Comparing prices properly
A lower shelf price does not always mean a lower cost per gram, item or use. Compare matching units, check offers and consider how much you will use.
What a learner can do afterwards
- Compare two products by unit price rather than by shelf price
- Work out the cost per wear or per use for something bought once and used often
- Spot one offer where buying more does not lower the unit price
1 · Read
Unit price is price divided by amount. Compare the same product in matching units. One kilogram is 1,000 grams. For a price per 100 g, divide the price by the number of 100 g portions: 600 g has six. You need both price and quantity. Use a calculator if needed, and keep extra decimal places until you compare.
Four identical pencils cost $3, while ten cost $8. Divide $3 by 4 to get $0.75 per pencil. Divide $8 by 10 to get $0.80 per pencil. The four-pack has the lower unit price. More pencils in a pack does not guarantee a lower price per pencil.
Cost per use is price divided by uses. A $72 backpack used 24 times costs $3 per use. A $50 one used 10 times costs $5 per use. Future uses are estimates; fewer uses raise the cost per use. To find the uses needed for a target cost, divide price by that target: $72 divided by $3 gives 24 uses.
A bulk offer can have a lower, equal or higher unit price. Compare equal amounts. If 1 litre of the same juice costs $3, four small bottles cost $12. A 4-litre bottle at $13 costs more for the same amount. If it cost $12, there would be no saving per litre.
Use the final price and the full quantity in an offer. If a coupon applies, subtract its discount before dividing. Include free items in the quantity: buying four pens and getting one free gives five pens. Divide the price paid by five. Check the offer's conditions before using it.
Compare products of similar quality and size. Rolls with different sheet counts are not equal amounts. Check whether the total price fits your budget and whether you will use the contents before they spoil. Online, read the stated quantity rather than judging from a photo.
In this example, four quarts make one gallon. A quart of milk costs $1.50, so four cost $6. A gallon of the same milk costs $5.20. The gallon is cheaper for the same amount. Comparing equal totals avoids rounding the price per quart.
Compare matching amounts or uses. Include discounts and free items, then check the total price and how much you will use.
Price-comparison lesson, questions and flashcards written for this learning dataset. Source records were studied for teaching methods and question formats.
Readings, questions and flashcards written for this learning dataset. Video reference: "Solving unit price problem" by Khan Academy.
More to read
Two ways to compare equal amounts
The same flour costs $2.40 for 300 g or $3.60 for 600 g. The smaller pack costs less at the checkout. To compare equal amounts, divide each price by its number of 100 g portions. The rates are $0.80 and $0.60 per 100 g.
You can also match the total quantities. Two small packs give 600 g for $4.80. One large pack gives 600 g for $3.60. Both methods show that the larger pack costs less for the same amount.
Check product quality and quantity before comparing. Two rolls need not contain the same number of sheets.
Read the quantities in an online listing
A product photo does not tell you the full order quantity. Read the listed size and number of items. Check the total you would pay, including required delivery charges, before comparing costs per item.
For a reusable purchase, divide the price by a realistic use count. A $54 item used 18 times costs $3 per use. With only nine uses, it costs $6 per use. Future use counts are estimates, so a predicted rate may change.
Compare similar products and check the total against your spending plan. A lower estimated cost per use does not make an unaffordable purchase fit your budget.
Think about what you will use
Check what you already have before choosing a pack. A larger pack may have a lower unit price while still costing more money overall.
Ten identical snack portions cost $5, while four cost $2.40. The rates are $0.50 and $0.60 per portion bought. If you need only four and the other six would spoil, the large pack would cost $5 to supply those four useful portions. The small pack supplies them for $2.40.
There is no pack size that always costs least for your needs. Compare the price and usable quantity, then check what you can afford.
Work out an offer's final unit price
Check which product and size an offer covers. A coupon may have an expiry date or a minimum purchase. Do not assume the shop allows two discounts together.
Suppose a 700 g pack costs $6.90 and a valid coupon takes $1.30 off. The final price is $5.60. Divide by seven 100 g portions to get $0.80 per 100 g, then compare that rate with the other pack's rate.
For a free-item offer, count every item supplied in the quantity. Check the final charge and whether you need the extra amount before buying.
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