Look over the operation manual for your lawn mower closely to see if you have a regular lawn mower or a mulching lawn mower.
Disconnect the spark plug wire and lay your lawn mower down on its side. Locate the center bolt that holds the blade to the lawn mower.
Take a tape measure and measure the entire length of the lawn mower blade diagonally from cutting edge to cutting edge.
Take this information with you to your local home improvement store or garden center and purchase the correct replacement blade.