3+ walls |
| Sanding breaks through to infill | Shell too thin for finishing | 1.4 mm or more on faces you'll sand |
Get it printed
Every Mandarin 3D quote is sliced the same way: 0.2 mm layers, 2 walls, 5 top and 3 bottom layers, 20% infill, in PLA or PETG. Live prices today, one piece, shipped by USPS anywhere in the US, before tax:
| Part | PLA | PETG |
|---|---|---|
| Drawer organizer bin, 100 x 75 x 40 mm | $11.43 | $12.74 |
| Desk name plate with stand, 180 mm long | $12.08 | $13.33 |
PETG is made to order in black or white and adds 3 to 5 business days.
Since the profile is fixed, the way to get a thicker top or floor is to model it in: a 2 mm floor in your CAD file prints as 2 mm of solid plastic, whatever the shell setting. Drop your STL, 3MF or STEP file in the box below for an instant price, or describe the part to the AI designer and say how thick the floor and lid should be.
Questions
How many top shell layers should I use?
Five at 0.2 mm layer height, about 1 mm of solid top. Go to 6-8 when infill is 10% or less, when layers are thinner than 0.15 mm, or when the top surface must look flawless.
How many bottom shell layers do I need?
Three or four at 0.2 mm (0.6-0.8 mm). The bottom sits on the bed, so it doesn't bridge over infill and needs fewer layers than the top. Use 4-5 for floors that must stay watertight or get sanded.
Is shell thickness or shell layers more important?
Thickness. The same 5 layers are 1 mm at 0.2 mm but only 0.6 mm at 0.12 mm. Most slicers let you set both and use whichever gives more, so set a thickness and the layer count follows the layer height.
Why does my top surface show gaps or bumps?
That is pillowing: the first solid layers sag between infill lines and too few layers follow to close it. Add one or two top layers, or raise infill, rather than raising temperature.