Real-Use Unit Cost Calculator

A worked example

A 12-serving box of protein powder costs $4.49 — about 37.4¢ per serving. The 48-serving tub is $11.99, or 25.0¢ per serving on the shelf tag. Bulk looks like an easy win: 33% cheaper.

But you've bought the big tub before, and you got bored of the flavour around the two-thirds mark. Realistically you'll finish about 70% of it.

The gap collapses from 33% to about 5%. And if you only get through half the tub, the real cost climbs to 50¢ a serving — the small box would have been the cheaper choice all along.

This is the same reason a $180 coat worn 60 times ($3 a wear) beats a $40 coat worn four times ($10 a wear), and why a gym membership at $45 a month is only cheap if you go.

Limitations

• The waste percentage is a guess, and it's the number that decides everything. Base it on what happened last time you bought this, not on what you intend this time.
• It doesn't model spoilage dates directly. For perishables, work out how many units you get through before the expiry date and convert that into a waste share.
• Storage, freezer space, and clutter aren't counted. Bulk buying has costs that don't show up in a per-unit figure.
• It assumes both options are otherwise identical. Different quality, concentration, or formulation makes a direct comparison unreliable — normalise for that first.
• Cash flow isn't considered. Cheaper per unit is little help if the larger size doesn't fit this week's budget.
• This is a budgeting estimate, not financial advice.

Frequently asked questions

How do I estimate the waste percentage honestly?
Look backwards, not forwards. If you've thrown out roughly a third of this product before, use 33% — not the 0% you're hoping for. For anything perishable, count how many servings you genuinely get through in the time before it turns, and treat the rest as waste.

Can I use this for things that don't spoil?
Yes. Waste doesn't have to mean rotting. For clothes, "units" are wears and waste is the share of the garment's life you won't use before it goes out of rotation. For subscriptions, units are months used versus months paid for.

What if I'd share the extra with someone?
Then it isn't waste — enter a lower waste share, since those units still get used. The calculation is about units consumed, not units consumed by you specifically.

Does this replace the shelf's unit price tag?
No, it corrects it. Shelf tags are accurate about what you're buying and silent about what you'll finish. Use both: the tag for a first pass, this for the sizes you're genuinely torn between.

Why do both columns default to different waste levels?
Because that's the usual real-world shape of the problem: small packages get finished, large ones often don't. If you reliably finish both, set both to 0% and you'll get the same answer the shelf tag gives.