Estimate materials needed for a school project.
A school project budget that lists only the materials always overruns, because something gets cut wrong, a print comes out crooked or a component fails. A wastage allowance of ten to twenty percent covers that, and it is applied to the materials only — printing and mounting are entered as a firm cost because they are usually bought once. Tax is applied last, to everything, which is the order retailers actually charge in and the reason a budget built on shelf prices comes in short.
Materials with allowance
Materials = items x average price; wastage = materials x allowance percentage
Total
Total = (materials + wastage + printing) x (1 + tax rate)
Ten percent for assembly from bought parts, twenty or more where you are cutting, painting or soldering for the first time. First attempts at an unfamiliar technique routinely consume double the material.
Substantially, for anything sold in bulk units — card, paint, adhesive and fixings are usually priced per pack far larger than one project needs. Splitting packs across a group is the single biggest saving available.