Grass Seed Calculator

Grass seed is sold and recommended by weight, but weight is a poor stand-in for what actually matters: how many seeds are going into the ground. Different species have wildly different seed sizes, so a pound of one grass can hold far more individual seeds than a pound of another.

Kentucky bluegrass seed is tiny — a single pound holds roughly 1 to 2 million seeds — so its new-lawn rate is only about 2.5 lb per 1,000 sq ft. Tall fescue seed is much bigger and heavier per seed, so it takes roughly 9 lb of seed, over three times as much by weight, to plant a comparable number of seeds over the same area. In other words, 2 lb of bluegrass can put down more actual seeds than 9 lb of fescue. That's why you can't compare seeding rates across species by weight alone — always use the rate for the specific grass type you're planting.

The math behind this calculator, in three steps:

Area (sq ft) = Length ft × Width ft
Rate = grass type's new-lawn rate (÷ 2 if overseeding)
Seed needed (lb) = (Area ÷ 1,000) × Rate

Dividing area by 1,000 puts it in the same units as the seeding rate on the bag, which is always given per 1,000 sq ft rather than per square foot, since the per-square-foot number would be too small to be useful.

Worked example: a 5,000 sq ft new lawn (a 100 ft × 50 ft area, for instance) planted with tall fescue, at its new-lawn rate of 9 lb per 1,000 sq ft:

5,000 ÷ 1,000 = 5
5 × 9 = 45 lb of seed for a new lawn
Overseeding the same area: 45 ÷ 2 = 22.5 lb

That 45 lb works out to fifteen 3 lb bags, three 20 lb bags, or just one 50 lb bag — which is why buying a single larger bag is usually cheaper and less wasteful than several small ones once your lawn is any real size.

A new lawn starts from bare, exposed soil, so every seed has open ground to germinate in and no competition from established grass. That's why new-lawn rates are set high enough to cover essentially the whole surface with seedlings.

Overseeding means spreading seed into a lawn that's already growing, usually to thicken thin turf or introduce a hardier grass type. Because most of the ground is already covered by living grass, overseeding only needs to fill the gaps — which is why the rate is typically half the new-lawn figure. For overseeding to actually work, though, the new seed needs to reach soil, not just sit on top of existing thatch. Aerating or mowing the lawn low (scalping) beforehand opens up seed-to-soil contact, which matters more for germination success than how much seed you put down.

It's tempting to assume a heavier hand with the spreader means a thicker, faster lawn — but seeding rates aren't conservative numbers with room to spare. They're calibrated so each seedling has enough space, light, and soil nutrients to grow into a healthy plant.

Overseed too heavily and seedlings crowd each other, competing for the same light and root space instead of filling in evenly. Dense, overcrowded sprouts also hold moisture longer at the soil surface, which raises the risk of fungal diseases like damping-off, especially in humid weather. The result of going over the recommended rate is often a thinner, patchier lawn than following the rate would have given you — not a thicker one.

Timing depends on which broad category your grass falls into. Cool-season grasses — tall fescue, Kentucky bluegrass, perennial ryegrass, and fine fescue, common across the northern and transition-zone US — establish best in early fall, when soil is still warm from summer but air temperatures are cooling and weed pressure is dropping. Spring is a workable second choice, though summer heat arrives before the grass is fully established.

Warm-season grasses like Bermuda, common in the South, do best seeded in late spring into early summer, once soil has warmed and the grass is heading into its active growing season. Seeding warm-season grass too early, into cool soil, often leads to poor, uneven germination.