Comparing prices fairly
Products come in different sizes, packs and deals. To compare them fairly, convert each price to a unit price (also called unit cost): the price for one unit.
Unit price = price ÷ number of units
Example: juice
| Option | Price | Size | Unit price |
|---|---|---|---|
| A | $2.49 | 1 L | $2.49/L |
| B | $4.29 | 2 L | $2.15/L |
| C | $5.99 | 3 L | $2.00/L |
Option C has the best value.
Choose a sensible unit
For small amounts, compare per 100 g or per 100 mL to avoid tiny decimals:
- Coffee: $8.99 for 340 g → $2.64 per 100 g
- Coffee: $13.49 for 600 g → $2.25 per 100 g
Comparing deals
| Deal | Effective unit price on $4 items |
|---|---|
| 25% off | $3.00 |
| Buy one, get one 50% off | $3.00 each when buying 2 |
| 3 for $10 | $3.33 |
| Buy 2, get 1 free | $2.67 |
Comparing different units
Convert first. A 1.5 kg bag for $6.00 vs a 750 g bag for $3.20:
- $6.00 ÷ 1,500 g = $0.40 per 100 g
- $3.20 ÷ 750 g ≈ $0.43 per 100 g
- The 1.5 kg bag is cheaper per gram.
Beyond unit price
The cheapest per unit isn’t always best:
- Waste: perishable food may spoil.
- Cash flow: a big pack costs more upfront.
- Quality: a better product may last longer.
- Total cost: shipping, membership fees or travel can cancel savings.
Making a comparison table
When shopping for something important (a phone plan, for example), list each option’s total cost over the same time period and what you get.
Worked examples
Example 1: 8 granola bars for $3.92 or 12 for $5.40?
$0.49 vs $0.45 per bar → 12-pack.
Example 2: A $60 jacket at 30% discount vs a $50 jacket at 10% off?
$42 vs $45 → the first is cheaper.
Example 3: Laundry soap: $11.99 for 48 loads or $17.99 for 80 loads?
$0.25 vs $0.22 per load → 80 loads.
Common mistakes
- Comparing total prices instead of unit prices.
- Mixing units (per g vs per kg).
- Ignoring whether you’ll actually use the larger amount.
Try it yourself
- 500 mL for $1.80 or 2 L for $6.00?
- 6 batteries for $7.20 or 10 for $11.50?
- A $25 shirt is 20% off; another is $21. Which costs less?
- Name one reason not to buy the bulk size.
Answers: 1) 2 L ($0.30 vs $0.36 per 100 mL) 2) 10-pack ($1.15 vs $1.20) 3) $20 sale shirt 4) e.g. it might spoil or you can’t store it
Quick recap
- Unit price = price ÷ quantity; compare in the same units.
- Turn deals and discounts into unit prices.
- Consider waste, storage and total cost as well.