The Unit Price Trap: When the Bigger Pack Costs You More

"Bigger is cheaper" is one of the expensive beliefs in a supermarket. Sometimes it is true. Sometimes the big pack costs more per gram and sometimes it costs more per use. Here is the math, plus six edge cases where the shelf label can mislead you. The only number that matters is the unit price: price ÷ quantity in the unit (per kg per litre per 100 sheets, per wash). In the EU shops generally have to show it on the shelf label for packaged goods usually in small print.

Edge case 1: The middle pack wins

Cereal three sizes:

Pack Price Per kg

500 g €3.20 €6.40

750 g €4.50 €6.00

1 kg €6.40 €6.40

The biggest box costs exactly the same per kg as the smallest. If you eat 1 kg of this cereal a week picking the pack saves about €21 a year on one product.

Edge case 2: Labels in units

Two pasta sauces, side by side:

Jar A: €1.89 for 250 g label says €0.76 / 100 g

Jar B: €6.50 / kg

Your brain sees "0.76" next to "6.50" and says A is cheaper. Convert: A is 0.756 × 10 = €7.56/kg. B is €6.50/kg 14 % cheaper.

Edge case 3: The multi-buy that's n’t the best deal

Shower gel, 250 ml bottles:

Single: €2.40 → €9.60/L

“3 for €6.00" → 750 ml for €6 = €8.00/L

One 750 ml bottle at €5.40 → €7.20/L

The promo discount is real (€2.00 each of €2.40) but the plain big bottle beats it by 10 %. A "deal" sticker only means cheaper than the small pack, not cheaper than everything else on the shelf.

Edge case 4: The tub you can’t finish

Yoghurt:

500 g at €1.60 → €3.20/kg

1 kg tub at €2.80 → €2.80/kg

The tub wins on paper.. If you only eat 700 g before the rest goes off you paid €2.80 for 0.7 kg, which is €4.00 per kg eaten. Worse than the pot.

The break-even is big unit price ÷ small unit price = the share of the big pack you must actually eat. Here 2.80 ÷ 3.20 = 87.5 %. If you’d throw away than 12.5 % of the tub buy the small one.

Edge case 5: Shrinkflation ( price, less product)

Crisps last year: 175 g for €2.49 = €14.23/kg. This year: 150 g for €2.49 = €16.60/kg.

The shelf price didn’t move,. You pay 16.7 % more per kg. The only way to notice is to track the unit price since the front of the pack won’t tell you.

Edge case 6: Count the rolls, not the "rolls”

Toilet paper:

Pack A: 12 rolls × 120 sheets = 1,440 sheets for €4.20 → €0.292 per 100 sheets

Pack B: 8 rolls × 200 sheets = 1,600 sheets for €4.40 → €0.275 per 100 sheets

Pack A has rolls and a lower sticker price but B is nearly 6 % cheaper where it counts. If the label only says "per roll " compare sheets yourself.

Bonus: Concentrated products

Laundry detergent:

1 L, 20 washes, €4.00 → €0.20/wash (€4.00/L)

1.5 L 25 washes, €5.40 → €0.216/wash (€3.60/L)

The big bottle is cheaper per litre but 8 % more expensive per wash. For products compare per use not per volume.

Cheat sheet

Compare unit prices, not sticker prices.

Convert to the unit before comparing.

For food ask: will I finish it? Use the break- formula.

Compare promos against the pack not just the small one.

For concentrates and rolls compare per use or, per sheet.