Overseeding Calculator

Overseeding versus starting a new lawn

Overseeding means spreading fresh seed over a lawn that is already there, to thicken it, fill bare patches and crowd out weeds. New seeding means building a lawn from bare soil. The two use the same seed but different amounts, because an existing lawn already has plants competing for space.

When you overseed, the goal is to add enough seed to fill the thin spots without smothering the grass that is already growing. That is why the rate is lighter, usually around 4 pounds per 1,000 square feet for a cool-season mix. When you start from scratch, you want the soil covered thickly from the first day, so the rate roughly doubles to 6 or 8 pounds per 1,000 square feet.

Getting the rate right saves money in both directions. Too little seed and the lawn stays thin, so you buy again next season. Too much seed and the seedlings crowd each other, the excess dies, and the money is wasted. The calculator above uses your lawn size and grass type to land in the recommended band.

The overseeding rate: pounds per 1,000 square feet

Seed rates are written as pounds per 1,000 square feet, because that is a size most homeowners can picture. The number depends on the grass, since a large seed like tall fescue needs more weight to cover the same ground than a tiny one like Kentucky bluegrass.

The working figures for overseeding an existing lawn:

• Kentucky bluegrass: 1 to 2 lb per 1,000 sq ft
• Tall fescue: 4 to 6 lb per 1,000 sq ft
• Perennial ryegrass: 4 to 6 lb per 1,000 sq ft
• Fine fescue: 2 to 3 lb per 1,000 sq ft
• Bermudagrass (warm season): 2 to 3 lb per 1,000 sq ft

Multiply the rate by your lawn area in thousands of square feet. A 5,000 square foot yard at 4 lb per 1,000 needs 5 × 4 = 20 pounds. Measure the lawn in sections, rectangles and triangles, and add them up, because a guess at the total is where most seed budgets go wrong.

Round up to the nearest bag when you buy, and keep a small amount of seed aside for touch-ups after the first mow, when thin spots tend to show up.

Worked example: overseeding a 5,000 square foot yard

A typical suburban front and back yard measures about 5,000 square feet, and you want to thicken a tall fescue lawn at 5 pounds per 1,000 square feet.

• Area: 5,000 sq ft = 5 units of 1,000
• Seed: 5 × 5 lb = 25 pounds
• In bags: one 25 lb bag, or two 12.5 lb bags
• Cost guide: at about $2 per pound, roughly $50 of seed

Split the seed into two passes and spread each at half the rate, one going north to south and one east to west. The crosshatch pattern covers the ground more evenly than a single pass, which leaves streaks where the spreader ran out. Then water lightly every day for the first two weeks so the seed stays moist.

Fall is the best window for cool-season grass, when the soil is still warm from summer but the air is cooling. That combination lets the seed germinate fast and gives the young plants time to root before winter.

Worked example: a small 1,500 square foot yard

Now a smaller city lot, 1,500 square feet, planted with fine fescue that is overseeded at 2.5 pounds per 1,000 square feet.

• Area: 1,500 sq ft = 1.5 units of 1,000
• Seed: 1.5 × 2.5 lb = 3.75 pounds
• In bags: about one sixth of a 25 lb bag
• At $2 per pound: roughly $7.50 of seed

Small yards are easy to overbuy for, because seed comes in fixed bag sizes. Buying a 5-pound bag covers this job with a little left for touch-ups, which beats a 25-pound bag that sits half-used in the garage. Store the leftover in a sealed container away from moisture and heat, and use it within a season.

Fine fescue tolerates shade and low water better than most grasses, which is why it suits small lots with mature trees. The lighter rate reflects its finer seed, so 3.75 pounds covers the same ground that 8 pounds of tall fescue would.

Grass seed rates by type

Overseeding and new-lawn rates for the common grasses, with the germination window so you can plan the timing.

Warm-season grasses like bermudagrass are often overseeded with annual ryegrass in fall so the lawn stays green through winter, then the ryegrass dies back as the bermuda wakes up in spring. Check the seed bag for the exact rate, since blends mix two or three grasses and the label reflects that.

Getting the seed to take

Seed is only half the job. Contact with soil, steady moisture and a little patience decide whether the lawn thickens.

Aerate first. Core aeration pulls small plugs of soil out and opens channels for the seed to drop into. Seed that lands on top of thick thatch dries out before it can root, which is the most common reason overseeding fails. Aerating right before you spread seed gives it the best start.

Keep it moist, not soaked. Water lightly once or twice a day for the first two weeks so the top inch of soil stays damp. A heavy soak washes seed into piles and can rot it. Once the grass is an inch tall, cut back to a deeper watering a couple of times a week.

Stay off it and hold the mower. New seedlings are fragile for the first three to four weeks. Mow only after the new grass is about 3 inches tall, and keep foot traffic light. The Penn State Extension turfgrass program and the USDA Natural Resources Conservation Service both publish regional guidance on seeding rates and timing. These figures are estimates for planning, so confirm the rate on your seed bag.