Grass Seed Calculator

The number that drives everything: pounds per 1,000 square feet

Grass seed is sold by weight and applied by area, so every rate on a bag is written as pounds per 1,000 square feet. A 25 pound bag of tall fescue at 7 pounds per 1,000 covers about 3,571 square feet, which is close to a 60 by 60 foot lawn. Change the grass and the same bag covers a different area, because a Kentucky bluegrass seed is a fraction of the size of a tall fescue seed.

Those are label rates for improved seeded varieties, per 1,000 square feet. Coated seed, which is common for warm season grasses, carries a filler weight of 30 to 50 percent, so the bag rate for coated bermudagrass runs higher than the raw seed figure. A blend is priced on its heaviest component: a bag that is 80 percent tall fescue and 20 percent bluegrass is applied at the tall fescue rate.

Read the rate off the bag when you have the bag in hand. Rates vary by variety and by region, and the Penn State Extension guide to lawn establishment is a useful cross check for cool season grasses.

New lawn versus overseeding: roughly double

Bare soil needs a full stand, so every square foot has to produce enough seedlings to close the canopy. An existing lawn only needs the thin spots filled, and the established plants already hold the soil and shade the ground. The new lawn rate therefore runs about 1.5 to 2 times the overseeding rate for the same species.

For tall fescue the label rates are 5 pounds per 1,000 overseeding and 7 for new seed, a 40 percent gap. For Kentucky bluegrass it is 1.5 against 2.5, which is close to double. A 5,000 square foot lawn of tall fescue needs 25 pounds to overseed and 35 pounds from bare soil.

The bag count is where the money shows up. 35 pounds is two 25 pound bags, so you buy 50 pounds and have 15 left over, or you buy one 50 pound bag. At 25 pounds you buy a single 25 pound bag and use all of it. Ordering the overseeding rate for a new lawn is the most common way a seeding job comes out thin, and it shows up six weeks later as a patchy stand that weeds move into.

The opposite mistake costs less but still wastes money. Overseeding at the new lawn rate puts twice the seed on ground that already has plants competing for water, and the extra seedlings die. The survivors are no denser than they would have been at the lower rate.

Seed size explains why the rates differ so much

A pound of Kentucky bluegrass holds about 2.2 million seeds. A pound of tall fescue holds about 227,000, roughly a tenth as many, because each seed is much larger. That is why bluegrass goes on at 2.5 pounds and tall fescue at 7 for the same square footage: the goal is a similar number of seeds per square foot, not a similar weight.

Run the numbers and the target becomes visible. Tall fescue at 7 pounds per 1,000 square feet puts about 1,590 seeds on every square foot. Kentucky bluegrass at 2.5 pounds puts about 5,500 there. Bluegrass seeds are smaller and slower to establish, so it takes more of them to close the ground, and the plant spreads sideways once it does.

Fine fescue sits in the middle at roughly 550,000 seeds per pound, which is why its new lawn rate is 4 pounds. Perennial ryegrass has large seeds like tall fescue, germinates in five to seven days, and is often blended at 10 to 20 percent to hold the soil while the slower grass comes up. Iowa State Extension's tall fescue guide covers the seeding depth and timing that go with these rates.

This is also why a bag of bluegrass feels like it will last forever and a bag of tall fescue disappears. The bluegrass bag covers nearly three times the area at its label rate, and it costs more per pound for the same reason.

Spreader, slit seeder or hand: the method changes the rate

A rotary spreader throws seed in a fan and covers a 10 to 12 foot swath, but the edges of the pattern are lighter than the middle. A drop spreader lays a band the exact width of the hopper, 18 to 24 inches, with no overlap and no throw, so it is slower but more even. Hand scattering is the least even of the three, and most people apply 15 to 20 percent more seed than they intend while leaving thin streaks behind.

A slit seeder is a different machine. It cuts shallow grooves, drops seed into them and closes the soil over the top, so the seed sits in contact with moist soil instead of on top of thatch. Crews that slit seed often cut the rate by about a quarter and still get a better stand than a broadcast application at full rate, which is why the machine rents for more per day than a spreader.

