Which Pack Is Actually Cheaper? How to Compare Unit Prices

One Calculation, Applied Consistently

Every "which is better value" question reduces to the same division:

Unit price = total price ÷ quantity

The arithmetic is trivial. What makes shelf comparison hard is that the two products in your hands are rarely quoted in the same unit, and the quantity that matters is not always the one printed on the label. Almost every mistake comes from one of those two things rather than from the division.

Pick one unit — per 100 g, per litre, per wash, per sheet — convert both products to it, and only then compare. Doing it in your head at the shelf is realistic if you round: £2.40 for 750 g is near enough £2.40 for 7.5 lots of 100 g, which is 32p.

Bigger Packs Are Not Reliably Cheaper

The assumption that buying larger always saves money is the single most expensive habit in a supermarket, because it stops people checking.

Pasta at £2.40 for 750 g is 32p per 100 g. The 1.2 kg bag at £3.60 is 30p per 100 g — the big pack wins, by about 6%.

Rice at £1.30 for 500 g is 26p per 100 g. The 1 kg bag at £2.80 is 28p per 100 g — here the small pack wins, by about 7%.

Nothing distinguishes these two situations from the front of the packet. Large sizes are often priced on the assumption that shoppers will not check, and promotions on small sizes routinely invert the usual order for a week at a time. The only way to know is to divide.

UK Shelf Labels Must Show a Unit Price

Under the Price Marking Order, retailers in the UK are required to display a unit price alongside the selling price on most grocery items, which is the small print in the corner of the shelf label. It is the quickest route to an answer when it is there.

Two cautions. Historically retailers had latitude over which unit to use, so the same shelf could show one product per 100 g and its neighbour per kilogram — a tenfold difference that looks like a bargain at a glance. Consistency has improved after pressure from the Competition and Markets Authority, but mixed units still appear, particularly across different brands of the same product.

More importantly, promotional tickets are where unit pricing is thinnest on the ground. A large yellow "2 for £5" label frequently replaces the standard shelf edge label rather than supplementing it, which removes the one number that would let you compare it with anything. Treat a promotional ticket as an invitation to do the division yourself.

What Multibuys Are Actually Worth

Offers are phrased to sound larger than they are. Each of these has a fixed effective discount, and they are worth knowing by heart:

Buy one get one free: two items for the price of one — 50% off
3 for 2: three items for the price of two — 33.3% off
Buy one get one half price: two items for 1.5 times the price — 25% off
4 for 3: 25% off

Note that "buy one get one half price" delivers half the saving of a 3-for-2, despite the word half appearing in it and not in the other. The discount applies to one item of two, not to the total.

"50% Extra Free" Is a 33% Discount

This is the most reliably misread claim in a supermarket, and there is a clean rule behind it.

Extra-free promotions hold the price and increase the quantity, so the saving is the extra quantity as a share of the new, larger total — not of the original:

Discount = X ÷ (100 + X)

20% extra free = 20/120 = 16.7% off
25% extra free = 25/125 = 20% off
50% extra free = 50/150 = 33.3% off
100% extra free = 100/200 = 50% off, which is the same as buy one get one free

The rule runs the other way too, and it explains why deep discounts are rarely advertised as extra free: matching a genuine 50% off would require giving away 100% extra.

An "extra free" pack is still worth having. It is simply worth a third less than the headline number suggests.

Choosing the Right Unit Changes the Answer

For rice and pasta, weight is obviously the right measure. For plenty of other products the obvious unit gives the wrong answer outright.

Detergent: per litre or per wash?

A standard 1.5 L bottle at £6.00 doing 38 washes costs £4.00 per litre and 15.8p per wash. A concentrated 1 L bottle at £6.50 doing 50 washes costs £6.50 per litre and 13.0p per wash.

By the litre, the concentrate looks about 63% more expensive. By the wash — the only thing you actually buy detergent for — it is about 18% cheaper. The shelf label will usually quote per litre.

Kitchen roll: per sheet or per square metre?

Sheet counts are printed prominently and sheet sizes are not, so they vary between brands.

Two rolls of 100 sheets at 23 × 24 cm for £2.50 works out at 1.25p per sheet, or 22.6p per m². Three rolls of 60 sheets at 27 × 24 cm for £2.70 is 1.5p per sheet, or 23.1p per m².

Per sheet, the first looks 20% better. Per square metre, which is what you are drying your hands with, it is better by about 2%.

The same reasoning applies to nappies by size rather than count, drinks by volume rather than by bottle, and anything sold as concentrate. Ask what the product is actually for, and measure that.

Waste Makes the Unit Price a Lie

Unit price assumes you use all of it. For fresh food that assumption is often wrong, and it overturns the result more decisively than any promotion.

Effective unit price = total price ÷ quantity actually used

Return to the rice: the 1 kg bag at £2.80 is 28p per 100 g. Throw away a quarter of it and you have paid £2.80 for 750 g of usable rice — 37.3p per 100 g, now 43% worse than the small pack you rejected.

UK households discard a substantial share of the food they buy, and oversized packs are a direct contributor. Before upgrading to the larger size of anything perishable, the honest question is not whether it is cheaper per gram but whether you will finish it.

Three other costs sit outside the division and deserve a thought: the storage the pack needs, the cash tied up in stock you will use over months, and freshness, since a large bag of coffee or flour degrades before the end.

Loyalty Prices and the Comparison That Matters

Member pricing has made shelf comparison harder, because two numbers now sit on the same label and only one applies to you.

If you hold the card, the member price is your real price and the only one worth using. If you do not, the higher figure is yours, however prominently the other is displayed. The comparison that is never valid is a member price for one product against the shelf price of another — which is exactly the comparison the layout encourages.

A Practical Method at the Shelf

Read the small print on the shelf label first. When both products are quoted in the same unit, the work is done.
If the units differ, convert to the smaller one. Per kilogram to per 100 g is a division by ten, which is a decimal point rather than a sum.
Round hard. £3.60 for 1.2 kg is 36 divided by 12, which is 30p per 100 g. Precision to the penny changes nothing when the gap is 5%.
Convert a promotion to a percentage using the figures above rather than trusting the wording.
Ask whether you will use it all before choosing the larger pack of anything fresh.

Conclusion

Unit pricing is one short division, and its value lies in applying it consistently rather than in the arithmetic. The habit catches the three things that cost real money over a year: large packs that are quietly more expensive per gram, promotions worth a third less than they sound, and bulk buys that end up in the bin.

Where the packs are quoted in units that do not line up — grams against ounces, litres against fluid ounces, millilitres against pints — convert before dividing rather than after. A Percentage Calculator turns any two unit prices into the gap between them, and the converters below will put two awkward pack sizes into the same units first.

Try our Percentage Calculator

Try our Weight Converter