The only number that compares two packs
A shelf gives you two figures and neither answers the question on its own. The price tells you what leaves your account. The pack size tells you what you get. Value is the relationship between them, and that is the unit price:
unit price = total price ÷ quantity
Expressed against a common measure — one kilogram, one litre, one item — so that a 400g pack and a 1kg pack become directly comparable numbers rather than two pairs of figures you have to hold in your head.
£2.50 for 400g → £6.25 per kg
£5.60 for 1kg → £5.60 per kg
The larger pack wins here by about 10%. It very often does. It does not always, and that is the entire reason this calculation is worth doing rather than assuming.
The bigger pack is not always cheaper
This is the assumption the tool exists to test, and it fails more often than most people expect.
Shelf surveys by consumer organisations have repeatedly found larger packs priced above the smaller ones beside them on a per-unit basis. It concentrates in a few predictable places:
Promotions on the small size. A temporary discount on the 400g tin can easily take it below the 1kg one per gram, while the shelf still implies the big one is the value option.
Premium and convenience ranges. Resealable pouches, single-serve portions and anything sold on convenience carry a margin that does not scale down with size.
Family and bulk packs on non-food lines. Cleaning products, toiletries and pet food are frequent offenders, partly because the packs look dramatically bigger without the quantity rising proportionally.
The shelf is not trying to deceive you in most of these cases. Pricing is set by category, by promotion cycle and by supplier deal, and per-unit consistency is not a constraint anyone is enforcing. The result is the same either way: the size that looks like value sometimes is not, and the only way to know is to divide.
Why the shelf label does not always help
Most of Europe and the UK require a unit price on the shelf edge, so in principle the work is done for you. In practice comparing them by eye is unreliable, for one specific reason: the displayed unit is not always the same across a category.
One product shows £ per 100g. Its neighbour shows £ per kg. A third, sold as a multipack, shows £ per item. Glancing between "£0.62" and "£5.60" tells you nothing at all until you notice they are measured against different things — and the figures are printed small, in the corner, in grey.
The regulations require a unit price. They do not, in every case, force a single unit across every comparable product on a shelf. That gap is where the eye comparison fails.
In the United States there is no federal requirement at all. Unit pricing is mandated by some states, encouraged by NIST's model regulation, and voluntary elsewhere, which is why shelf labels vary so widely between chains and between states.
Entering both figures here puts them on one basis, which is the entire job.
Shrinkflation is a price rise you cannot see
A pack gets smaller. The price does not change. Nothing on the shelf looks different.
This is a price increase, and it is specifically designed not to register as one, because shoppers remember prices far better than they remember pack weights. A cereal box dropping from 500g to 440g at the same price is a 13.6% rise, and almost nobody notices it happening.
The unit price is the only figure that catches it. The shelf price is unchanged; the per-kilogram figure has jumped.
If there is one habit worth taking from this page: note the unit price of the handful of things you buy every week. A jump with no change in shelf price is shrinkflation, every time, and it is invisible by any other method.
Multibuys need setting up correctly
Promotions are where unit pricing earns its keep, because the offer is usually stated in a form designed to sound better than it is.
To compare a multibuy, enter it as one line: the total you would actually pay, and the total quantity you would actually receive.
Three for the price of two on 400g tins at £2.50 each:
- Price: £5.00 (you pay for two)
- Quantity: 1200g (you get three)
- Unit price: £4.17 per kg
Now compare that against the 1kg tin at £5.60 — which is £5.60 per kg. The multibuy wins here. Run the same exercise on a two-for-£6 offer where the single price is £3.20 and it does not.
Two things to watch:
"Half price" against what. A promotional saving is measured against a reference price that may have been in place briefly or may itself be above the usual selling price. The unit price of what you will pay is a fact; the saving is a claim.
Quantity you would not otherwise buy. Three tins at a good unit price is only a saving if you were going to use three tins. If one goes out of date, the real unit price is what you paid divided by what you used.
Comparing things that are not the same kind
The tool refuses to compare a weight against a volume, and that refusal is deliberate. £2 per kilogram and £3 per litre are not comparable numbers, and a tool that cheerfully ranked them would be producing nonsense with a confident face.
Where you genuinely need to cross the boundary, supply the missing link yourself:
Count against weight. Twelve bread rolls and an 800g loaf compare only once you know what a roll weighs. Weigh one, multiply, enter that.
Concentrate against ready-to-use. Compare the made-up quantity, not the packaged one. A 500ml concentrate that makes 5 litres is competing on those 5 litres.
Density-dependent goods. A litre of oil and a kilogram of oil are different amounts, and the conversion depends on the specific oil. If a recipe or a use case works in one measure, convert everything to that one.
Unit price is an input, not the decision
The cheapest per kilogram is not automatically the right purchase, and treating it that way is its own error.
Waste cancels the saving completely. A 10% better unit price on a pack where you throw away a quarter is a 15% worse outcome. For anything perishable, the real question is cost per unit consumed.
Storage has a cost. Bulk that fills a cupboard you needed for something else, or that has to be decanted, is not free.
Larger packs change behaviour. There is reasonably consistent evidence that people consume more of something when they have more of it in the house, which for some categories is precisely not what you wanted.
Cash flow is real. A better unit price that requires £40 now rather than £6 now is not available to everyone every week, and that is a legitimate reason to buy the smaller size.
Unit price answers one question well: which of these is cheaper per gram. Whether cheaper per gram is the thing you should optimise depends on what you are buying and how you will use it.