π± Grass Seed & Fertilizer Calculator
How much seed to buy, and how much fertilizer to feed it.
Seed Needed
How to Use This Calculator
Enter your lawn size, pick your grass seed, and tell the calculator whether this is a brand-new lawn or an overseeding job. You will get the pounds of seed, the pounds of fertilizer product, and the number of bags to buy.
Enter the Lawn Area in square feet. Pace off the lawn in feet, multiply length by width, and subtract driveways, patios, and planting beds.
Choose your Seed Type: tall fescue for heat tolerance and durability, Kentucky bluegrass for a dense, self-repairing lawn, or perennial rye for fast establishment.
Set the Job Type. A new lawn needs the full seeding rate; overseeding an existing lawn needs about half the rate because you are filling thin spots, not starting from bare soil.
Enter the Fertilizer N (%), the first number on your fertilizer bag. The calculator converts it to product pounds using the 1 pound of nitrogen per 1,000 sq ft rule.
Seeding and Fertilizer Formulas
Seeding rates per 1,000 square feet: tall fescue 7 lb new, 3.5 lb overseed; Kentucky bluegrass 2.5 lb new, 1.5 lb overseed; perennial ryegrass 8 lb new, 4 lb overseed. The fertilizer math uses the nitrogen content of the product: you need 1 pound of actual nitrogen per 1,000 square feet, so divide 1 by the nitrogen fraction. A 10-10-10 bag is 10 percent nitrogen, so 1 / 0.10 = 10 pounds of product per 1,000 square feet. A 24-0-10 bag is 24 percent nitrogen, so 1 / 0.24 = 4.2 pounds of product per 1,000 square feet. Seed is typically sold in 10-pound bags and fertilizer in 50-pound bags, so the calculator rounds each result up to whole bags.
Worked Example
A 5,000 square foot new tall fescue lawn, feeding with 10-10-10 fertilizer. Seed: 5,000 / 1,000 x 7 = 35 pounds of seed. At 10-pound bags, that is 4 bags (35 / 10 = 3.5, rounded up). Nitrogen needed: 5,000 / 1,000 x 1 = 5 pounds of nitrogen. Product: 5 / 0.10 = 50 pounds of 10-10-10. At 50-pound bags, that is exactly 1 bag. If the same lawn were an overseeding job, the seed would drop to 5,000 / 1,000 x 3.5 = 17.5 pounds, or 2 bags of seed, while the fertilizer stays the same.
Practical Lawn Tips
Seed in early fall for cool-season grasses: soil is warm enough for fast germination and weed pressure is low. Test your soil pH first; grass thrives at 6.0 to 7.0, and lime or sulfur is cheap insurance. Water new seed lightly two to three times a day until it sprouts, then shift to one deep watering per week. Mow tall fescue at 3 to 4 inches, never cutting more than one third of the blade at once. Do not apply weed killer for at least 6 to 8 weeks after seeding, since most herbicides also kill young grass seedlings. Store leftover seed in a cool, dry place; sealed seed stays viable for two to three years.
Frequently Asked Questions
How much grass seed do I need per 1,000 square feet?
For a new lawn: tall fescue 6 to 8 pounds, Kentucky bluegrass 2 to 3 pounds, and perennial ryegrass 8 to 10 pounds per 1,000 square feet. Overseeding an existing lawn needs roughly half those amounts. Using less than the recommended rate gives thin, weedy turf; using more wastes seed and causes weak, disease-prone grass.
What does the 1 pound of nitrogen per 1,000 sq ft rule mean?
The standard lawn feeding is 1 pound of actual nitrogen per 1,000 square feet per application. The fertilizer bag shows its nitrogen percentage as the first N-P-K number: a 10-10-10 bag is 10 percent nitrogen, so you need 10 pounds of product per 1,000 square feet to deliver 1 pound of nitrogen. A 24-0-10 bag only needs about 4.2 pounds for the same feeding.
Should I fertilize when I seed?
Yes. Use a starter fertilizer with extra phosphorus (the middle N-P-K number) at seeding time, because phosphorus drives root development. Keep traffic off the area and water lightly two to three times a day until germination, then switch to deeper, less frequent watering.
Can I mix grass seed types?
Yes, seed blends are common and often outperform single varieties. A popular mix is 80 to 90 percent tall fescue with 10 to 20 percent Kentucky bluegrass: the fescue establishes fast and tolerates heat, while the bluegrass fills in thin spots. Calculate the total at the fescue rate, then split the pounds by your blend ratio.
How many fertilizer bags do I need to buy?
Fertilizer is usually sold in 50-pound bags. A 50-pound bag of 10-10-10 covers 5,000 square feet at the standard 1-pound-of-nitrogen rate. The calculator converts your lawn area into product pounds and rounds up to whole bags, since you cannot buy a fraction of a bag.