The volume of seed needed to plant a new lawn varies by the type of grass. The label of your seed or seed blend should report the volume in proportions of pounds per square feet. A typical measurement is three pounds per 1,000 square feet. Bent grass varieties take a little less, while ryegrass takes a bit more.
It is recommended that you use a mechanical spreader to plant your grass seed. Sowing the seed by hand results in uneven distribution of the seed and disappointing final results. A good spreader is adjustable, with a variety of settings for both seed and later fertilizer use. The spreader is used by walking back and forth over the area in one direction and then repeating the process in a perpendicular manner.
If you overseeding an existing lawn, the volume of seed needed will be less than that used for the original planting. The seed vendor will recommend a specific coverage for each specific variety or blend. It is usually in the range of one-half to two-thirds the volume of seed used for starting a lawn.