To calculate price per 100g, divide the total price by the pack weight in grams, then multiply by 100. For price per 100ml, do the same using millilitres. For price per item, divide the total price by the number of items. Keep the unit and currency the same for every option you compare.
The formulas
| Comparison | Calculation |
|---|---|
| Price per 100g | Price ÷ weight in grams × 100 |
| Price per 100ml | Price ÷ volume in millilitres × 100 |
| Price per kilogram | Price ÷ weight in grams × 1,000 |
| Price per litre | Price ÷ volume in millilitres × 1,000 |
| Price per item | Price ÷ number of equivalent items |
Enter the numbers in this order in your phone calculator. If you enter pounds, the answer is in pounds; if you enter pence, it is in pence. This page gives calculator instructions and worked examples rather than an interactive calculator.
Worked example: compare two pack weights
Suppose Pack A costs £2.40 for 300g and Pack B costs £3.50 for 500g. These are invented examples, not current retail offers.
- Pack A: £2.40 ÷ 300 × 100 = £0.80 per 100g.
- Pack B: £3.50 ÷ 500 × 100 = £0.70 per 100g.
Pack B is cheaper by weight, despite costing more at checkout. It saves 10p for each 100g compared with Pack A. But if you only need a small quantity and would waste the rest, Pack A can still be the more sensible purchase.
Worked example: compare bottle sizes
Using hypothetical prices, a 750ml bottle at £3 costs £3 ÷ 750 × 100 = £0.40 per 100ml. A 1-litre bottle at £3.60 costs £0.36 per 100ml. The larger bottle is cheaper per unit of volume.
Convert litres to millilitres first if using the 100ml formula: 1 litre is 1,000ml. Do not divide one price by 750 and the other by 1 while pretending both answers use the same unit.
How to compare a multibuy
Use the full required spend and the full quantity included. For an illustrative offer of three 250g packs for £6, the total is 750g, making the price £6 ÷ 750 × 100 = £0.80 per 100g. Compare that with the actual price per 100g of the alternative.
Do not divide the multibuy price by the weight of just one pack. If the discount requires membership, only use it in your comparison if you qualify. If packs have different sizes, add their weights individually.
When price per item works better
For items used one at a time, divide by the number of equivalent items. A hypothetical 40-item pack at £6 costs 15p per item. A 60-item pack at £8.40 costs 14p per item.
The word “equivalent” matters. Different sizes, concentrations or usage instructions can make a simple item count misleading. Compare the amount needed for the same job if that information is supplied reliably, and keep quality or suitability differences separate from the arithmetic.
How to convert shelf-label units
If one price is per kilogram and another is per 100g, divide the kilogram price by 10 to put them on a per-100g basis. The same conversion works between price per litre and per 100ml. For example, £8 per kilogram is £0.80 per 100g.
The Competition and Markets Authority’s unit-pricing guide explains why larger packs, promotional offers and multipacks are not automatically the best value. Checking a common unit helps you test the claim for yourself.
Include costs and waste
A lower unit price is useful, but your budget still pays the whole bill. If delivery is a cost incurred solely to obtain a particular order, include it when comparing that purchase. For a mixed shop, avoid adding the full delivery cost independently to every item.
Keep the quantity you will use in mind. If a hypothetical £5 pack contains 1,000g but you only use 500g, you effectively spent £1 per 100g consumed. Unit pricing of the amount bought does not automatically account for waste.
Common mistakes to avoid
- Mixing pounds and pence.
- Comparing grams with millilitres without an appropriate conversion.
- Ignoring the number of packs in a bundle.
- Using an unavailable loyalty discount.
- Rounding too early: keep extra decimal places until the final comparison.
For a real brand example, see our Pepsi comparison. For the wider decision about quantity, read our Amazon bulk-buying guide.
Research checked 25 October 2026. All calculations above are illustrative, not live product prices.
Check the result without starting again
Multiply your unit price back by the quantity to catch a calculator error. For the £3.50, 500g example, 70p per 100g multiplied by five gives £3.50. If your answer is £350 or 0.7p, pounds and pence have been mixed.
For a bundle, add every included pack before dividing. Two 750ml bottles contain 1,500ml, so an illustrative £5 bundle costs about 33.3p per 100ml. This is a quantity calculation; it does not establish that different concentrations do the same job.
