Why infill weight estimates run lighter than the slicer
How infill density maps to filament use, why walls and solid layers add grams this estimate skips, and when a quick volume calc is close enough.
What infill percentage really controls
Infill is the internal lattice a slicer prints inside the solid shell of a part, and its percentage sets how much of that interior volume is plastic versus air. At 20% roughly a fifth of the inside is filled, which is why the estimate multiplies the model volume by 0.20. This tool treats the whole model as if it were only infill, so weight = volume x infill fraction x density. That keeps the math transparent and gives you a floor for how much filament the print will use.
Where the missing grams come from
Real prints are not pure infill. Every part has perimeter walls, a solid floor and ceiling of top and bottom layers, and often supports and a brim, all printed at or near 100% density. On a small or thin-walled model those solid regions can outweigh the sparse infill entirely, so the slicer number lands well above this estimate. On a large, blocky part the walls are a thin skin around a big infilled core, and the gap between the two figures shrinks. That is why the tool warns it reads low and is closest for chunky shapes.
From grams to meters of filament
Weight and length are two views of the same plastic. To get length the tool divides the weight by the density to recover a solid volume, then divides by the cross-section of the filament. For 1.75 mm stock that circle is about 2.405 square millimeters, so 24.8 grams of PLA works out to roughly 8.32 meters. This is useful for checking whether a partly used spool has enough left, but it assumes 1.75 mm filament, so multiply-through differently if you run 2.85 mm.
When a quick estimate is enough
For choosing between spools, budgeting a print or confirming you will not run out mid-job, a volume-and-infill estimate is fast and good enough, especially if you add a margin for the walls and supports it omits. When you need an accurate cost or a precise material reservation, slice the actual model and read the slicer's own weight and length, which account for every wall, layer and support. Think of this calculator as the back-of-the-envelope pass that happens before you open the slicer.