How unit prices are calculated
CheaperPack does one simple thing carefully: it puts every pack in the same unit, divides price by size, and shows you the numbers it used.
The formula
For each product:
- Convert the package size to the unit you want to compare by, such as kilograms.
- Divide the price by that size. The result is the price per unit, for example the price per kilogram.
- The product with the lowest price per unit is the best value.
price per unit = price ÷ size in that unit
For a reference amount such as 100 g, CheaperPack divides by the size in grams and multiplies by 100. Changing the reference amount (per kg, per 100 g, per lb or per oz) changes the numbers shown but never which product is cheapest.
Worked example (made-up prices, in US dollars)
- Pack A: $3.49 for 750 g750 g = 0.75 kg (1 g = 0.001 kg)$3.49 ÷ 0.75 kg = $4.65 per kg
- Pack B: $4.49 for 2 lb2 lb = 0.9072 kg (1 lb = 0.4536 kg)$4.49 ÷ 0.9072 kg = $4.95 per kg
Pack A is the better value, even though Pack B is the bigger pack: Pack B costs $0.30 more per kg. Buying 2 lb at Pack A’s unit price would cost $4.22 instead of $4.49.
What the differences mean
- Difference per unit: how much more a product costs per kg (or per litre, item and so on) than the best value, also shown as a percentage of the best value’s unit price.
- Difference for the pack: how much less you would pay for the same amount at the best value’s unit price. This is the saving per pack, assuming you could buy that exact amount at the cheaper rate.
- Ties: when products have exactly the same unit price, all of them are marked as the best value.
Unit conversions
CheaperPack uses exact, internationally defined conversion factors. Sizes are converted only within the same kind of measurement.
Weight
| Unit | Equals |
|---|---|
| 1 g (grams) | 1 g |
| 1 kg (kilograms) | 1,000 g |
| 1 oz (ounces, weight) | 28.349523125 g |
| 1 lb (pounds) | 453.59237 g |
Volume
| Unit | Equals |
|---|---|
| 1 ml (millilitres) | 1 ml |
| 1 cl (centilitres) | 10 ml |
| 1 L (litres) | 1,000 ml |
| 1 US fl oz (US fluid ounces) | 29.5735295625 ml |
| 1 UK fl oz (UK fluid ounces) | 28.4130625 ml |
| 1 US gal (US gallons) | 3,785.411784 ml |
Count
| Unit | Equals |
|---|---|
| 1 item (count) | 1 item |
Why weight and volume aren’t compared
A gram is a unit of weight and a millilitre is a unit of volume. Turning one into the other needs the product’s density: a litre of water weighs about 1 kg, but a litre of cooking oil weighs about 0.92 kg and a litre of honey about 1.4 kg. Rather than guess, CheaperPack asks you to use the same kind of unit for every product. Many packs show both weight and volume, so you can usually pick one both labels share.
Ounces need particular care: oz is a weight (28.35 g), while fl oz is a volume, and the US fluid ounce (29.57 ml) differs from the UK one (28.41 ml).
Why currencies aren’t converted
Exchange rates change constantly, and CheaperPack doesn’t connect to an exchange-rate service. So every product must be priced in the same currency. The currency setting changes how amounts are formatted, not their value.
Rounding
Calculations use the full precision of the numbers you enter. Results are rounded only for display, with extra decimal places for small unit prices (such as a price per gram) so that two different prices never look identical. The steps shown under each product are rounded too, so recalculating them by hand may differ in the last digit.
When the lowest unit price isn’t the best buy
- You won’t use the whole pack before it expires.
- You don’t have room to store a large pack, or the money to spend up front.
- The products aren’t really equivalent, such as a different quality or concentration.