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The Bigger Pack Is Often the More Expensive One Per Unit

It happens often enough to have a name — the quantity surcharge. Convert everything to one unit and the shelf stops being able to hide it.

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Offer formats you can add as a fifth field

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How the calculation works

Three sizes, priced per 100 g Small — 500 g 0.498 cheapest Large — 1 kg 0.429 Family — 1.8 kg 0.388 Here the big pack wins. Often it does not — which is the whole reason to check.

How to Use This Tool

Put one product per line as name, price, size, unit. Sizes can be in grams, kilos, millilitres, litres or a plain count, and they are all converted to whatever you choose to compare in.

The bigger pack is not automatically cheaper

Most shoppers treat "buy the big one" as a rule, and retailers know it. When the larger size is priced above the smaller one's unit rate, the effect has a name — a quantity surcharge — and it is common enough that unit-price labelling is legally required in many countries specifically to expose it.

It is not usually deception so much as pricing to a habit. Larger packs also carry real costs that scale oddly, and a size that only one supplier makes has no direct competitor to be priced against. Either way the only defence is dividing, which takes a moment and is what this does.

Multibuy offers only apply if you buy the offer

"3 for 2" reduces the unit price by 33.3%, but only if you take three. Take one and you pay the shelf price, which is what the unit-price label on the shelf usually shows — the label describes the single item, not the deal.

Add a fifth field to a line to fold the offer in:

  • 3for2, 2for1, 5for4 — buy the first number, pay for the second.
  • -25% — a straight discount.
  • x2 — a multipack, where the size given is per item.

The result then shows what you actually pay per unit if you take the whole offer, and how much you have to spend to get there.

750 ml at 2.10, offered as 3 for 2 buy 1 0.280 per 100 ml buy 3 0.187 per 100 ml Same shelf, same label. The label shows the first number, and the offer only exists at the second — which costs 4.20 up front instead of 2.10.
A cheaper unit price that requires spending more is a real trade-off, not a free win.

What the arithmetic cannot see

A calculator compares price per gram. It does not know:

  • Waste. A cheaper rate on more food than you will finish is not a saving. For anything perishable, the honest unit is price per gram eaten, and nobody can compute that for you.
  • Drained weight. Tinned goods are often priced on gross weight while the useful quantity is the drained weight, which can be barely half.
  • Concentration. Two detergents at the same price per litre are not comparable if one is twice as concentrated. Compare washes, not litres.
  • Cash flow. The cheaper unit rate frequently requires spending more today.

Unit price is the right starting point and it is not the whole answer.

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Frequently Asked Questions

Is the bigger pack always cheaper per unit?
No, and the exception is common enough to have a name — the quantity surcharge. Larger sizes are sometimes priced above the smaller one's unit rate precisely because shoppers assume otherwise, which is why unit-price labelling is legally required in many countries.
How do I compare prices in different units?
Convert everything to a single unit before dividing. Grams and kilos, millilitres and litres — pick one and be consistent. Comparing a price per 500 g against a price per kilo without converting is the mistake the shelf label is designed to prevent.
How does a 3 for 2 offer change the unit price?
It reduces it by 33.3%, but only if you take all three. Buy one and you pay the shelf price. The unit-price label on the shelf normally shows the single-item rate, so the deal is invisible on the label.
What is a quantity surcharge?
When a larger pack costs more per unit than a smaller one of the same product. It arises partly from pricing to shopper habits and partly from real cost differences, and the only reliable way to spot it is to divide price by size yourself.
Should I always buy the lowest unit price?
Not always. A cheaper rate on more than you will use is not a saving — for perishables the honest measure is price per gram actually eaten. Cash flow matters too, since the better rate usually requires spending more today.
Why do tinned goods compare badly on price per gram?
Because the printed weight is often the gross weight including liquid, while the useful quantity is the drained weight, which can be little more than half. Compare drained weights where they are given, or the tin with more brine will look like the better buy.

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