Infill Cost Calculator

Why infill does not scale linearly

Doubling infill does not double the filament, and it does not double the print time either. Two reasons:

• Walls are not infill. Perimeters, top layers and bottom layers are printed solid regardless of the infill percentage. On a typical part those solid regions are a fifth to a third of the material, so they do not move when you change infill.
• Time is driven by travel and cooling, not just volume. Dense infill does add time, but faster nozzle speeds on the infill pass and the fact that solid top layers take longer than sparse infill means the relationship is not proportional.

The result is that 20% to 40% sounds like a doubling and usually lands somewhere near a third more material.

The approximation this tool uses

To compare two settings you need one number from your slicer and two you supply. The model's weight at 100% infill gives you the total volume of material the part would need if it were solid. The shell share is how much of that total the slicer already prints solid. What remains scales with infill:

At 20% infill with a 25% shell share, that is 180 × (0.25 + 0.75 × 0.20) = 72 g. At 40%, 180 × (0.25 + 0.75 × 0.40) = 99 g.

This is an approximation and it is honest about it. Real slicers add supports, purge, brim and adaptive infill, and the shell share genuinely varies with wall count and how flat the part is. Use the calculator to see whether a setting change is worth investigating, then take the real numbers from your own slicer preview.

Worked example: 20% versus 40%

A 180 g solid model, a quarter of the material in walls and skins, a $22 spool of 1000 g at $0.022 per gram, print times of 6.0 and 7.4 hours and a machine rate of $0.20/h:

Two things worth noticing. The material increase is 27 grams, not the 36 grams a naive doubling would suggest. And the machine hours contributed about a third of the extra cost even though the model only got 37% heavier — because the extra infill slows the print down as well as consuming plastic.

When more infill actually helps

Infill increases stiffness, but it does so on a curve that flattens fast. Going from 10% to 25% usually transforms how a part feels. Going from 50% to 100% doubles the material and time for a change most people cannot detect by hand.

Two rules that hold up in practice:

• Walls do more than infill. Adding a perimeter increases stiffness and strength more cheaply than raising infill, up to a point. If you are stiffening a functional part, try three or four perimeters at 15% infill before you consider 60% infill.
• Infill pattern matters as much as percentage. Gyroid and cubic at the same percentage behave differently in compression and take different amounts of time. If you are optimising, change one variable at a time.

If the part is decorative, the honest answer is usually 10 to 15% and spend the saving on a better surface finish.

Important assumptions

The shell share default of 25% is an estimate. If your profile uses four perimeters and five top and bottom layers, the solid share is higher than that; if it uses two perimeters, it is lower. The single most useful thing you can do is slice the same model at 100% infill and at your normal setting, and compare the actual filament figures — that gives you your real shell share for that geometry.

Print times are your own slicer figures, not calculated here. The calculator does not attempt to model how a slicer will schedule infill; it takes the times you give it and prices them.