Pure live seed: what the tag changes
Pure live seed (PLS) is the share of a bag that is the seed you paid for and will germinate. The tag gives the two figures behind it, purity and germination. Multiply them and you have PLS.
LSU AgCenter's Pure Live Seed Calculation shows the steps. Its example tag reads 0.48 purity and 0.80 germination, so PLS is 0.38. An area of 20,000 sq ft at 1 lb PLS per 1,000 sq ft needs 20 lb PLS, and 20 divided by 0.38 is 52.6 lb of seed from the bag. The calculator does not round PLS first: 0.48 × 0.80 is 0.384, and the same area comes out at 52.08 lb.
Penn State Extension's Lawn Establishment page works in percent. Pure seed of 92 % times germination of 80 %, divided by 100, gives 74 % PLS, and 100 / 74 gives 1.35 lb of seed for each pound specified in the mixture. Unrounded, the product is 73.6 % and the factor is 1.36.
The tool runs the same step on your tag. At the starting values, 90 % × 85 % is 76.5 % PLS, so each live pound takes 1.31 lb from the bag. A low purity or germination figure raises that factor, and you see it before you buy.
Two extension tables, two units
The rate menu holds two extension tables. Each row keeps its publisher, its region and its printed unit.
LSU AgCenter (Louisiana) prints pounds of PLS per 1,000 sq ft: annual ryegrass 5 to 10, bahiagrass 5 to 10, bermudagrass 0.75 to 1, carpetgrass 1 to 1.5, centipedegrass 0.5 to 1, perennial ryegrass 3.5 to 10 and zoysiagrass 1 to 2. A footnote marks both ryegrasses as cool-season grasses with a rate for overseeding warm-season turf.
Penn State Extension (Pennsylvania), Table 1 prints a "Seed Rate (lb/1000 sq ft)" column with no PLS label. In sun and well-drained soil: turf-type tall fescue 6 to 8, Kentucky bluegrass 2 to 3, perennial ryegrass 4 to 5. In shade: fine fescues 4 to 5, and rough bluegrass 2 to 3 for full shade. For overseeding or renovation: perennial ryegrass 4 to 6. Its mixtures, by weight: Kentucky bluegrass 80 to 90 % with perennial ryegrass 10 to 20 % at 2 to 3; Kentucky bluegrass 40 to 60 %, fine fescues 20 to 40 % and perennial ryegrass 10 % at 3 to 4; and for partial shade, fine fescues and Kentucky bluegrass at 40 to 50 % each with perennial ryegrass 10 % at 4, or turf-type tall fescue at 6 to 8.
Since that column carries no PLS label, a Penn State row gets two lines in the results: the printed pounds taken as seed from the bag, and the same pounds taken as live seed, corrected by your tag. The second line is our reading of Penn State's 100 / %PLS step, not a statement of the table.
Price per pound of live seed
A cheaper bag can cost more per pound of seed that grows. The comparison divides each bag's price per pound by that bag's own PLS, then ranks the bags.
Penn State's Lawn Establishment page gives a worked example. Seedlot A sells at $0.95 per lb with 85 % purity and 60 % germination: 51 % PLS, or $1.90 per lb of PLS. Seedlot B sells at $1.50 per lb with 99 % purity and 90 % germination: 89 % PLS, or $1.65 per lb of PLS. The full-price lot is the cheaper live seed. Penn State rounds 100 / 51 to 2 and 100 / 89 to 1.1; without that rounding the calculator gets $1.86 and $1.68, in the same order.
Those are the publisher's illustration prices, not market prices. The calculator holds no seed price of its own: the ranking appears once you type the price, weight, purity and germination of two or three bags. For each one it also returns the whole bags this lawn takes at the selected rate, and what they cost.
Checking what the spreader puts down
The bag count assumes the spreader delivers the rate. The UMass Extension sheet on rotary spreader calibration says the procedure is the same for any free-flowing granular material, seed included, and calls a spreader calibrated within ±5 % of the recommended rate. Its example is fertilizer: a 15 ft swath over 30 ft, so 450 sq ft, with 1.1 lb collected, checked against 5.00 lb per 1,000 sq ft and a band of 4.75 to 5.25.
Run through this check, those figures give 2.44 lb per 1,000 sq ft, under the band for one pass. UMass and Penn State's fertilizer spreader page both describe two passes at right angles, with the spreader set for half the rate. At 2.5 per pass, the same 2.44 falls within the band. That last step is our arithmetic on the UMass figures. UMass Table 1 also lists 3.5 to 4.5 ft of spacing for seed during rotary spreader operation.
In the tool, enter the strip you marked, the seed you weighed before and after, the rate you want and the number of passes. With the starting values, 0.4 lb on a 5 × 10 ft strip is 8 lb per 1,000 sq ft per pass, against 2 per pass for 4 lb in two passes. That is four times the target: the setting is too open.
Measuring the lawn and rounding to bags
Up to three shapes make the lawn: rectangles, circles and triangles. Beds, a patio or a path inside them go in as bare square feet and come off the total. The starting layout is a 50 × 40 ft rectangle plus a triangle with a 20 ft base and a 10 ft height, 2,100 sq ft in all.
