How do I calculate unit prices when package sizes are different?
Quick answer
Divide each package’s price by its amount, using the same unit for every option. For example, compare dollars per kilogram rather than dollars per package. Convert grams to kilograms first, then compare the results: the lower unit price costs less for that stated amount. Check that the products have comparable contents, concentration, and usable quantity; a lower number alone may not mean better value.
The question, the details, the next step.
I’m looking at the same kind of product in two different package sizes. What’s the simplest way to compare the prices?
Calculate the price per shared unit. Divide the package price by its quantity, and make sure both quantities use the same unit—such as dollars per kilogram, per litre, or per 100 grams.
Formula: unit price = package price ÷ package quantity.
For example, if a package costs $3.60 for 900 g, convert 900 g to 0.9 kg. Then $3.60 ÷ 0.9 = $4.00 per kg. Do the same for the other package, then compare the per-kilogram figures. The key is to use the same unit and comparison basis for both.
Can we work through two packages, and what if the labels use different units?
Suppose one package is $3.60 for 900 g and the other is $5.18 for 1.4 kg. Assuming the products are comparable and the prices use the same currency:
- Smaller package: 900 g = 0.9 kg; $3.60 ÷ 0.9 = $4.00 per kg.
- Larger package: $5.18 ÷ 1.4 = $3.70 per kg, rounded to the nearest cent.
The larger package is about 30 cents less per kilogram, although it costs more at checkout. For labels using different units, convert both amounts to one shared unit before dividing. Write down each original amount and its conversion so you can check the arithmetic. If a label shows a unit price, check that it uses the same unit and basis as your calculation.
Does the lower unit price always mean I should choose that package?
No. Unit price compares cost on a particular basis; it doesn’t by itself establish which package is better value for you. Check whether the products have comparable contents, concentration, and usable amount. A comparison by weight may not tell you how many uses or servings you get.
For example, if two hypothetical products differ in concentration, cost per litre of packaged liquid may not show how much cleaning solution each can make. You would need comparable information about usable amount or intended dilution to calculate a fairer cost per use. If the needed information isn’t available, record the figures but mark the comparison as incomplete rather than treating the lower number as a definite better value.
Can you give me a worksheet I can fill in while shopping?
Copy this and fill in the same comparison basis for both options:
Product or contents: [what each option is] Basis: [kg / g / litre / other shared unit] Option A: price $[___]; amount [___]; converted amount [___]; unit price $[___] per [unit] Option B: price $[___]; amount [___]; converted amount [___]; unit price $[___] per [unit] Differences to check: [concentration, contents, usable amount, or other] Result: [lower calculated unit price / not directly comparable]
For each option, divide its price by its converted amount. Round both results to the same number of decimal places for readability. Keep the unrounded calculations if the results are very close, since rounding can make small differences look equal. If you need to compare cost per serving, use, or finished amount, use matching information for both packages; don’t guess at missing details.
A practical checklist
Choose one shared unit, such as kilograms or litres.
Convert each package quantity to that unit before dividing.
Divide each package price by its converted quantity.
Check that contents and concentration are comparable.
Note when usable amount or another important difference is unknown.
A few more useful answers
Should I compare per 100 grams or per kilogram?
Either works if you use the same unit for every option. Choose whichever makes the figures easiest to compare; the ranking will be the same.
How should I handle a sale price?
Use the price you would actually pay for each package, including any discount you can verify. Then divide by the package quantity as usual.
What if two unit prices are almost the same?
Compare the unrounded calculations before deciding that one is cheaper. Also check whether the products and usable quantities are truly comparable.