A 500 g pack for 450 or a 1 kg pack for 820? A "3 for 2" or a 15% discount? The shelf price can't tell you. The unit price can.
unit price = price ÷ quantity
To compare quickly, use the Unit Price Calculator.
Pick one unit and stick with it
Convert everything to the same unit before you compare:
| Product type | Compare per | Formula |
|---|---|---|
| Rice, flour, lentils, snacks | 100 g or 1 kg | price ÷ grams × 100 |
| Milk, oil, juice, shampoo | 1 litre | price ÷ ml × 1,000 |
| Eggs, nappies, tea bags | 1 piece | price ÷ count |
| Detergent | 1 wash | price ÷ washes on the pack |
Worked examples
Bigger pack, better deal?
| Pack | Price | Per 100 g |
|---|---|---|
| Basmati rice, 500 g | 450 | 90.0 |
| Basmati rice, 1 kg | 820 | 82.0 |
| Basmati rice, 3 × 1 kg pack | 2,400 | 80.0 |
The big pack wins here, but only by about 11% over the smallest. If you won't use 3 kg before it goes stale, the "saving" is wasted.
When the smaller pack is cheaper
| Pack | Price | Per 100 g |
|---|---|---|
| Biscuits, 400 g | 95 | 23.75 |
| Biscuits, 450 g "new bigger pack" | 110 | 24.44 |
The bigger pack costs more per gram. This happens all the time: the small pack is on promotion, or the "family size" just has a higher price per gram.
Multi-buy vs discount
Juice is 200 a litre.
- Offer A: 3 for the price of 2 → you pay 400 for 3 L → 133 / L, about 33% off.
- Offer B: 25% off one bottle → 150 / L.
A is cheaper per litre, but only if you'd actually drink 3 litres. Buying more than you need is how multi-buys make money for the shop.
Shrinkflation
Same price, smaller pack:
| Before | After | |
|---|---|---|
| Pack size | 200 g | 180 g |
| Price | 100 | 100 |
| Per 100 g | 50.0 | 55.6 |
The price didn't change, but you're paying 11% more per gram. Check the weight, not just the price, on products you buy often.
Common tricks to watch for
- Different units on shelf labels. One label shows price per 100 g, the next per kg. Convert before comparing.
- "Drained weight" vs net weight for tinned food. Compare drained weight if you're throwing the liquid away.
- Concentrated products. For detergent and squash, compare per wash or per serving, not per ml.
- Bundles with something you don't want. A "free" item you'll never use isn't a discount.
- Percentages that stack differently than they sound. "Extra 20% free" (you get 1.2× for the same price) is about 16.7% off the unit price, not 20%. Check with the Percentage Calculator.
The 10-second habit
At the shelf, use your phone's calculator:
price ÷ grams × 100
Do it for the two packs you're choosing between. After a few weeks you'll know the normal unit price of what you buy, and real deals will be obvious.