Average unit price for mixed purchases: divide totals, not prices
Find the average cost per kilogram when you buy different quantities at different unit prices.
Method
The average price paid per unit is total cost divided by total quantity. A plain average of two displayed unit prices is appropriate only when the purchased quantities are equal. If one purchase is much larger, its price should contribute more to the combined average. This is a weighted average in which the quantities supply the weights.
Convert both quantities to the same unit before adding them. A price per 100 g and a price per kilogram cannot be averaged as displayed. Multiply each unit price by its matching quantity to obtain cost, add the costs, and divide by the total quantity. Keep the currency the same as well. If delivery or other charges belong to the comparison, include them once in total cost.
- InputsCurrency · Products
- MethodUnit price = product price ÷ quantity. Savings compare each product with the lowest unit price for the same unit.
- ResultUnit price
Example only; replace these assumptions with your own values.
Worked example
Suppose you buy 2 kg at 6 currency units per kg and another 3 kg at 10 per kg. The costs are 12 and 30, for a total of 42. The total quantity is 5 kg, so the average paid price is 8.4 per kg. The unweighted average of 6 and 10 is 8, which understates the actual average because more quantity was bought at the higher price.
Unit price can compare individual packs using their total prices and quantities. For a combined purchase, add the pack prices and quantities first, then divide. If quantities are recorded as 2,000 g and 3,000 g, total cost divided by 5,000 gives 0.0084 per gram; multiply by 1,000 to express the same result as 8.4 per kilogram. The numeric result changes with the unit, but the economic comparison does not.
2 × 6 + 3 × 10 = 42
42 / (2 + 3) = 8.4 per kg
(6 + 10) / 2 = 8
- 2 kg at 6/kg
- 6 per kg
- 3 kg at 10/kg
- 10 per kg
- Combined average
- 8.4 per kg
(2 × 6 + 3 × 10) ÷ 5 = 8.4 per kg.
Checks and limits
Use the quantity that represents what you are actually comparing. Gross package weight may include packaging, and edible quantity may differ from total weight. Prices before and after a discount or tax should use the same basis. A low combined average does not tell you which purchase was cheaper or whether the larger quantity was needed. Keep both the individual comparisons and the combined result visible.
Related calculators
Source: OpenStax