Figuring out how much grass seed to plant is one of the last decisions you need to make before seeding your new lawn---or over-seeding an old one. Fall is the best time to plant but if you must start in the spring, prepare the topsoil but don't plant until all soil has warmed to 60 degrees Fahrenheit. Whether you buy a packaged blend of seed or buy seed specially mixed for your area, there are a number of questions to answer and steps to take before seeding.
Measure your lawn with a tape and helper or by using your property survey. Divide an irregular parcel into rectangles to make area easier to figure. Compute the area of your yard by multiplying the length times the width of each rectangle and adding the results together for a total area. Subtract the area of your garden or large flower borders.
Buy the right mix. Find a grass that grows well in your climate area (cool season, warm season or transitional). Look for varieties that match the conditions in your yard (sun, shade, wet, dry). If you choose grass that is not suited to your lawn, be prepared not only to seed more heavily but also to wait longer for germination and over-seed next season.
Check the package. Most boxes and bags will show a "seeding rate" in pounds of seed per 1000 square feet of lawn. If the seed is a proprietary mix from a local seed company or garden center, it should be marked with a seeding rate.
Adjust seed rates for mixes. The Purdue-University of Illinois (PUI) turf science programs recommend a seed rate of 2 pounds per 1000 square feet for Kentucky bluegrass in full sun or shade. Kentucky bluegrass germinates slowly and perennial ryegrass or fine fescue is often added for high foot-traffic areas. Depending on the mixture, you may need 3 to 5 pounds per 1000 square feet for such mixtures.
Make adjustments for how the lawn will be used. PUI recommends 6 to 9 pounds per 1000 square feet of turf-type tall fescue for a lawn in full sun fertilized at a rate of 3 to 4 pounds of nitrogen per 1000 square feet per year. The seeding rate for tall fescue for roadsides where it receives no fertilizer and is mowed infrequently is 8 to 10 pounds per 1000 square feet---and a KY31 variety is recommended.
Calibrate your spreader to accurately drop or broadcast the proper amount of grass seed. If you can't interpret the calibration on the spreader, measure the area covered by the spreader, put a pound of seed in the hopper and run the spreader for about 100 square feet. Measure the seed remaining in the hopper; subtract to find the amount used and project the amount of seed that would be used in 1000 square feet.