Cost Per Unit Calculator
Work out the price per gram, millilitre, or item and see which pack is actually the better deal.
Product B is the better value — about 4.8% cheaper per g.
Cost per unit is price ÷ quantity. A $3.50 bag holding 500 g costs $3.50 ÷ 500 = $0.0070 per gram; a $5.00 bag of 750 g costs $5.00 ÷ 750 ≈ $0.0067 per gram. The bigger bag wins — it’s about 4.8% cheaper per gram despite the higher sticker price.
What cost per unit means
The unit price is what one gram, millilitre, ounce, or single item actually costs you. Shelf tags show the total pack price, which makes packs of different sizes hard to compare at a glance. Dividing the price by the quantity strips out the pack size so you compare like with like — the only fair way to tell which option gives you more for your money.
Divide the pack price by how many units it contains; the option with the lower unit price is the better value.
Worked example
Two bags of the same rice: Product A is $3.50 for 500 g, Product B is $5.00 for 750 g.
- 1 Put both prices in the same units. Both bags are measured in grams, so no conversion is needed here.
- 2 Find the unit price of Product A. $3.50 ÷ 500 g = $0.0070 per gram.
- 3 Find the unit price of Product B. $5.00 ÷ 750 g ≈ $0.0067 per gram.
- 4 Compare the two unit prices. $0.0067 is lower than $0.0070, so Product B costs less per gram.
- 5 Work out how much cheaper. ($0.0070 − $0.0067) ÷ $0.0070 × 100 ≈ 4.8%, so B is about 4.8% cheaper per gram.
Unit-price tips that catch people out
Small habits that stop the shelf tag from fooling you.
| Tip | Why it matters |
|---|---|
| Convert to the same unit first | A $2 / 500 g pack and a $2 / 1 kg pack look level until you notice one is grams and one is kilograms — always match units before dividing. |
| Watch shrinking pack sizes | A cheaper sticker often hides a smaller pack; the unit price exposes the real cost when a 1 L bottle quietly becomes 900 ml. |
| Compare per use, not per pack | For detergent or capsules, divide by loads or doses rather than weight — that is the unit you actually spend. |
| Round to the same decimals | Unit prices are tiny, so compare them at the same number of decimal places to avoid misreading $0.007 as cheaper than $0.0067. |
| Use the store’s own unit price | Shelf tags list a unit price, but sometimes in different units per brand; recompute when they don’t match. |
Bigger packs are not always cheaper
Buying in bulk usually lowers the unit price, but not always. Promotions on small packs, multi-buy deals, and premium “family size” pricing can flip the maths, so the large box occasionally costs more per unit than the small one. The unit price is the only reliable check. And a lower unit price is only a saving if you’ll use it all — a giant pack that expires or goes stale before you finish it is more expensive than the small one you actually consume.