A 400-gram bag of oats costs $2.40. A 750-gram bag costs $3.75. The small bag takes less money at the checkout. The larger bag buys more oats for every dollar. Which number helps depends on whether you are comparing the cost of oats or deciding what fits your budget this week.
Put the packages on the same scale
Divide the price by the amount in the package. The 400-gram bag costs $2.40 divided by 400, or $0.006 per gram. Multiply by 100 and you get $0.60 per 100 grams. The 750-gram bag costs $0.50 per 100 grams. The arithmetic compares identical quantities even though the packages differ.
Shelf labels often do this calculation for you. The U.S. National Institute of Standards and Technology recommends using a consistent unit across comparable products. If one label says $6 per kilogram and another says $0.50 per 100 grams, convert before comparing. A kilogram contains ten lots of 100 grams, so $0.50 per 100 grams equals $5 per kilogram.
Recalculate after a size or price change
Imagine the $2.40 bag shrinks from 400 grams to 360 grams. At the same shelf price, the unit price rises from $0.60 to about $0.67 per 100 grams. The till still charges $2.40. The amount that price buys has changed. Check the actual quantity when familiar packaging looks much the same.
A promotion can also change the order. If the small bag costs $1.80 for 400 grams, its unit price is $0.45 per 100 grams. Use the price you can actually get and read any conditions before doing the calculation. When comparing a multipack, count the total quantity you must buy to get that price.
Match the number to the decision
The UK Competition and Markets Authority’s consumer research found that people considered waste and the amount they would actually use, particularly for fresh products. In this example, buying the larger bag makes sense if the extra oats are useful. A small bag might be the better purchase for someone who needs only a little or has $3 left to spend.
- Match comparable products and quantities before dividing.
- Look at both the unit price and the cash you need at checkout.
- Take account of storage, likely waste and any conditions on the price.
Sources & further reading
- NIST unit pricing guidance
Defines unit prices and describes how size changes alter the price per quantity.
- UK CMA consumer research on grocery unit pricing
Reports how shoppers used unit pricing and how considerations such as waste affected comparisons.