Overseeding Calculator
Calculate how many pounds of seed you need to overseed an existing lawn, at half the new-lawn seeding rate, plus total cost.
How this calculator works
Overseeding thickens an existing lawn rather than establishing turf from bare soil, so it needs roughly half the seed density of new seeding. This calculator uses a coverage rate of 500 square feet per pound of seed, equivalent to about 2 lbs per 1,000 square feet.
Formula: Seed (lbs) = Lawn area (sq ft) ÷ 500 sq ft per lb.
Worked example
A 50 ft × 20 ft lawn area (1,000 sq ft):
- Seed needed: 1,000 ÷ 500 = 2.00 lbs
- At $4/lb, that’s $8.00.
Coverage reference
| Lawn area | Seed needed |
|---|---|
| 25 × 10 ft | 0.50 lbs |
| 50 × 20 ft | 2.00 lbs |
| 100 × 40 ft | 8.00 lbs |
How to measure your lawn
Measure the length and width of the lawn area you’re thickening, and use this calculator rather than the new-lawn grass seed calculator whenever you’re filling in an existing, mostly-established lawn instead of starting from bare dirt. Bare or heavily thinned patches larger than a few square feet often do better treated as new seeding at the higher rate.
Frequently asked questions
Why does overseeding use a different rate than a new lawn?
A new lawn is seeded from bare soil and needs a dense rate to establish full coverage from scratch. Overseeding fills gaps in a lawn that's already mostly established, so it only needs about half the new-lawn rate — this calculator uses 500 square feet of coverage per pound, versus 250 sq ft per lb for new seeding.
What's the best time of year to overseed?
Early fall is the top window for cool-season lawns — warm soil and cooling air speed germination while weed pressure and heat stress drop off. Late spring is the better window for warm-season grasses, once soil has warmed but before summer heat peaks.
Do I need to prep the lawn before overseeding?
Mowing shorter than usual, raking out thatch, and aerating compacted soil beforehand all dramatically improve seed-to-soil contact, which is the single biggest factor in overseeding success. Skipping prep and just scattering seed over existing turf usually gives disappointing germination.
How much should I water after overseeding?
New seed needs consistently moist (not soaked) soil for the first couple of weeks, typically light watering once or twice a day rather than one deep soak. Once seedlings are up and mowed a couple of times, you can transition back to your lawn's normal deeper, less frequent watering schedule.
Should I pair overseeding with topdressing?
Yes — a thin topdressing layer after overseeding helps hold moisture around the seed and smooths minor low spots, which often improves germination rates. Check the lawn topdressing calculator for the sand or compost quantity that pairs with the area you're overseeding here.
Estimates only. Verify quantities with your supplier before purchasing.