Comparing prices of products of differing amounts can be hard. 

• Is $74 for a box of 40 cheaper than $120 for a box of 85?
• Is $98.99 for 200 feet of cable cheaper than $30 for 50 feet?
• Is $26.54 per square yard cheaper than $3.05 per square foot?
• Is $2.45 for 2 quarts cheaper than $6.00 for a gallon?
• Is $52.25 per pound cheaper than $0.65 per ounce?

Here's a calculator that makes it A LOT EASIER! 

The SHOPPING CALC compares prices for products with different amounts and prices.

• Price Compare by Product Price and Number of Items in the Product
• Price Compare by Product Price and Length of Item in Product
• Price Compare by Product Price and Area of Item in Product
• Price Compare by Product Price and Volume of Item in Product
• Price Compare by Product Price and Weight or Mass of Item in Product

It doesn't matter what you're comparing.  Sometimes there are different numbers of items in a box, different servings in a container, different lengths of fabric in a roll or in the case on the right concrete in two different weights.  The picture below shows how you can compare price and weight of the two items and it tells you how they compare.  In the example below, the larger bag (80 lbs) was 2.5% cheaper by weight.  Maybe that's not enough to sway you, but savings add up with time.