Unit price is the only honest way to compare packs
Supermarkets price the shelf, not your basket. A ¥23.90 pack next to a ¥12.90 pack looks like the expensive option until you divide by the contents: 500 g at ¥12.90 is ¥25.80 per kilogram, while 1 kg at ¥23.90 is ¥23.90 per kilogram, so the 500 g pack costs 7.9% more per gram. That division — unit price = price ÷ pack size — is the whole method, and it is the same calculation as the small print on the shelf edge label, just done for you and for any combination of units.
Mixed units, one denominator
Packs rarely come in the same unit, so normalise first: 1 kg = 1,000 g, 1 L = 1,000 mL, 1 lb = 453.6 g, 1 oz = 28.35 g. Chinese shops mostly quote 元/kg for packaged goods and 元/500 g for produce, while US and UK shelves use price per ounce or per pound — all of these are the same idea with a different denominator. Once everything is converted to a common base (grams, millilitres or pieces), the cheapest option is simply the smallest number. Weight, volume and count never mix: ¥ per litre tells you nothing about ¥ per kilogram, so this tool compares only within the same measure and says so when you feed it mixed units.
When the big pack is not the better buy
- Promotions flip the ranking. A 2-for-1 on the small size, or a temporary 30% cut, regularly beats the bulk pack. Re-check the unit price each time, because the ranking changes with every promotion.
- Waste is a real cost. A 5 kg sack of flour at the best unit price is only cheaper if you finish it. Effective cost is
price ÷ (size × share you actually use): if a third of a bulk pack spoils, a 12% unit-price advantage disappears. - Storage and cash flow. Buying a year of something locks up money and shelf space; a 7% saving on a ¥24 item is ¥1.70 a pack, which is real over a year but rarely worth a stockpile you cannot store.
- Count items hide shrink. For items sold by piece (eggs, rolls, nappies), compare price per piece, and check the pack count rather than the pack size — 12 rolls of 60 sheets is not 12 rolls of 100.
Yearly extrapolation: why small gaps matter
Unit-price differences look trivial per pack and stop being trivial per year. A ¥0.02 per-100 g gap on a household that uses 60 kg of rice a year is ¥12; on coffee at 30 kg a year it can be several hundred yuan. The tool multiplies the per-unit saving by the quantity you actually consume in a year, and also shows how many packs of the winner that is — the point at which "cheaper per gram" turns into a cupboard full of food nobody finishes. The right answer is usually the cheapest unit price among the packs you can realistically use before they go off, bought on promotion when the promotion is genuinely cheaper.
Frequently asked questions
How do I calculate the unit price?
Divide the price by the pack size, then normalise to a unit you can compare: price ÷ grams for weight, price ÷ millilitres for liquids, price ÷ count for pieces. ¥23.90 for 1 kg is ¥23.90 per kg, or ¥2.39 per 100 g, and that is the number to compare.
Is the bigger pack always cheaper per unit?
No. Bulk packs usually win, but promotions on smaller sizes regularly undercut them, and sometimes the "value pack" is simply priced with a nicer label. The only reliable check is the unit price, recalculated whenever prices or promotions change.
How do I compare 500 g with 1 kg?
Convert both to the same unit, typically grams: 500 g stays 500 and 1 kg becomes 1,000 g. Then divide each price by its grams. ¥12.90 ÷ 500 = ¥0.0258 per gram (¥25.80 per kg) against ¥23.90 ÷ 1,000 = ¥0.0239 per gram (¥23.90 per kg), so the 1 kg pack wins at ¥23.90 per kg (the 500 g pack is 7.9% more per gram).
How do I compare a price per 500 g with a price per kg?
Both are unit prices with different denominators. A price of ¥12.90 per 500 g is ¥25.80 per kg, so multiply the per-500 g figure by two to compare it with a per-kg shelf label. Many markets quote produce per 500 g and packaged goods per kilogram, so the conversion comes up constantly.
Should I include food waste in the comparison?
Yes, if you might not finish the pack. Effective cost is price ÷ (size × the share you actually use). A pack with a 10% unit-price advantage but 30% waste is more expensive per usable gram than the smaller pack.
How do I compare items sold by the piece?
Use price per piece, and check what a piece contains. Eggs, rolls and nappies are straightforward, but for paper goods and similar items the pack often changes sheet count between sizes, so compare price per 100 sheets rather than price per roll.
Why does the calculator ask for a yearly quantity?
Because unit-price gaps are small per pack and meaningful per year. Two cents per 100 g is ¥12 on 60 kg of rice a year and hundreds of yuan on coffee. Entering your real consumption turns the comparison into the number that actually affects your budget.
Are weight and volume interchangeable when comparing?
No. A kilogram and a litre are different measures, so a price per kg cannot be ranked against a price per litre unless you know the density of the product. Compare within a single measure, or convert using a known density first.