Compare products by their true unit price — per 100 g, per kilogram, per litre or per item — with mixed metric and imperial pack sizes normalised for you. Runs entirely in your browser — your numbers never leave this device.
| Item | Standard unit price | Bulk unit price | Verdict |
|---|---|---|---|
| Family rice sack | $0.80 per 100 g | $8.00 per kg | ★ Best value |
| Store brand rice, small bag | $0.90 per 100 g | $9.00 per kg | 12.5% more than the best |
| Small rice pack | $1.10 per 100 g | $11.00 per kg | 37.5% more than the best |
Shelf labels quote whichever unit makes the pack look cheapest. Normalising everything to the same base — per 100 g, per kg, per litre — is what makes the bigger pack's real saving (or the small pack's quiet premium) visible.
| Unit | Category | In base units |
|---|---|---|
| kilogram (kg) | Mass → grams | 1,000 g |
| pound (lb) | Mass → grams | 453.59237 g |
| ounce (oz) | Mass → grams | 28.349523125 g |
| litre (L) | Volume → millilitres | 1,000 ml |
| US gallon (gal) | Volume → millilitres | 3,785.411784 ml |
| fluid ounce (fl oz) | Volume → millilitres | 29.5735295625 ml |
| item | Count → items | 1 item |
Two packs of the same product, priced differently and sized differently, cannot be compared by their shelf prices — the bigger number is not the worse deal, and it is not automatically the better one either. The fix is to normalise: divide each price by its quantity to get a price per standard amount, then compare those. This calculator accepts two to five items with their names, prices, quantities and units, converts every quantity into its category's base — grams for mass, millilitres for volume, the count itself for items — and reports each product's price per 100 g (or 100 ml) and per kilogram (or litre), ranked cheapest first with the best value flagged.
The conversion table does the imperial half of the work exactly: a pound is 453.59237 grams by definition, an ounce a sixteenth of that, a US gallon 3,785.411784 millilitres. So the 16-ounce pack at $6.40 in the worked example is 453.59 grams of product, costing $1.41 per 100 g — a figure you can set directly against a metric pack's per-100 g price without mental arithmetic at the shelf. The percent-more column then prices the gap, not just the ranking: the small rice pack costs 37.5% more per gram than the best item, a number that lands differently from 'it ranked third'.
Comparison is like-for-like, and the tool enforces it: mass is never ranked against volume, because a per-gram price and a per-litre price measure different goods — if your list mixes categories, each category gets its own best and its own ranking. The other judgement the calculator deliberately leaves to you is quality. Unit price tells you what the quantity costs, not what the product is worth: a cheaper-per-gram cheese you enjoy less is not a saving, and a concentrated product used at half the dose beats its unit price on paper. Use the ranking to see the shelf clearly, then decide with your eyes open — and if a discount is in play, the Discount Calculator linked below works out the reduced price to enter here first.
Because per-gram prices are unreadably small — $0.009 a gram communicates nothing at a glance. Per 100 g (the convention on most supermarket shelf labels) and per kilogram give numbers in the range people can compare: the rice in the worked example reads as $0.80, $0.90 and $1.10 per 100 g. The underlying price is identical; only the presentation changes, and the tool shows both the 100-unit and the 1,000-unit figure.
Usually, but not always — which is the entire reason this calculator exists. Promotions on small packs, bulk pricing quirks and shrinkflation (the pack quietly getting smaller at the same price) all produce cases where the mid-size or even the small pack wins per unit. In the worked example the biggest pack does win, but the smallest is not second — it is the worst value on the shelf by 37.5%.
No, and the tool will not pretend otherwise: grams and millilitres measure different things, and converting between them needs the product's density, which varies (a litre of honey weighs about 1.4 kg; a litre of oil about 0.92 kg). Mixed lists are ranked within each category separately, each with its own best value, rather than in one meaningless league table.
Yes — they are definitions, not measurements. The pound is defined as exactly 453.59237 grams, the ounce as a sixteenth of a pound (28.349523125 g), and the US gallon as exactly 3,785.411784 millilitres. The 16-ounce example lands on exactly one pound for that reason. Note that these are US fluid measures; the imperial gallon used historically in the UK is a different, larger unit and is not one of the options.
Work the offer out first, then enter the effective price. If a pack is 20% off, its shelf price here should be the discounted figure — the Discount Calculator linked below computes it, including stacked discounts. For buy-one-get-one offers, the honest entry is the combined pack at the combined price you actually pay: two 500 g packs for $4.50 is one entry of 1,000 g at $4.50.
Because unit price prices quantity, not fit. A 5 kg sack with the best per-gram price is a poor buy if half of it goes stale, if you cannot store it, or if the product itself is one you like less. The ranking answers one question honestly — what does each option cost per standard amount — and your household answers the rest.
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