Use one unit for both packages
Divide each package’s price by the amount inside it. For a liquid measured in ounces, compare dollars per ounce; for dry goods measured by weight, compare dollars per pound or ounce; for counted items, compare dollars per item.
If labels use different units, convert them first. One pound contains 16 ounces, so a price per pound can be divided by 16 to express it per ounce.
An illustrative example
Sample prices only: a 24-ounce bottle at $3.60 costs $0.15 per ounce ($3.60 ÷ 24). A 40-ounce bottle at $5.60 costs $0.14 per ounce ($5.60 ÷ 40). On these sample numbers, the larger bottle has the lower price per ounce.
That calculation does not make the larger bottle the right choice for everyone. Consider whether you have room to store it, whether you will use it before quality declines, and whether the extra amount fits your budget today.
Check what the label leaves out
Package size is only one part of value. Compare the same formulation, concentration, count, or usable amount; a “per item” label can hide meaningful differences in size or performance.
- Include a promotion only if you can buy the required quantity and will use it.
- Consider membership, delivery, or travel costs when they apply to the purchase.
- Avoid counting food or household products you are likely to waste as a saving.