Unit Price Calculator

Compare two packages even when they use different units — ounces against grams, litres against fluid ounces — and see which one is actually cheaper.

🛒 Six Shelf Tricks

ShrinkflationSame box, less inside
"Family size"Often dearer per unit
MultibuyCheck it beats the single
Eye levelThe dearest shelf
Odd sizesMade hard to compare
Loyalty priceCompare to the normal one

What the Shelf Label Won't Tell You

Bigger is not automatically cheaper. Retailers know shoppers assume it is, and the larger pack is dearer per unit often enough that checking pays.
In the United States, unit pricing on the shelf label is mandatory in only a handful of states — elsewhere it is voluntary, and often absent.
Even where labels exist, two products beside each other may use different bases: per ounce on one, per 100 count on the next. That defeats the whole purpose.
Shrinkflation hides a price rise as a size cut. The shelf price stays put while the pack goes from 500 g to 450 g, which is a 10% increase per gram.

One Number That Makes Packages Comparable

Unit price is what a product costs per gram, per millilitre or per item, and it exists because package sizes are not designed to be compared. A 12-ounce box beside a 500-gram bag beside a 1.5-litre bottle gives you three prices and three sizes with nothing in common. Reducing all of them to a price per 100 grams or per 100 millilitres puts them on the same footing, and the comparison takes a second instead of a minute of mental arithmetic in a busy aisle. It is the single most reliable way to tell whether a deal is a deal, and it is why consumer-protection rules in the EU and the UK require it on the shelf.

Why Mixed Units Are the Hard Part

The arithmetic is simply price divided by size, but only after both sizes are in the same unit — and that conversion step is where the aisle comparison collapses. A 12-ounce box is 340.2 grams, so at $4.29 it works out at $1.26 per 100 grams; a 500-gram bag at $6.99 is $1.40 per 100 grams, making the smaller American box the better buy despite looking dearer. Getting there means converting ounces to grams first, which nobody does standing up with a trolley. This calculator does the conversion for you, which is the only reason a mixed-unit comparison is practical at all. The one rule it cannot bend: weight compares with weight and volume with volume, because grams and millilitres only coincide for water.

How Packaging Defeats Comparison

Shrinkflation: the price holds while the contents shrink. A 500 g pack becoming 450 g at the same price is a 10% rise per gram, invisible on the shelf tag.
"Family size" and "value pack" are marketing terms with no legal meaning. They are frequently dearer per unit than the standard size.
Non-round sizes — 11.5 oz, 383 g — exist partly to make cross-brand comparison hard without a calculator.
Multibuys like 3 for 2 are worth checking: the effective discount is 33%, which a single item on promotion sometimes beats.
The most profitable products sit at adult eye level. The cheaper unit price is usually on the top or bottom shelf.
For anything perishable, the cheapest unit price is worthless if half of it is thrown away. Buy the size you will finish.

Unit Pricing Rules Around the World

European Union: Directive 98/6/EC requires a unit price on the shelf for most pre-packaged goods, in a consistent unit within a product category.
United Kingdom: the Price Marking Order requires unit pricing, and 2024 guidance tightened how promotional prices must show it.
United States: there is no federal requirement. A small number of states mandate it and the rest leave it voluntary.
Where labels exist, the base unit is not always consistent between neighbouring products, which quietly breaks the comparison.
Online listings often omit unit price entirely, so the trolley is where the calculation moves.
When in doubt, work in price per 100 g or 100 mL. Those are the bases European labels use and they keep the numbers readable.

Worked Examples

Where the bigger box loses

12 oz at $4.29 vs 500 g at $6.99

12 oz = 340.2 g, so $4.29 ÷ 3.402 = $1.26 per 100 g
500 g at $6.99 = $1.40 per 100 g
The smaller box is 10% cheaper per gram

Option A wins despite the lower size

Spotting shrinkflation

Old: 500 g for $6.99 = $1.398 per 100 g
New: 450 g for $6.99 = $1.553 per 100 g
Same shelf price, 11% more per gram

A price rise the tag never shows

Frequently Asked Questions

How do I calculate unit price?

Divide the price by the size, then scale to a readable base. A 500 g bag at $6.99 is $0.014 per gram, which reads better as $1.40 per 100 g. Comparing two products means putting both on that same base first.

Can I compare ounces with grams?

Yes — that is what this calculator is for. It converts both to grams before dividing, so a 12-ounce box and a 500-gram bag can be compared directly without you doing the conversion in the aisle.

Is the bigger package always cheaper?

No, and that assumption is exactly what retailers rely on. Larger packs are frequently dearer per unit, particularly ones labelled family size or value pack — terms that carry no legal meaning at all.

Why can't I compare weight with volume?

Because grams and millilitres only line up for water. A litre of oil weighs about 920 grams and a litre of honey about 1,420, so dividing a price by the wrong measure gives a number that means nothing. Compare weight with weight and volume with volume.

What is shrinkflation?

A price rise disguised as a size cut. The shelf price stays the same while the contents drop — 500 g becomes 450 g — so the cost per gram climbs about 11% with nothing on the tag to show it. Unit price is the only way to catch it.

Do shops have to show unit prices?

In the EU and the UK, yes for most pre-packaged goods. In the United States there is no federal requirement and only a few states mandate it, which is why the aisle comparison so often falls to the shopper.