Now some of my readers may pooh-pooh my use of a calculator when shopping, saying they can simple look at the unit pricing on the item shelves of the supermarket. Oh yeah? I needed to get 13 gallon kitchen trash bags the other day. Consider the following choices:

The unit pricing label tells us that the 45 count package that normally costs $15.09 per hundred bags is on sale for $11.09 per hundred.

The unit pricing label on the 120 count package informs us that it only costs $9.41 per hundred bags. Ostensibly it seems we do not need to use a calculator at all. The 120 count package seems cheaper and if we have enough money, it would appear to be a wise decision to buy the larger package. But we have to look closer. The bags only appear to be similar in size and thickness.

The dimensions for the bags in the 45 count package are 24" by 27.375" by .9 mils thick. Doing all the math - it turns out we are paying 0.1875 cents per cubic inch of kitchen bags.

The dimensions for the bags in the 120 count package are 23.75" by 28.375" by .69 mils thick. Doing all the math - it turns out we are paying 0.2023 cents per cubic inch of kitchen bags.

The company appears to be giving you more material in the larger package for a lower unit price. The reality is that they are giving you much less material at a higher price. Unit pricing labels do not always tell the whole story.

The above could not be figured out without a slide rule or calculator.