How to Use This Tool
Enter the area, pick the species, and say whether the soil is bare or you are thickening existing turf. The rate field updates from the species but stays editable, because bag recommendations vary.
Why the species matters more than the area
Seeding rate is driven by seed size and by whether the plant spreads. Kentucky bluegrass has very small seed and spreads by rhizomes, so 2 lb covers 1,000 sq ft and the plants fill the gaps themselves. Tall fescue seed is several times larger and the plant is a bunch grass that does not spread — every plant has to be sown, so it needs 8 lb for the same area.
That is a fourfold difference between two grasses that both get sold as "lawn seed", and a sixteenfold range across the species list. No single number can be right.
area 5,000 sq ft = 5.00 thousands tall fescue new lawn, 8 lb per 1,000 = 40.0 lb PLS 85% germ x 95% purity = 80.75% = 49.5 lb bag weight bags at 10 lb = 5 bags, 0.5 lb spare same lawn, overseeding at 4 lb = 20.0 lb same lawn, bluegrass new at 2 lb = 10.0 lb
Overseeding is half, and for a reason
A new seeding has to establish ground cover from nothing, competing with weeds and holding soil against rain. Overseeding lands into turf that already does that work, so the rate is consistently about half. Using the new-lawn rate to overseed wastes seed and creates competition that thins the seedlings you were trying to add.
Pure live seed is on the bag and nobody reads it
Every bag lists germination and purity percentages. Multiply them for the fraction of the bag weight that is viable seed: 85 per cent germination times 95 per cent purity is 80.75 per cent. To put down 40 lb of live seed you therefore need about 49.5 lb of bag.
This is also how a cheap bag stops being cheap. A discount mix at 70 per cent germination and 85 per cent purity is only 59.5 per cent viable, so it needs a third more weight than a premium bag to achieve the same stand.
More seed does not mean a thicker lawn
Seedlings compete for light, water and space. Sowing at double the rate produces dense weak seedlings that thin themselves out, often through damping off, and the result is patchier than the correct rate would have been. The rate on the bag is an optimum, not a minimum.
Timing beats quantity
Cool-season grasses — bluegrass, fescues, ryegrass — establish best in late summer to early autumn, when soil is warm and weed pressure is falling. Warm-season grasses like Bermuda and Zoysia want late spring. Sowing the right amount at the wrong time fails more reliably than sowing slightly too little at the right time.
What this does not cover
Starter fertiliser, soil preparation, straw or mulch cover and irrigation are all separate, and all of them matter more to the outcome than getting the seed weight exactly right.
