Compare bulk pack cost against single unit price.
The sticker unit price of a bulk pack ignores what you throw away. Dividing the pack price by the units you actually use gives the real cost per unit, and comparing it with the single price shows whether buying big still wins. The break-even waste percentage is the useful threshold: waste more than that and the bulk pack is the more expensive option.
Waste-adjusted unit cost
Real cost per unit used = pack price / (units x (1 - waste/100)); break-even waste = (1 - pack unit cost / single unit price) x 100
When your expected waste exceeds the break-even figure shown here. For perishables that threshold is often surprisingly low — a 20% discount is erased by 20% spoilage.
It does in practice: packs that will not fit tend to get opened, decanted and forgotten, which raises real waste. Reflect that by entering a higher waste percentage.
Anything you paid for and did not use — expired product, damaged units and quantities given away because you could not get through them.