Well, here's a great reason to pay attention. The other day, I spotted a sale on Tropicana Orange Juice at the local grocery store. The tag said "Save $2.00" with a sale price of $5.99. Sounds pretty good...a large (89 fl. oz.) $7.99 container of orange juice for just $5.99. On the shelf below, there was a half gallon container (64 fl. oz.) of the same exact brand priced at $3.69. It was not on sale, so surely the larger "on sale" container was the cheapest, right? (hint: if it was, I wouldn't be writing about it)
Let's do the math...if we reduce the price down to a common unit, we can see which one is really cheaper. In this case, the store chose to display unit prices by the quart (32 fl. oz.). In the case of the large container, 89 fl. oz. divided by 32 fl. oz. in a quart = 2.78 quarts. $5.99 divided by 2.78 = $2.15 per quart.
For the half gallon, 64 fl. oz. divided by 32 fl. oz. in a quart = 2 quarts. $3.69 divided by 2 = $1.85 per quart. So the large container on sale is actually 16% more expensive!
But you would have already known that (without a calculator) because your local supermarket tells you that on every tag. You just need to remember to look.
Well, it happened again. This time it was time to buy Triscuits. First look at the 13 oz. "Family Size" with a price of $3.59 (or $4.42 per pound). Then, on the shelf above, was a sale on the regular size (9.5 oz.) with a price of 2 for $5.00 (or $4.21 per pound). Keep watching those unit prices!