Infill: Infill is the internal structure an FDM printer lays down inside a part's walls, set as a density percentage and a pattern such as grid, gyroid or honeycomb.
FDM parts are rarely solid. The slicer prints a few outer walls (perimeters) and top and bottom skins, then fills the inside with a lattice. Density runs from 0% (hollow) to 100% (solid); 15-25% is a common default for visual parts, with more for parts that carry load.
What it means for the price
Infill changes both material and time, but not in proportion. Walls and skins take a fixed amount whatever the infill, so going from 20% to 40% does not double the cost of the part. On small parts the walls dominate; on large, chunky parts the infill does. Strength does not rise in step with density either: adding a perimeter often helps more than adding infill.
Example: a 100 cm³ block whose walls and skins come to 25 cm³ uses about 25 + 75 × 0.2 = 40 cm³ of plastic at 20% infill, and 25 + 75 × 0.4 = 55 cm³ at 40%. The figures are an example.
Common confusions
The infill percentage is not the share of the part’s volume that is plastic, because the walls and skins are always solid. Pattern matters too: gyroid and honeycomb behave differently under load from a simple grid. Read how FDM print time is estimated or see the FDM process page.
Go deeper: FDM print time estimation: why volume does not predict it