Comparing Prices and Unit Costs

What you will learn

• Find the unit cost of an item.
• Compare two packs fairly.
• Know that cheaper per unit is not always better.

Before you start: you should be able to divide decimals and convert pounds to pence. See Multiplying Prices and Sharing Bills.

The idea

A bigger pack usually costs more, but is it better value? To compare fairly, find the price for one unit: one item, one litre or $100$ g. This is the unit cost.

Method

1. Convert the price to pence if that is easier.
2. Divide the price by the amount to get the cost of one unit.
3. Compare the unit costs. Lower is better value.

Not always the best choice

A cheaper unit cost only helps if you will use it all. A large pack that goes off before you use it wastes money.

Worked examples

Example 1: Per 100 g

A 500 g pack costs £1.20.

1. $120$p for $500$ g.
2. $120 \div 5 = 24$p per $100$ g.

Answer: 24p per 100 g

Example 2: Per item

$12$ pens cost £5.40.

1. $540 \div 12 = 45$.

Answer: 45p each

Example 3: Better value

24p per 100 g or 22p per 100 g?

1. Compare: $22 < 24$.

Answer: the second pack

Tip: compare in the same unit: both per 100 g, or both per item.

Why it matters

Unit costs help you compare prices in shops, online and on bills. Discounts also change the price, see discounts and sale prices.