Grass Seed Calculator

Background.

For 5,000 square feet of new lawn, Kentucky bluegrass needs about 12.5 pounds of seed and turf-type tall fescue about 35. Neither figure is a misprint. Seeding rates are written by weight, but what a lawn needs is a number of seeds on each patch of soil, and bluegrass seed is so small that a pound of it goes much further than a pound of the far heavier fescue. Sow bluegrass at a fescue rate and much of the bag is wasted on seedlings crowding each other out; sow fescue at a bluegrass rate and you get a thin stand that weeds fill before the grass closes in.

So the calculation starts from the species. Each row in the list is a published row from a university extension table: Penn State Extension's seeding-rate tables for home lawns, golf turf, low-maintenance ground and temporary cover, and Michigan State University Extension's fall establishment guidance for overseeding existing turf. Published rates come as ranges, so you also choose the low end, the midpoint or the high end. The rate multiplied by your area in thousands of square feet gives the pounds of seed.

The seed label then tells you how much of those pounds can grow. Purity is the share of the bag's weight that is the seed you are paying for; germination is the share of that seed which sprouted in a laboratory test. Multiplied together and divided by 100 they give pure live seed, the measure of seed quality the USDA's Natural Resources Conservation Service uses. A lot at 96% purity and 88% germination is 84.48% live; one at 80% and 70% is only 56%, and it would take about 47.8 pounds of it to deliver the live seed a reference-quality lot puts down in 35. Running two bags through the calculator turns a price-per-pound comparison into a comparison of what each bag can actually grow.

The rows cover cool-season grasses only: the bluegrasses, fescues, ryegrasses and bentgrass of northern lawns. Warm-season species such as bermudagrass and zoysiagrass have rates that change with whether the seed is hulled, unhulled or coated; for those, take the figure from your own state extension service and enter it as a custom rate.

What is grass seed calculator?

A seeding rate is the weight of seed a published table recommends for a given area — in North American turf guidance, almost always pounds per 1,000 square feet. It differs by species because seed size differs, and by job: new seeding goes onto bare, prepared soil where every seed can reach the ground, while overseeding goes into existing turf whose plants already occupy most of the space, so overseeding rates are generally lower. Pure live seed (PLS) is the percentage of a seed lot's weight that is both the labelled grass and able to germinate, calculated as purity × germination ÷ 100. Comparing a bag's PLS with a reference quality shows how much more or less of that bag you need than the table rate implies.

How to use this calculator.

1. 01Measure only the area you will seed, in square feet, leaving out the house, driveway, paths and planting beds. Split an irregular lawn into rectangles, multiply length by width for each and add them up.
2. 02Pick the row for your species and job: a new-seeding row for bare, prepared soil, an overseeding row for thickening an existing lawn, or the renovation or temporary-cover rows where they fit. For a mixture, use the mixture row if one matches, otherwise the row for the main species.
3. 03Choose where in the published range to sit. The midpoint suits most prepared home lawns; go low on a good seedbed with reliable watering, high where you need fast cover on a slope or a well-trodden area.
4. 04Copy purity and germination from the bag's label. Leave the reference pure live seed at 76.5% unless you have reason to assume a different quality baseline.
5. 05Enter the bag weight and the price per pound (bag price ÷ bag weight) to see bags to buy, pounds ordered and seed cost.
6. 06For a warm-season grass, or a rate from a current state table, choose the custom row and enter that rate in pounds per 1,000 sq ft.

The formula.

The rate R comes from the chosen row: its low value, its high value, or the midpoint (low + high) ÷ 2. Bulk seed is then S = R × A ÷ 1,000, where A is the area in square feet, because extension rates are written per 1,000 sq ft. The same rate per acre is R × 43.56, since an acre is 43,560 square feet in NIST Handbook 133's table of units.

Pure live seed is PLS = p × g ÷ 100, with p the label purity and g the germination, both in percent, and the live pounds inside the bulk weight are S × PLS ÷ 100. The adjusted figure, S′ = S × PLS₀ ÷ PLS, answers a different question: how many pounds of this particular lot would put down the same live seed as S pounds of a reference lot at PLS₀. The reference defaults to 76.5%, Purdue's preferred label minimums — purity above 90% and germination above 85% — multiplied out: 90 × 85 ÷ 100. The calculator assumes the published bulk rates were written for seed of roughly that quality, so a better lot needs slightly less than the table rate and a poorer one needs more.

Bags are N = ⌈S ÷ w⌉ for a bag weight w, rounded up because part bags are not sold, and the pounds ordered are N × w. Seed cost is S × price per pound, priced on the pounds the rate calls for rather than the pounds ordered, so two species can be compared without the jump that comes from crossing a bag boundary.

One convention is worth knowing. Conservation seedings specified by the USDA quote rates in pounds of pure live seed and divide by PLS to find bulk pounds; turf tables, including those used here, quote bulk pounds of ordinary-quality seed. The calculator follows the turf convention and shows the PLS figures alongside rather than mixing the two.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Penn State Extension (Landschoot, P.), "Turfgrass Seed and Seed Mixtures", published 1 January 1997. Section II seeding-rate tables — home lawns, heavy-use areas, low-maintenance turf, golf, renovation and temporary cover. Primary source for every establishment rate row. Retrieved 2026-07-30; open access.
2. [2]Purdue University Turf Science and University of Illinois Turfgrass Program, AY-25-W / IL-IN TW 2, "Purchasing Quality Grass Seed for Your Home Lawn". Table 1 (species seed rates), Table 2 (preferred label ranges: purity above 90%, germination above 85%, inert below 8%, tested within 9 months), and the establishment-cost comparison scenarios. Used as the independent check on Penn State's rates and as the source of the 76.5% reference pure live seed default. Retrieved 2026-07-30; open access PDF.
3. [3]Michigan State University Extension (Frank, K.), "Six steps to fall turfgrass establishment", 26 August 2011. Source of the two overseeding rate rows: Kentucky bluegrass 1.5 to 2 lb per 1,000 sq ft, and perennial ryegrass or fine fescues 3 to 5 lb per 1,000 sq ft. Retrieved 2026-07-30; open access.
4. [4]USDA Natural Resources Conservation Service, Plant Materials Program, "Understanding Pure Live Seed (PLS)". Source of percent PLS = (percent germination × percent purity) ÷ 100 and of the bulk-from-PLS conversion. Retrieved 2026-07-30; open access PDF, though the fetch timed out at 60 seconds on our side — the formula is corroborated by the NRCS "Seed Quality" note and by AY-25-W's purity and germination definitions.
5. [5]National Institute of Standards and Technology, Handbook 133 – 2026, Appendix E, "General Tables of Units of Measurement". Units of Area: 1 acre = 43,560 square feet, used for the per-acre restatement. Retrieved 2026-07-30; open access PDF, read in full.
6. [6]USDA Natural Resources Conservation Service, "Seed Quality" plant materials note. Corroborating source for the purity, germination and pure live seed definitions used on this page. Retrieved 2026-07-30; open access PDF.