Comparing the ticket price of two grocery packs can be misleading. A larger pack is not always cheaper, and a discount is not necessarily the best value once pack size, loyalty pricing, and waste are considered. This guide shows how to calculate the cheapest price per unit, compare grams, litres, and individual items, and keep a simple worksheet you can reuse whenever supermarket prices change.
Overview
Unit price shopping means converting different pack sizes into the same measurement. Instead of asking whether a £2 pack is cheaper than a £3 pack, you ask how much each option costs per 100g, per litre, per item, or another useful unit.
The basic formula is:
Unit price = total price ÷ quantity
For grocery price comparison, shoppers often use:
- Price per 100g: useful for food sold by weight.
- Price per litre: useful for drinks, detergent, and other liquids.
- Price per item: useful for toilet rolls, nappies, batteries, bin bags, and multipacks.
- Price per serving: useful when pack sizes differ but the number of servings matters more than weight.
Unit pricing is most useful when the products are genuinely comparable. Check that the brand, product type, concentration, quality, and included quantity are similar. A lower price per 100g may not represent better value if one product has a different recipe, strength, or usable yield.
How to estimate the cheapest price per unit
1. Record the shelf price and pack quantity
Write down the actual price you would pay, not just the regular price printed on the shelf. Include a loyalty-card price only if you are eligible for it and intend to use it. For a temporary promotion, note the offer conditions, such as buying two packs or purchasing a minimum basket value.
Then record the net quantity. For weight, use grams; for liquid, use millilitres or litres; and for multipacks, count the individual units.
2. Convert each option to a common unit
If a product weighs 750g, divide its price by 7.5 to calculate the price per 100g. If a bottle contains 1.5 litres, divide the price by 1.5 to calculate the price per litre. If a box contains 24 items, divide the price by 24 to find the cost per item.
For a quick weight formula:
Price per 100g = price ÷ weight in grams × 100
For liquids:
Price per litre = price ÷ volume in litres
For multipacks:
Price per item = pack price ÷ number of items
3. Compare the results, not the pack prices
The lowest calculated figure is the cheapest price per unit, assuming the products meet your needs. Round figures only after doing the calculation. Rounding too early can make two close options appear identical.
4. Check the promotion carefully
A multibuy offer changes the calculation only when you buy the required quantity. If one pack costs £2.40, but two cost £4, use £4 as the total price for two packs and divide by the combined quantity. Do not treat a multibuy as a saving if you would not normally use the extra product before it expires.
Coupons, discount codes, and store discounts can also change the result. Apply a product-specific discount to the relevant pack before calculating the unit price. If a coupon reduces the whole basket or requires a minimum spend, calculate the overall basket cost separately rather than assigning the full saving to one item.
Inputs and assumptions
A reliable unit price comparison depends on using consistent inputs. Keep the following checklist with your shopping notes:
- Same measurement: compare grams with grams, litres with litres, and items with items.
- Net quantity: use the amount stated on the packaging, not the outside dimensions or an approximate fill level.
- Comparable products: check flavour, strength, concentration, size, and product specification.
- Real checkout price: include a valid loyalty price, coupon, or qualifying promotion only when its conditions apply.
- Usable quantity: consider waste, preparation losses, and whether all of the product will be used.
- Extra costs: for online orders, include delivery or service charges when comparing the total basket cost.
Packaging can use different conventions. A product may be listed in kilograms while another shows grams, or one drink may show millilitres while another uses litres. Convert before comparing: 1 kilogram equals 1,000 grams, and 1 litre equals 1,000 millilitres.
It is also worth separating “cheapest unit price” from “lowest total spend.” A large pack may offer a better price per 100g but require more money at checkout. If your budget is tight or the product is perishable, the smaller pack can still be the sensible choice.
Worked examples
The figures below are illustrative examples. Replace them with the prices and quantities you see during your own shop.
| Option | Pack price | Pack size | Calculation | Unit price |
|---|---|---|---|---|
| A | £1.80 | 500g | £1.80 ÷ 500 × 100 | £0.36 per 100g |
| B | £2.70 | 800g | £2.70 ÷ 800 × 100 | £0.3375 per 100g |
| C | £3.20 | 1kg | £3.20 ÷ 1,000 × 100 | £0.32 per 100g |
Option C has the lowest price per 100g in this example. Option A has the lowest total price, however, so the best choice depends on both value and how much product you need.
For a multipack, imagine six rolls priced at £3.30 and a twelve-roll pack priced at £6.00. The first option costs £0.55 per roll, while the second costs £0.50 per roll. The larger pack is cheaper per unit, but only if the pack fits your storage space and you expect to use the rolls before buying another pack.
For a temporary discount, imagine a 750ml detergent bottle priced at £3, reduced to £2.40 with a valid store offer. At the regular price, the calculation is £3 ÷ 0.75, or £4 per litre. At the offer price, it is £2.40 ÷ 0.75, or £3.20 per litre. Record both figures if you want to judge whether the promotion is a meaningful saving against alternatives.
Use a reusable unit-price worksheet
A simple worksheet prevents rushed decisions in the aisle. Create columns for product name, checkout price, quantity, measurement, standard unit, unit price, promotion conditions, and notes. Enter at least two comparable options before choosing.
For example:
- Product: the exact item and variant.
- Price paid: the price after an eligible discount.
- Quantity: grams, litres, or item count.
- Standard unit: per 100g, per litre, or per item.
- Result: the calculated unit price.
- Practical note: expiry date, storage, minimum purchase, or likely waste.
A calculator app or spreadsheet can do the arithmetic, but the decision still needs a common-sense check. A cheap unit price is not a saving if it leads to unused food, duplicate purchases, or an offer you would not otherwise buy.
When to recalculate
Recalculate whenever the underlying inputs change. Supermarket prices, pack sizes, loyalty offers, clearance labels, and multibuy conditions can all alter the result. A product can become more expensive per unit even when its shelf price looks unchanged if the quantity has been reduced.
Review your comparison when:
- a regular price or loyalty price changes;
- a coupon, promo code, or multibuy offer starts or ends;
- the product moves to a different pack size;
- you switch between shopping in-store and online and delivery costs apply;
- your household usage, storage space, or budget changes;
- you are comparing seasonal, clearance, or limited-time deals.
Before each grocery shop, choose the products you buy regularly and refresh only those calculations. Start with high-use items, then check unusual promotions. For wider supermarket planning, pair unit-price notes with a weekly shopping list and a firm budget. If you use price-drop alerts, recalculate the unit price when an alert arrives rather than assuming the marked-down item is automatically the best deal.
Finally, keep the winning comparison until your next shop. That gives you a practical benchmark for spotting genuine value without relying on a large “save” label. For related ways to assess checkout savings, see our discount calculator guide and learn how to check whether a coupon code is genuine before applying it.