At Penn State's tall fescue rate of 6 to 8, that lawn takes 12.6 to 16.8 lb if the printed pounds are bag seed, or 16.47 to 21.96 lb if they are live seed at 76.5 % PLS. Bags round up, never down: in 20 lb bags that is 1 bag, or 1 to 2. Each line also gives the area one bag covers. See the rounding rules behind the bag count, or price sod rolls and pallets for the same lawn instead.
Grass seed questions
How do you calculate how much grass seed you need?
Multiply the area by the rate and divide by 1,000 to get pounds of live seed, then divide by the PLS from your tag to get pounds from the bag. In LSU AgCenter's example, 0.48 purity × 0.80 germination = 0.38 PLS; 20,000 sq ft at 1 lb PLS per 1,000 sq ft is 20 lb PLS, or 52.6 lb of seed. Without rounding PLS, the calculator gives 52.08 lb.
How much will a 50 lb bag of grass seed cover?
It depends on the rate and the tag: coverage per bag is the bag weight × 1,000 divided by the bulk rate. With LSU AgCenter's bahiagrass rate of 5 to 10 lb PLS per 1,000 sq ft and a tag at 76.5 % PLS, a 50 lb bag covers 7,650 to 3,825 sq ft. Change the species or the tag and the result line updates.
How many square feet will 20 lbs of grass seed cover?
At the starting values, Penn State's turf-type tall fescue rate of 6 to 8 lb per 1,000 sq ft, read as printed, gives 3,333.33 to 2,500 sq ft for 20 lb. Read as live seed at 76.5 % PLS, the same 20 lb covers 2,550 to 1,912.5 sq ft. The calculator shows both because the Penn State column has no PLS label.
How many pounds of grass seed per 1000 sq ft for overseeding?
Penn State Table 1 lists perennial ryegrass at 4 to 6 lb per 1,000 sq ft for overseeding or renovation of lawns, printed without the PLS label. LSU AgCenter marks annual ryegrass at 5 to 10 and perennial ryegrass at 3.5 to 10 lb PLS per 1,000 sq ft as rates for overseeding warm-season turf. For another species, enter the rate you have chosen.
How much grass seed do I need per 1,000 square feet?
Pick the species in the rate menu. LSU AgCenter (Louisiana) gives pounds of pure live seed per 1,000 sq ft, from 0.5 to 1 for centipedegrass to 5 to 10 for bahiagrass. Penn State (Pennsylvania) gives pounds per 1,000 sq ft without the PLS label, from 2 to 3 for Kentucky bluegrass to 6 to 8 for turf-type tall fescue. For Penn State rows, the tool shows those pounds as bag seed and as live seed.
How much grass seed do I need to overseed 1 acre?
With Penn State's overseeding rate for perennial ryegrass, 4 to 6 lb per 1,000 sq ft, one acre of 43,560 sq ft takes 174.24 to 261.36 lb as printed, or 227.76 to 341.65 lb if those pounds are live seed at 76.5 % PLS. In 50 lb bags, that is 4 to 6 bags, or 5 to 7.
Can I use this as a grass seed calculator for overseeding?
Yes. Choose Penn State's overseeding or renovation row (perennial ryegrass), one of LSU AgCenter's ryegrass rows (rates for overseeding warm-season turf), or type your own rate and say whether it is live seed or bag seed. The tag correction, the bag count and the spreader check work the same way.
How much grass seed do I need per acre?
Multiply the rate per 1,000 sq ft by 43.56. For LSU AgCenter's bahiagrass rate of 5 to 10 lb PLS per 1,000 sq ft, one acre needs 217.8 to 435.6 lb of live seed, or 284.71 to 569.41 lb from bags at 76.5 % PLS. Enter a 220 × 198 ft rectangle and pick any row to get the acre figure for another species.
How many 50 lb bags of grass seed do I need per acre?
On one acre with a tag at 76.5 % PLS, LSU AgCenter's bahiagrass rate of 5 to 10 lb PLS per 1,000 sq ft takes 6 to 12 bags of 50 lb. Penn State's overseeding rate for perennial ryegrass, 4 to 6, takes 4 to 6 bags read as printed, or 5 to 7 read as live seed.
How much grass seed do I need per square foot?
The extension tables give rates per 1,000 sq ft, not per square foot. The calculator divides the rate by 1,000 and multiplies by the area you measured, then rounds up to whole bags, so a small patch gets the same rate as a large lawn.
Where this estimate stops
- Seed prices come from you. The Penn State prices above are the publisher's worked example.
- Penn State Table 1 prints its rates without the PLS label, so both readings stay on screen.
- Overseeding rates cover perennial ryegrass (Penn State) and the two ryegrasses over warm-season turf (LSU AgCenter); for any other species, enter your own rate.
- Each table is regional: Louisiana for LSU AgCenter, Pennsylvania for Penn State.
- The tool computes quantities; time to emergence and lawn care after seeding are outside its scope.
- The spreader check follows the UMass test-strip method on the strip you measure and tells you which way to move the setting.