What Are Top and Bottom Layers? Choosing Solid Layer Count for a Clean Finish

The flat surfaces that cap the top and bottom of a 3D print are not just cosmetic skins. Top and bottom layers are the solid shells that seal off the honeycomb infill underneath, and getting the count right is the difference between a smooth, sealed part and one pocked with little sagging dimples. Most slicers ship with a sensible default, but the moment you change layer height or print something that needs to hold water, that default stops being right.

Quick Answer

Use 3 to 5 solid layers on both the top and bottom, aiming for roughly 0.8mm to 1mm of total solid thickness. At a 0.2mm layer height that means 4 to 5 layers. Too few and you get pillowing; more layers buy you a cleaner finish and water-tightness, with diminishing returns past about 1mm.

Why Solid Layers Exist At All

Infill is hollow on purpose. A 20 percent gyroid or grid pattern saves filament and print time, but it leaves gaps the nozzle has to bridge over when it reaches the top of the part. The top solid layers are printed across that open structure, and the first one or two are effectively bridging across infill gaps. If you only give the slicer one or two top layers, the plastic sags into those gaps before it cools, leaving the dimpled, cratered look known as pillowing.

The bottom layers do the same job in reverse. They sit against the build plate, so they pick up its texture, but they still need enough solid passes to form a flat, rigid base rather than a thin film that telegraphs the infill above it.

Setting The Count By Thickness, Not Layer Number

The trap is thinking in layers when you should think in millimetres. Most slicers, including Cura and PrusaSlicer, now let you specify a solid thickness and they calculate the layer count for you. Set roughly 0.8mm to 1mm and the software adapts when you change layer height.

If you only set a fixed number, here is what happens: 4 top layers at 0.1mm height gives you a thin 0.4mm skin that pillows, while the same 4 layers at 0.28mm gives a chunky 1.12mm cap that wastes time. Always sanity-check that your layer count times your layer height lands near 1mm.

When To Add More Layers

A few jobs justify pushing past five solid layers:

• Water-tight parts like vases, planters, or funnels need 5 or 6 layers plus a few extra perimeters and higher infill so there is no porous path through the wall.
• Parts that will be sanded and painted benefit from extra top layers, because sanding removes material and you do not want to break through into infill.
• Low infill prints (10 percent or less) leave bigger gaps to bridge, so add a top layer or two to stop sagging.

You can browse printers and the maintenance bits that keep finishes clean through the 3D printing range at Evetech, and spare nozzles and tools live in the accessories best sellers.

Frequently Asked Questions

How many top layers stop pillowing?

Three to five solid top layers, totalling around 0.8mm to 1mm, reliably stops pillowing at standard infill. If you still see dimples, raise infill density to 20 percent or more so the layers have less open space to bridge.

Should top and bottom layer counts match?

They can differ. Bottom layers print against a flat plate so they rarely sag, meaning 3 is often enough there, while tops bridge over infill and benefit from one or two more.

Does layer height change the count I need?

Yes. Thinner layers need more of them to reach the same solid thickness. Set a millimetre target instead of a fixed number so the slicer recalculates whenever you change height.

Will more solid layers slow my print much?

Each added solid layer adds full-coverage passes only at the very top and bottom of the part, so the time cost is small on tall prints and more noticeable on short, flat ones.