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Lawn Seed Calculator
Use this free lawn seed calculator to work out how much grass seed your lawn needs before you buy. Enter your lawn area, pick your grass type and its seeding rate, choose new lawn or overseeding, and it gives you the seed in pounds and kilograms, how many bags to buy, and the estimated cost.
Picking a grass fills in its usual mid-range rate. Adjust the rate if your seed bag recommends a different one; the bag rate always wins.
| Amount | Value |
|---|---|
| Lawn area | 5,000 sq ft |
| Seeding rate used | 7 lb / 1,000 sq ft |
| Seed before waste allowance | 35 lb |
| Waste allowance (10%) | 3.5 lb |
| Seed needed | 38.5 lb (17.5 kg) |
| Bags to buy (25 lb) | 2 bags (50 lb) |
| Estimated cost | $134.75 |
| Bag size | Bags needed |
|---|---|
| 5 lb bags | 8 |
| 10 lb bags | 4 |
| 25 lb bags | 2 |
| 50 lb bags | 1 |
How it works
Grass seed is sold by the pound, but it is spread by the square foot, so the whole calculation is a rate problem. Every grass type has a seeding rate in pounds per 1,000 square feet, and the calculator multiplies that rate by your lawn's area. A 5,000 square foot lawn at 7 pounds per 1,000 square feet needs 5 × 7 = 35 pounds of seed.
The rate depends on the grass because grass seeds are different sizes. The rates show it: bluegrass needs only 2 to 3 pounds per 1,000 square feet and bermuda 1 to 2 pounds, while perennial ryegrass needs 5 to 9 pounds and tall fescue 6 to 8 pounds. The calculator fills in the middle of each grass's range when you pick it, so tall fescue starts at 7 pounds and bluegrass at 2.5 pounds.
Seed (lb) = area in sq ft ÷ 1,000 × rate in lb per 1,000 sq ft
Overseeding rate = new-lawn rate ÷ 2
Seed with waste = seed × (1 + waste %)
Bags to buy = seed ÷ bag size, rounded up
Cost = seed × price per pound
Overseeding is the same math at half the rate. When you overseed, you are thickening a lawn that already exists, filling thin patches and adding new varieties, so the existing grass does half the work and you only need half the seed. A tall fescue lawn that needs 35 pounds for a new seeding needs only 17.5 pounds to overseed.
Measuring the lawn is the part people get wrong most often, and it is worth doing carefully. For a rectangle, multiply length by width: 100 feet by 50 feet is 5,000 square feet. For an L-shaped or irregular lawn, split it into rectangles, work out each one, and add them up.
Bags are the last step, and they are a rounding problem. Seed is sold in whole bags, so you divide the pounds needed by the bag size and round up. Needing 38.5 pounds with 25 pound bags means 38.5 ÷ 25 = 1.54, so you buy 2 bags, or 50 pounds.
Worked example
You are seeding a new tall fescue lawn that measures 100 feet by 50 feet, using a seeding rate of 7 pounds per 1,000 square feet, a 10 percent waste allowance, and seed at $3.50 per pound.
Step 1: Area — 100 × 50 = 5,000 square feet.
Step 2: Seed before waste — 5,000 ÷ 1,000 × 7 = 35 pounds.
Step 3: Add the waste allowance — 35 × 1.10 = 38.5 pounds.
Step 4: Convert to kilograms — 38.5 × 0.4536 = 17.5 kg.
Step 5: Bags — 38.5 ÷ 25 = 1.54, rounded up to 2 bags of 25 pounds.
Step 6: Cost — 38.5 × $3.50 = $134.75.
The result is 38.5 pounds, or 17.5 kilograms, of seed, 2 bags of 25 pounds, and an estimated cost of $134.75. Overseeding the same lawn would need half the rate, 3.5 pounds per 1,000 square feet, giving 17.5 pounds before waste, or 19.25 pounds with the 10 percent allowance.
FAQ
How much grass seed do I need per 1,000 square feet?
It depends on the grass type. Kentucky bluegrass needs 2 to 3 pounds per 1,000 square feet, perennial ryegrass needs 5 to 9 pounds, tall fescue needs 6 to 8 pounds, and bermuda and zoysia each need only 1 to 2 pounds. The differences come from seed size: bluegrass and bermuda seeds are tiny, so a pound contains far more seeds than a pound of big fescue or ryegrass seed. Pick your grass in the calculator and it fills in the usual rate for you.
How much seed do I need for overseeding?
Overseeding uses about half the rate of a new lawn, because you are thickening grass that already exists rather than starting from bare soil. A tall fescue lawn that needs 7 pounds per 1,000 square feet for a new seeding needs about 3.5 pounds per 1,000 square feet for overseeding. A 5,000 square foot fescue lawn needs 35 pounds of seed for a new lawn but only 17.5 pounds to overseed it. Pick the overseeding mode in the calculator and it halves the rate for you.
How many bags of grass seed should I buy?
Divide the pounds you need by the bag size and round up to a whole bag, because seed is only sold in whole bags. If you need 35 pounds and buy 25 pound bags, 35 divided by 25 is 1.4, so you buy 2 bags, which gives you 50 pounds. The extra seed is not wasted: keep it in a cool, dry place and use it to patch bare spots next season. The calculator shows the bag count for 5, 10, 25, and 50 pound bags so you can compare.
Should I buy extra grass seed?
Yes, buying about 10 percent extra is sensible for a new lawn. Some seed is lost to birds, wind, and washout in the first rain, and some spots always come up thin. A little extra seed on hand lets you patch those spots right away instead of buying another bag. The calculator adds a 10 percent waste allowance by default, and you can set it to zero if you are overseeding an established lawn or spreading seed with a slit seeder that places it precisely.
What is the best grass seed for a sunny lawn?
For a sunny lawn in a cool climate, tall fescue or perennial ryegrass are the usual choices: fescue handles heat, drought, and foot traffic well, and ryegrass germinates fast and looks fine. In a warm climate, bermuda and zoysia are the standard sun-loving grasses, and both need far less seed per square foot. For shade, look for a fine fescue blend instead. Whatever you choose, match the grass to your climate first and your sun second, because the wrong grass will struggle no matter how much seed you spread.