How To Calculate Unit Price

The unit price is what one pack costs per single unit of the thing inside it — a gram, a millilitre, a sheet, a wash. It is the only figure that lets you compare two differently sized packs honestly, because it strips away the packaging and leaves the rate. A 750 g pack at 4.50 has a unit price of 0.006 per gram, and a 1.5 kg pack at 9.00 has exactly the same rate even though the totals differ by double. Once you hold that rate, the question of which pack is cheaper has an unambiguous answer.

Totals mislead because packs are sold in whatever size the maker chose, and those sizes rarely line up. One bag holds 750 g, another 1 kg, a third 12 oz, and comparing 4.50 with 5.60 directly tells you only which number is larger, not which purchase is better value. Converting everything to a common unit — grams, or more conveniently per 100 g or per kilogram — puts the packs on the same footing. Only then does the arithmetic reflect the goods rather than the labels printed on them. The same applies when the pack formats differ, as with a multipack of small sachets against one large tub, because the only bridge between two shapes is a shared unit.

Supermarkets print a small unit price figure on the shelf edge for exactly this reason, usually in a corner of the label beneath the headline price. It is calculated as the price divided by the quantity, normalised to a fixed reference such as 1 kg, 1 litre or 100 g. The reference differs by product because the useful unit differs: liquids are quoted per litre, dry goods per kilogram, and some jurisdictions mandate per 100 g so the numbers stay short. Those labels are regulated in many countries, which is why they are trustworthy when present — but they only exist for the pack in front of you, not for the rival you left in the other aisle.

To compute a unit price by hand, divide the price by the quantity and keep the units attached to the answer. 4.50 divided by 750 g is 0.006 per gram, which you can then scale upward: 0.6 per 100 g, or 6.0 per kilogram. The reciprocal is just as useful and often more intuitive, because 750 g for 4.50 means 166.6666667 g for each unit of currency, so a larger number there signals a better deal. Both forms describe the same rate, so pick whichever your mental arithmetic prefers and apply it consistently to every pack.

Shrinkflation is the quiet version of a price rise, where the pack shrinks while the shelf price stays put. A cereal box that falls from 500 g to 450 g at the same 3.20 has quietly raised its unit price from 0.64 to about 0.711 per 100 g, a jump of roughly 11% that no headline announces. Because the sticker price is unchanged, the only way to notice is to read the net weight and recompute the rate yourself. This is precisely why the unit price, not the price, is the honest measure of what a shop is charging you. Comparing a current 450 g pack with last month's 500 g one at the same price reveals the increase instantly, whereas comparing two identical sticker prices reveals nothing at all.

It is tempting to assume the larger pack always wins, and often it does, but not by any rule. Shelf position and turnover matter: a big bag may sit at eye level and sell slowly, while the small one is discounted weekly and moves fast, so the price gap reflects logistics rather than an inherent economy of scale. In the worked comparison the 1 kg pack at 5.60 beats the 750 g pack at 4.50, yet a 500 g pack at 3.20 is dearer at 0.64 per 100 g than the 750 g pack's 0.6. The larger size helps only when the division actually says so.

Discounted near-expiry stock complicates the picture, because the true cost depends on how much you will realistically use before it spoils. A 1 kg pack knocked down to 4.00 looks excellent at 0.4 per 100 g, until you admit you will throw half of it away, at which point the effective rate doubles to 0.8 per 100 g and beats nothing. The unit price on a perishable item is only valid if the whole pack gets consumed, so scale the rate by the fraction you expect to waste before comparing it with anything else on the shelf.

Promotions such as 20% extra free or three for two are unit-price questions in disguise. Twenty per cent extra on 750 g gives 900 g, so a 4.50 pack now yields 0.5 per 100 g instead of 0.6 — a genuine improvement, but one you can confirm only by recomputing rather than trusting the banner. Three for two on identical packs multiplies quantity by three and price by two, which is a one-third cut in unit price, again something the label rarely states outright. The habit worth building is to treat every promotion as a fresh division problem.

Finally, unit price is a necessary measure but not a sufficient one, because it says nothing about quality, durability or fit. A cheaper per-litre paint that needs three coats can cost more per finished wall than a dearer one that covers in two, and a cheaper per-kilogram cut of meat with heavy trim wastes the apparent saving. Use the unit price to eliminate packs that are genuinely poor value, then let quality, storage and your realistic usage decide among the survivors. The calculation narrows the field; it does not make the decision alone.