The lawn overseeding calculator tells you how much grass seed to buy based on your lawn area and grass type, plus the best time of year to overseed by climate region.
Best time to overseed
How to Overseed Your Lawn
Overseeding is the most cost-effective way to thicken a thinning lawn. Unlike lawn renovation (full kill and reseed), overseeding works with your existing turf to fill gaps, improve density, and crowd out weeds through competition.
Preparation is 80% of success
Mow the existing lawn short (2-3 inches), remove thatch if over 0.5 inches thick, and core aerate before seeding. The holes created by aeration dramatically improve seed-to-soil contact. Apply starter fertilizer at seeding. Keep the seedbed moist for 2-3 weeks until germination completes — this is the most critical period.
Frequently Asked Questions
What is overseeding?
Overseeding means spreading grass seed over existing turf to thicken thin areas, fill bare patches, or introduce a new grass variety. Unlike seeding bare ground, overseeding works with the existing lawn without stripping it first.
What is the overseeding rate for different grasses?
Kentucky Bluegrass: 1-3 lbs per 1,000 sq ft. Perennial Ryegrass: 4-8 lbs. Tall Fescue: 6-10 lbs. Bermudagrass: 1-2 lbs. Zoysia: 2-3 lbs. These are overseeding rates (lower than new seeding rates). Rates vary by seed quality.
When is the best time to overseed?
Cool-season grasses (fescue, rye, bluegrass): late summer to early fall (late August to mid-October in northern US). Warm-season grasses (bermuda, zoysia, St. Augustine): late spring to early summer. Overseeding outside these windows risks poor germination.
Should I aerate before overseeding?
Yes — core aeration immediately before overseeding dramatically improves results. The holes allow seed-to-soil contact, which is critical for germination. Without aeration, much of the seed rests on thatch and doesn't germinate.
Is this calculator free?
Yes, completely free with no signup required.