Unit Price

Two bags of sunflower seeds on the shelf: 500 g for 12, or 800 g for 18. The big bag costs 6 more — a loss? The tags alone cannot say. You need both bags measured on the same ruler, and that ruler is the unit price.

One ruler for all deals

The unit price answers a single question: how much per 1 kilogram (or per liter, per piece)? The method is one step: unit price = total price ÷ quantity. Run both bags through it:

500 g for 12: $12 \div 0.5 = 24$ per kg;
800 g for 18: $18 \div 0.8 = 22.5$ per kg.

The big bag wins by 1.5 per kg. Check it as whole bags too: at the small bag's rate, 800 g would cost $24 \times 0.8 = 19.2$ , but the big bag sells for 18 — buying big saves 1.2. Once the units match, the verdict is instant.

Worked out: who wins

Unify the units first: 500 g = 0.5 kg, 800 g = 0.8 kg. Then divide each: $12 \div 0.5 = 24$ and $18 \div 0.8 = 22.5$ . Since $22.5 < 24$ , the big bag has the lower unit price — cheaper by 1.5 per kg.

Is bigger always cheaper?

"Buy more, pay less per unit" is a common experience — not a law. Some stores run the small pack as a special, flipping the result: a 200 g pack on sale for 5 works out to 25 per kg, while a 500 g pack at 14 costs 28 per kg — the small pack wins this round. Never trust the words "value size"; compute the unit price and then decide. It takes ten seconds.

InteractivePrice Lab

Original priceDiscount 30%30

200

-30%

140

You save

Big packs often stack a promotion on top. Play with the price and the discount, divide the discounted price by the quantity, and compare again — once discounts enter, the winner can flip at any time. How discounts work lives at Discounts & Percentages.

Mismatched units, broken comparison

Shelf labels often quote "per 100 g" while packs come in 250 g or 500 g, and drinks are sold by the milliliter. Align the units before comparing:

a label of 3.5 per 100 g means 35 per kg;
a 1.5 L bottle and a 330 ml can must both become per-liter prices;
"12 per bag, contains 3 snack packs" — ask how much each small pack weighs first.

Unaligned units, wasted effort

Comparing "500 g for 12" against "100 g for 3" by staring at 12 and 3 is meaningless — converted, it is 24 per kg versus 30 per kg, and the big pack is actually cheaper. Unify first, compare second.

Mental math shortcuts

Dividing by 2 becomes multiplying by 2: 500 g is exactly half a kilo, so per-kg price = price × 2. The 12 bag is 24 per kg;
Dividing by 5: $18 \div 5 = 36 \div 10 = 3.6$ — double, then shift the decimal one place;
Scale both sides by 10: $24 \div 0.8$ becomes $240 \div 8 = 30$ , suddenly easy.

Master these three and the supermarket turns into a free mental-math gym.

500 g is a mental-math friend

Packs come sized as 100 g, 250 g and 500 g for a reason. Memorize three multipliers: ×2 for 500 g, ×4 for 250 g, ×10 for 100 g — and the per-kg price pops out at a glance.

Check yourself

Quick quiz

1. 500 g costs 12. What is the price per kilogram?
2. Which has the lower unit price: 800 g for 18, or 500 g for 12?
3. A label reads 2.8 per 100 g. What is that per kilogram?