A low shelf price does not always mean a lower cost. This unit price shopping guide shows you how to compare pack sizes, multibuys, discounts and different measures so you can identify the best value for your actual needs.
Overview
Unit pricing turns different-sized products into a like-for-like comparison. Instead of asking whether a £1 pack is cheaper than a £3 pack, you compare the cost of each item, 100g, litre or use. The result is a more useful measure of value, particularly when a promotion makes the headline price look attractive.
The cheapest option overall is not automatically the best choice. A larger pack may have a lower price per 100g but cost more at the checkout. It may also be unsuitable if you cannot use it before it is wasted, or if you are comparing products with different qualities, strengths or contents. The aim is to combine unit price with practical considerations rather than follow the biggest percentage discount.
This method works for groceries, cleaning products, toiletries, stationery, party supplies and many other purchases. It is also useful when comparing online deals, store coupons and clearance offers. If a voucher changes the final price, calculate the unit cost after applying the discount, not before.
How to estimate
Use this simple process whenever two or more products appear to compete on value:
- Choose the comparison measure. Use price per item for multipacks, price per 100g for products sold by weight, price per litre for liquids, or price per use when the number of uses is clear.
- Record the final payable price. Start with the shelf or online price, then subtract a valid coupon or discount that applies to that specific product. Include delivery or mandatory charges if you are shopping online.
- Record the quantity. Check the net weight, volume, number of items or stated uses. Do not rely on the outer packaging size.
- Apply the formula. For price per item, divide total price by number of items. For price per 100g, divide the price by the total grams and multiply by 100. For price per litre, divide the price by the total millilitres and multiply by 1,000.
- Compare like with like. Put the results beside each other using the same unit and round only at the end. The lower unit price is usually the stronger value if the products meet the same need.
Core formulas:
- Price per item = total price ÷ number of items
- Price per 100g = total price ÷ total grams × 100
- Price per litre = total price ÷ total millilitres × 1,000
- Price per use = total price ÷ realistic number of uses
For a quick worksheet, write down the product name, pack size, shelf price, discount, final price, comparison unit and calculated unit price. This prevents a large pack or bold sale label from influencing the decision before the numbers are visible.
Inputs and assumptions
Good comparisons depend on accurate inputs. Check these details before deciding that a deal is better:
- Net contents: Compare the weight or volume inside the pack, not the size of the packaging. Some products contain a mixture of units or multiple compartments.
- Usable quantity: A product may include liquid, concentrate, refills or accessories. Use the quantity that provides the benefit you are buying.
- Product equivalence: Similar-looking products may differ in concentration, specification, durability, materials or performance. A lower price per 100g is not a complete comparison if one item is a different formulation.
- Promotion conditions: A multibuy may require purchasing more than one item. Calculate the cost of the complete offer and check whether all the items will be used.
- Coupon restrictions: A promo code may exclude sale items, require a minimum spend or apply only once. Read the terms before reducing the advertised price.
- Travel and delivery: Add delivery charges, collection fees or extra transport where they affect the real cost. A lower online unit price may not remain lower after these charges.
- Waste and storage: A larger pack is poor value if it expires, dries out, breaks or cannot be stored safely before use.
Be careful with price-per-use estimates. The number of uses can vary with portion size, product strength and personal habits. If the label does not provide a credible usage figure, treat your estimate as a planning aid rather than an exact result.
For household or grocery shopping, a practical decision rule is to compare three figures: the unit price, the amount you must pay today and the likely waste. This gives a better view than focusing on any single number.
Worked examples
Example 1: Comparing pack sizes by weight
Assume Pack A costs £1.80 for 300g and Pack B costs £3.00 for 600g. Pack A costs £1.80 ÷ 300 × 100, which is £0.60 per 100g. Pack B costs £3.00 ÷ 600 × 100, which is £0.50 per 100g. Pack B has the lower unit price, although it requires a higher checkout payment.
If you only need 300g and the second pack is likely to be wasted, the larger pack may not be the better real-world choice. If you can use or store the full amount, its lower price per 100g is more meaningful.
Example 2: Comparing a multibuy with individual items
Assume one item costs £1.20 and a three-item offer costs £3.00. The individual route costs £1.20 per item. The multibuy costs £3.00 ÷ 3, or £1.00 per item, saving £0.20 per item when all three are needed. If you only need two, compare the cost of buying two individually with the full £3.00 offer. The advertised saving may not justify purchasing an unwanted third item.
Example 3: Applying a discount code
Assume an online pack costs £8 and a valid £2 voucher applies without an extra charge. The adjusted price is £6. If the pack contains 12 items, the final unit price is £6 ÷ 12, or £0.50 per item. If delivery costs £3 and no other items are being ordered, the effective cost becomes £9, or £0.75 per item. The discount is still relevant, but delivery changes the comparison.
For more help judging whether a percentage discount or fixed-value coupon suits your basket, see how to choose between percentage-off and fixed-amount coupons. When comparing discount retailers, a store comparison table can also provide a useful starting point, but calculate the unit price for the exact pack you intend to buy.
When to recalculate
Unit prices should be recalculated whenever one of the inputs changes. Repeat the comparison when a retailer changes the pack size, a promotion starts or ends, a coupon has different terms, or delivery charges change. This matters during weekly grocery shops, seasonal events, clearance sales and limited-time deals, when the same product may appear at several prices.
Recheck before buying in bulk, especially if your household needs have changed or storage space is limited. A previous calculation may no longer apply after a recipe change, a new product size or a different discount condition. Online listings can also show several variants together, so confirm that the selected size matches the price used in your calculation.
Before checkout, use this five-step action list:
- Write down the exact pack size and final price.
- Choose one common unit, such as item, 100g or litre.
- Calculate the price after coupons, multibuys and unavoidable charges.
- Check quality, quantity, expiry, storage and likely waste.
- Buy the lowest practical unit cost, not simply the lowest headline price.
Unit pricing is most useful as a repeatable habit. Keep a small note on your phone or use a calculator while shopping, then update it whenever prices or promotions change. Combined with careful coupon checking and a realistic view of what you will use, this approach can make cheap deals easier to judge and budget decisions more consistent.
