Lepus timidus on lawn grass.
image by Banangraut/Commons.wikimedia.org
There are dozens of varietals of grass seed on the market and the size and weight of individual grass seeds vary widely. As grass seed is measured and sold by the pound, coverage rates are determined largely by the size of the seed. Measuring the square footage of the area you wish to seed is the first step. That information can be used to consult the grass seed package of your varietal of choice which will help you to determine the coverage rate and the amount of seed to purchase.
Measure the length and width of the soil expanse you wish to seed for grass. Multiply the measurements to arrive at the square footage of the area to be covered with seed. Note that figure and take it with you to the nursery or home improvement center when you purchase your grass seed.
Select your preferred varietal of grass seed or a blend of two or more seeds to suit your needs. Consider sun and shade conditions at your site, climate and the durability of the lawn under regular foot traffic, if that will be an issue.
Consult the package label of your chosen grass seed to determine the distribution rate of the seed per square foot. Multiply the distribution rate of the chosen seed varietal by the square footage measurement of your space to arrive at the number of pounds of grass seed you will need to purchase to cover your square footage in a lush lawn.