Whichever method you use, split the seed in half and go over the lawn twice at right angles. That single habit removes most of the streaking that people blame on the seed, and it costs nothing but ten minutes.

Worked example: a 5,000 square foot front yard

Measure the lawn first. A lot that is 100 by 80 feet is 8,000 square feet, but the house, driveway, beds and walkways take a bite. Measure the grass as a set of rectangles, multiply length by width for each, and add them up. A typical suburban front yard comes out between 3,000 and 6,000 square feet.

Take 5,000 square feet of tall fescue, seeded from bare soil in September. The rate is 7 pounds per 1,000 square feet, so 5,000 divided by 1,000 is 5, and 5 times 7 is 35 pounds of seed. That is two 25 pound bags, or one 50 pound bag with 15 pounds left for touch ups.

Now check the seed tag. A bag listed at 90 percent purity and 85 percent germination is 76.5 percent pure live seed, so only 76.5 percent of the weight can grow. To land 35 pounds of viable seed you need 35 divided by 0.765, or 45.8 pounds of bag weight. That is the difference between buying 50 pounds and coming up short, and it is the number most people skip.

Water is the other half of the job. New seed has to stay moist for two to three weeks, which usually means a light watering twice a day rather than one long soak. Tall fescue germinates in 7 to 14 days, Kentucky bluegrass in 14 to 21, so a bluegrass lawn that looks bare at day ten is on schedule rather than failing.

Reading the seed tag before you buy

Every bag sold in the United States carries a tag with four numbers that matter: purity, germination, crop seed and weed seed. Purity is the percentage of the weight that is the named grass. Germination is the percentage of those seeds that will sprout. Multiply the two and you have pure live seed, the only figure that tells you what you are paying for.

Weed seed above 0.5 percent is a warning sign, and other crop above 1 percent usually means a cheap annual grass is padding the bag. The variety name matters too. A bag labeled tall fescue with no variety is a commodity blend, while a named variety has been tested for disease resistance, color and wear tolerance.

Two more fields are worth a glance. The test date tells you how old the germination figure is, since seed loses vigor in a hot garage, and the lot number lets you buy the same seed next year if it worked. A 40 pound bag priced 30 percent below the rest of the shelf is usually old seed with a germination number in the sixties, which means you need 40 percent more of it.

Timing changes the rate you can get away with

Cool season grasses, which means bluegrass, fescue and ryegrass, establish best when the soil is warm and the air is cooling, so late August through mid October is the window in most of the country. Seed put down then has months of root growth before summer arrives. Spring seeding works, but the new grass meets its first summer with a shallow root system and needs more water to survive it.

Warm season grasses want the opposite: late spring and early summer, when soil is above 65 degrees Fahrenheit. Dormant seeding, done after the ground freezes in late fall, relies on the freeze and thaw to work the seed into the soil, and it needs about 25 percent more seed because some of it is lost to birds, runoff and rot.

Weather in the first week matters more than the calendar. Seed that goes down before a week of rain beats seed that goes down at the perfect date before a heat wave, and a light straw cover of one bale per 1,000 square feet holds moisture without smothering the seedlings.

Five ways the calculation goes wrong

Most thin lawns trace back to one of five mistakes, and none of them is about the arithmetic.

• Using the overseeding rate on bare soil, which leaves the stand 30 to 40 percent short.
• Measuring the lot instead of the lawn, which overshoots by the size of the house and driveway.
• Ignoring pure live seed, which can add 30 percent to the bag weight you need.
• Spreading in one pass, which puts twice the seed in the overlap strips and half in the gaps.
• Covering the seed with too much soil or mulch, when a quarter inch is the target and a half inch stops most seedlings.

The calculator above handles the first three. The last two are about how you walk and how deep you rake, and no formula will fix them.

One more habit pays off. Weigh what is left in the hopper halfway through the job. If you have used more than half the seed at the halfway point, your spreader setting is too wide, and stopping to recalibrate is cheaper than buying another bag.