The larger pack is better value only if its price per comparable unit is lower and you use enough of it. Compare the same product, convert both quantities to one unit, then divide each price by its quantity.
Compare your two packs in the unit-price calculator. The examples below use invented shop prices to explain the arithmetic; they are not current offers.
First, put both packs on the same scale
Suppose the same rice is sold in these sizes. Convert 750 g to 0.75 kg before comparing.
| Pack | Checkout price | Quantity | Price per kg |
|---|---|---|---|
| A | £3.60 | 0.75 kg | £4.80 |
| B | £6.40 | 1 kg | £6.40 |
Pack A costs 25% less per kilogram: (6.40 − 4.80) ÷ 6.40 × 100. It also requires less money at checkout. The larger pack loses on both measures in this example.
Use kilograms for both weights, litres for both volumes, or a count for identical items. A price per 100 g cannot be compared directly with a price per kilogram. Do not compare grams with millilitres without a valid product-specific conversion.
A sale label does not settle the comparison
If Pack B gets a 20% discount, its price becomes £5.12 per kilogram. Pack A is still £4.80 per kilogram—6.25% lower than the discounted alternative.
Enter the price you would actually pay after an eligible discount. For a members-only offer, check whether you qualify. Keep any extra delivery charge or minimum-spend condition in view; a cheaper product does not automatically make the whole order cheaper. The discount calculator can check the reduced price.
Now check how much will be used
A different bulk pack costs £8.40 for 2 kg: £4.20 per kilogram. That beats Pack A if you use it all. If only 1.5 kg is used, the cost per kilogram actually used becomes £5.60: 8.40 ÷ 1.5. The unused quarter reverses the apparent saving.
This is a planning scenario, not a food-safety rule. Use the product's storage instructions and date information when deciding how much to buy.
| Question | What to compare |
|---|---|
| Which pack has the lower shelf unit price? | Price ÷ total quantity |
| Which costs less for what I use? | Price ÷ quantity actually used |
| Which fits this week's budget? | Total checkout price |
A quick check before buying
- Compare products with the same purpose and relevant quality.
- Put the quantities in the same unit.
- Use the price you qualify to pay.
- Check the amount you can use and store.
- Choose using both unit cost and total spending.
The formula is currency-independent: use one currency throughout. The CMA reference explains the comparison principle; the prices, discount example and usable-quantity scenario here are original worked examples. There is no assumption that bigger, smaller or branded products always win.