Grass Seed Calculator: How Much Grass Seed Do I Need?

How to measure your lawn for grass seed

1. Sketch the lawn. Draw the yard and mark the house, driveway, walks, patio, and planting beds. Only the ground you will seed counts, so leave those out.
2. Split it into shapes. Measure rectangles as length × width. For a triangle, multiply the two sides that meet at the square corner and divide by 2. For a circle, use 3.14 × radius × radius.
3. Measure bare patches on their own. If you are only repairing spots, measure each patch instead of the whole lawn. University of Illinois Extension recommends the new-lawn rate for bare spots, so leave the overseeding box unchecked for them.
4. Add it up and enter the total. For an irregular area you have already totaled, enter the square footage as the length and 1 as the width.

Example: patching three bare spots

Say a tall fescue lawn has three bare patches: 6 × 5 ft (30 sq ft), 4 × 4 ft (16 sq ft), and 8 × 3 ft (24 sq ft), or 70 sq ft in all. Enter 70 as the length and 1 as the width, leave the overseeding box unchecked, and the calculator returns about 0.4 to 0.6 lb of tall fescue seed (70 ÷ 1,000 × 6 = 0.42 and 70 ÷ 1,000 × 8 = 0.56). Patch jobs take very little seed.

Grass seed rates per 1,000 sq ft

New-lawn rates are Penn State Extension's for single-species stands; overseeding rates are University of Illinois Extension's. The last two columns apply them to the calculator's 1,500 sq ft default lawn.

Match the grass to the site. Penn State recommends 100 percent fine fescue for heavy shade on well-drained soil, and lists turf-type tall fescue for sun, partial shade, and heavy-use areas.

Extension rates differ slightly

Here is how three extension services compare for new lawns, in pounds per 1,000 sq ft:

The ranges overlap, and our calculator uses Penn State's figures for new lawns. If you buy a seed mix, the bag lists its own rate; follow the label.

When to plant grass seed

For cool-season grasses, Purdue Extension says the best time to seed is late summer to early fall, when adequate soil moisture, warm soil, and limited weed pressure favor seedlings: August 15 to September 15 in the northern half of Indiana and Illinois, and September 1 to 30 in the southern half. Penn State Extension calls late summer the optimum time in most of Pennsylvania, and University of Minnesota Extension recommends mid-August to mid-September there.

Spring is harder. Purdue describes spring seeding as difficult and often unsuccessful, and Penn State notes that spring seedlings face the heat and drought of summer. For dates where you live, check with your local extension office.

This calculator is for cool-season grasses

The four choices in the calculator (tall fescue, Kentucky bluegrass, perennial ryegrass, and fine fescue) are cool-season grasses, and the rates and timing in this guide apply to them. If your lawn is a warm-season grass such as bermudagrass, zoysiagrass, or centipedegrass, don't use these numbers. Follow your state extension service or the rate printed on the seed bag, and multiply your square footage by that rate yourself.

New lawn vs. overseeding

A new lawn starts from bare, prepared soil. Overseeding thickens turf that is already there. The steps differ, and so do the rates.

Seeding a new lawn

1. Test the soil. Purdue Extension recommends a soil test from the site to set fertilizer rates.
2. Prepare the seedbed. Purdue suggests working the soil 4 to 6 inches deep with a tiller, mixing in fertilizer or other amendments, then raking to finish grade just before seeding. If you need to build up or level the soil first, our topsoil calculator sizes that order.
3. Spread in two directions. Purdue advises a drop spreader set very low, with half the seed sown one way and the other half at right angles. Penn State notes that seeding in two directions leaves fewer skips.
4. Cover lightly and roll. Penn State says a gentle pass with a rake should cover most seeds with about 1/4 inch of soil. University of Minnesota Extension suggests raking so that 10 to 15 percent of the seed still shows, then rolling for good seed-to-soil contact.
5. Mulch lightly. Purdue suggests one bale of clean, weed-free straw per 1,000 sq ft, spread thin enough that about half the soil still shows.

Overseeding an existing lawn

University of Illinois Extension pairs overseeding with core aeration in fall, its pick for the best time to aerate:

1. Core aerate. Core (plug) aerators pull out plugs of soil about the diameter of a pencil. Spike aerators can press soil together around the holes and cause more compaction.
2. Time it with soil moisture. Aerating is easiest when the soil is moist from rain or irrigation, but skip it when the ground is too wet.
3. Leave the cores. They stay on the surface and break up over time.
4. Seed at the overseeding rate. The aeration holes give the seed better contact with the soil. Use 3 to 5 lb per 1,000 sq ft for fescue and ryegrass or 1.5 lb for bluegrass, and the new-lawn rate on any bare spots.

Watering new grass seed

Illinois Extension says regular watering is essential for about two weeks after overseeding, sometimes once or twice a day. For a new lawn, Purdue calls for watering two to four times daily depending on the weather, keeping the top 0.5 to 1 inch of soil moist without overwatering, then watering less often and more deeply once seedlings are 2 inches tall.

Be patient with bluegrass: Illinois Extension notes it can take two to four weeks to germinate, which may mean a longer watering period. Purdue says to start mowing once the first seedlings are tall enough, at 1.5 inches for Kentucky bluegrass, perennial ryegrass, and fine fescue and 2 inches for tall fescue.

Common mistakes to avoid

• Adding extra seed "just in case." Penn State warns that an overly dense stand from high seeding rates can bring more disease problems and mower scalping. Stay within the range.
• Using the new-lawn rate to overseed. On the default lawn that means 9 to 12 lb of tall fescue instead of 4.5 to 7.5 lb.
• Using a single-species rate for a mix. A bluegrass rate applied to a tall fescue blend comes up far short. Read the bag.
• Letting the seedbed dry out. Keep up regular watering for at least the first two weeks, and longer for bluegrass.