“Buy two and save” sounds persuasive, but shelf labels alone don't tell you whether you're spending less for what you'll actually use. The most useful comparison is often price per usable unit, not price per pack.
Compare like with like
First convert both options into the same unit: kilograms, litres, individual items or a consistent smaller unit such as 100 g. Then divide the pack price by its quantity. A larger packet may have a lower unit price, but it is not automatically a bargain if you do not need it.
Worked example in rand
Imagine that a regular 1 kg pack costs R45. A promotion offers 2 kg for R75.
| Option | Price | Quantity | Effective price per kg |
|---|---|---|---|
| Regular pack | R45 | 1 kg | R45.00 |
| Promotion, all used | R75 | 2 kg | R37.50 |
| Promotion, only 75% used | R75 | 1.5 kg | R50.00 |
If you use all the promotional quantity, the unit price is about 16.7% lower than normal. But if a quarter is wasted, your cost per consumed kilogram becomes R50, which is higher than buying the regular pack.
Ask four questions before buying
- Do the sizes match? Look for grams, litres or a count of actual units.
- Will I use it all? Consider expiry dates, household consumption and storage.
- Is the baseline price realistic? A claimed discount may compare against an inflated reference price.
- Does the promotion force extra spending? Buying more than you need can increase your total bill even if the unit price is lower.
Don't forget differences in quality
Price per unit works best for genuinely comparable products. A lower-priced item may have different ingredients, durability or nutritional value. Our calculator isolates the price-and-quantity part of the decision; the final choice remains yours.
A practical shopping habit
When you're undecided, compare the ordinary and promotional unit prices, then estimate what fraction of the promotional amount you'll actually use. That last step is where many apparent savings disappear.