Why the shelf label is not enough
EU shops must print a unit price next to the selling price — per kilogram, per litre, sometimes per 100 g. It works until the two products you are comparing use different bases, or one of them is on a promotion that the label ignores, or you are looking at an online listing that shows no unit price at all. Then it is back to arithmetic: price divided by quantity, both in the same unit.
500 g at 3.20 → 3.20 / 0.5 kg = 6.40 per kg 750 g at 4.99 → 4.99 / 0.75 kg = 6.65 per kg ← the bigger pack costs more
The large-pack trap
“Family size” is priced on the assumption that you will not check. In supermarket audits the bigger pack is more expensive per unit in roughly one case in five, and the gap is widest on items where the brand is strong: coffee, washing powder, pet food. Multipacks are the other classic — six cans bundled together at more than six times the single price. The only defence is the unit price, and the only unit price you can trust is the one you worked out yourself.
Pieces, servings and concentrate
Some products cannot be compared by weight. Dishwasher tablets, batteries and nappies are bought by the piece — choose “pcs” and compare per piece. Concentrated products are the hardest: a litre of concentrate that makes 40 washes is not comparable to a litre of standard liquid that makes 20. Compare per wash, per serving or per dose, using the manufacturer’s own count as the quantity.
Why one shop prints per 100 g and the next per kilogram
The unit price on a shelf label is not a courtesy, it is a requirement: EU law makes traders show both the selling price and the price per unit of measurement, so that two packs of different size can be compared without arithmetic.
The unit itself is where shops differ. The rule reaches for a single unit — kilogram, litre, metre, square metre — but for small quantities member states may allow 100 g or 100 ml, and many do. That is the whole reason the same yoghurt is 0.42 per 100 g in one chain and 4.20 per kilogram in another: identical price, different denominator.
So the first move when comparing is not to trust the labels but to put them on the same unit. That is what this calculator is for, and it is also why it accepts grams and kilograms in the same comparison.
Where the comparison quietly stops being one
Drained weight. A jar of olives sold at 340 g gross may hold 180 g of olives. Per-kilogram prices computed on the gross weight compare the brine as well.
Concentrates. Detergent at double strength costs twice as much per litre and washes twice as many loads. The honest denominator is the wash, not the litre — the same trap catches syrups, cleaning products and coffee capsules.
Pieces of different size. Price per piece compares nothing when the pieces differ: two rolls of kitchen towel at the same price per roll can differ by half in sheets and by more in sheet size. Where a per-piece price is offered, check what one piece is.
What you will actually finish. The larger pack usually wins on unit price and loses on waste. Half a discarded jar is a 100 % markup on what you ate, and no unit price on any shelf shows it.
Frequently asked questions
Yes — pick the unit each product is labelled in and the calculator converts both to per kilogram. The same goes for millilitres and litres. Weight against volume is not compared, because a litre of one thing does not weigh a kilogram.
Directive 98/6/EC requires it for all products sold to consumers by weight, volume, length or area, with national exemptions for very small shops and for items where it would be meaningless. Most countries extend it to online shops.
No. A lower unit price only saves money if you use all of it. Compare the price of what you will actually consume, not of what you will throw away.
Both are lawful. EU price indication rules ask for a single unit — kilogram or litre as a rule — but allow member states to permit 100 g or 100 ml for small quantities. The prices are the same number in different clothes: 0.42 per 100 g is 4.20 per kilogram.
Usually, not always, and shops know you assume it. Compare the actual unit prices rather than the pack sizes — and then compare that with what you will finish before it spoils, because waste is a cost the shelf label does not carry.