How to compare products by unit price so you know which package size is really cheaper

The only fair way to compare two sizes of the same product is by unit price: the total price divided by the quantity, expressed per 100 g, per litre, per item or per use. Bigger packs are usually cheaper per unit, but not always, and the label makes it hard to tell because sizes come in ounces on one shelf and grams on the next. This guide shows you how to do the conversion and the division by hand, and when the cheapest unit price is still the wrong buy.

The basic calculation

Unit price is price ÷ quantity. The only decision is which unit to express it in. Per gram or per millilitre gives tiny fractions that are hard to read, so retailers and this guide use per 100 g, per kilogram, per litre or per item, whichever produces numbers between roughly 0.10 and 50.

A 250 g jar of coffee for 4.79 costs 4.79 ÷ 250 × 100 = 1.916 per 100 g. Round to 1.92 for reading, but keep the extra digit while you compare, because two products can be a few tenths of a cent apart. Work out every product in the same unit before ranking them; comparing a per-ounce price to a per-100 g price tells you nothing.

Convert to one unit first

Mixed units are where most mistakes happen. Convert every quantity to a common base, grams for weight and millilitres for volume, then divide. These are the factors the unit price comparison calculator uses internally.

Label unit	Equals	Base unit
1 kg	1,000	g
1 oz	28.3495	g
1 lb	453.592	g
1 L	1,000	ml
1 fl oz (US)	29.5735	ml
1 pint (US)	473.176	ml
1 quart (US)	946.353	ml
1 gallon (US)	3,785.41	ml

Example: a 12 oz bag of coffee for 8.99 against a 500 g bag for 12.49. The 12 oz bag weighs 12 × 28.3495 = 340.2 g, so it costs 8.99 ÷ 340.2 × 100 = 2.643 per 100 g. The 500 g bag costs 12.49 ÷ 500 × 100 = 2.498 per 100 g. The metric bag is cheaper by about 5.8%, even though its shelf price is higher and its weight sounds similar.

Two cautions. Fluid ounces and weight ounces are different units, and a “16 oz” jar of something thick may be labelled by weight, not volume. And you cannot compare weight with volume at all; the answer would depend on how dense the product is. The calculator warns you rather than producing a number if you mix the two.

A worked example with three sizes

Suppose the same ground coffee is sold in three sizes. Convert each to grams, divide, and multiply by 100 to get the price per 100 g.

Size	Price	Grams	Price per 100 g	vs cheapest
250 g jar	4.79	250	1.916	+19.8%
500 g jar	8.49	500	1.698	+6.2%
1 kg bag	15.99	1,000	1.599	cheapest

Step by step for the middle row: 8.49 ÷ 500 = 0.01698 per gram, times 100 = 1.698 per 100 g. The “vs cheapest” column is (1.698 − 1.599) ÷ 1.599 × 100 = 6.2%. The 250 g jar costs almost 20% more per gram than the 1 kg bag, which is typical: the price gap between the smallest and largest sizes is usually far wider than the gap between the two larger ones.

Multipacks work the same way once you multiply out the total quantity. Six 330 ml cans for 4.50 contain 1,980 ml, so 4.50 ÷ 1,980 × 100 = 0.227 per 100 ml. A 2 L bottle of the same drink for 1.89 is 0.095 per 100 ml, less than half the price, which is the premium you pay for portioning.

When the bigger pack is not the better buy

Unit price tells you the cost of what you buy, not the cost of what you use. A 1 kg bag of coffee that goes stale after you have drunk 600 g of it has an effective price of 15.99 ÷ 600 × 100 = 2.665 per 100 g, worse than the small jar. The same applies to fresh food you will not finish before the date, to paint and sealant that skins over, and to anything you tie up cupboard space storing for months.

Ask three questions before choosing the largest size: will I use it all before it spoils, do I have somewhere to keep it, and can I afford the higher outlay now. If any answer is no, the middle size is often the sensible compromise. Promotions also reorder the ranking. When the small size is on a 25% off deal, run the numbers again rather than assuming; the discount calculator gives you the sale price to feed into the comparison.

Bonus sizes and shrinkflation

A “20% extra free” flash is only meaningful against the same price. Check the unit price of the bonus pack against the regular one and against the next size up, because manufacturers sometimes raise the shelf price at the same time and the bonus disappears.

Shrinkflation is the reverse: the price stays put while the contents shrink. If a 500 g pack for 4.00 quietly becomes 450 g for 4.00, the unit price moves from 0.80 to 0.889 per 100 g, an increase of 11.1% that never appears on a price tag. The only way to notice is to record the price per 100 g rather than the shelf price, and to glance at the net weight each time the packaging changes.

Per-use pricing

For some products weight and volume are the wrong measure entirely. Laundry detergent is sold by the number of loads, coffee pods by the pod, and dishwasher tablets by the tablet, and a concentrated formula can be cheaper per wash while costing more per litre. Compare on the unit you actually consume.

A 40-load bottle of detergent for 9.99 is 0.2497 per load; a 64-load bottle for 14.49 is 0.2264 per load, about 10% less. For coffee pods, a box of 10 for 4.99 is 0.499 per pod and a box of 30 for 13.49 is 0.45 per pod. The calculator has an “items” unit for exactly this: enter the number of loads or pods as the quantity and read the price per item, or per 100 items for very cheap things like sheets and wipes.

Quick checklist

• Convert every size to the same base unit before dividing.
• Express the result per 100 g, per litre or per item, and keep three decimals while comparing.
• Multiply out multipacks to their total quantity.
• Check the net weight on bonus packs and after a packaging redesign.
• Compare consumables per use, not per litre.
• Only take the biggest pack if you will finish it and can store it.