Formula Used
The calculator converts every section into square feet. It subtracts excluded spaces before applying treatment coverage. This keeps every later calculation consistent and comparable.
Effective Area = (Gross Lawn Area − Excluded Area) × Treatment Percentage Base Seed = Effective Area ÷ 1,000 × Application Rate Viability Adjusted Seed = Base Seed ÷ (Germination × Establishment) Final Seed = Viability Adjusted Seed × (1 + Total Allowances) Bags Required = Ceiling(Final Seed ÷ Bag Weight) Total Cost = Materials + Labour + Delivery + Tax
How to Use
Choose the measurement system matching your lawn measurements. Add every lawn section and enter its dimensions. Exclude permanent surfaces that will not receive seed.
Select the grass, purpose, and current lawn condition. Use recommended rates or enter a supplier rate. Adjust germination, losses, waste, and spreader overlap percentages.
Enter package sizes, material prices, labour, and watering assumptions. Review the calculated seed, bags, materials, and cost. Confirm local recommendations before purchasing expensive project materials.
Example Data
| Input | Example value | Purpose |
|---|---|---|
| Lawn dimensions | 50 ft × 30 ft | Creates a 1,500 sq ft rectangle |
| Grass seed | Tall fescue | Uses the preset tall-fescue rate |
| Lawn condition | Moderate thinning | Uses the standard condition multiplier |
| Germination | 85% | Compensates for non-germinating seed |
| Bag size | 10 lb | Calculates whole bags and remaining seed |
| Currency | GBP | Shows project estimates using pounds sterling |
Overseeding Rate Reference
| Grass type | Base rate | Common use |
|---|---|---|
| Kentucky bluegrass | 2.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Perennial ryegrass | 5.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Fine fescue | 4.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Tall fescue | 7.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Bermudagrass | 1.5 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Zoysiagrass | 2.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Bahia grass | 5.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Centipede grass | 1.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
| Custom grass-seed mixture | 5.0 lb per 1,000 sq ft | Adjust for purpose, condition, and supplier guidance |
Rates are planning estimates, not universal agronomic prescriptions. Cultivars and regional practices can require different quantities. Always compare the result with the seed label.
Lawn Preparation Checklist
- Mow lower than normal without scalping existing grass.
- Remove leaves, thatch, stones, and loose surface debris.
- Aerate compacted soil before spreading seed.
- Apply seed in crossing passes for even coverage.
- Rake lightly to improve seed-to-soil contact.
- Keep the upper soil layer consistently moist.
- Reduce watering frequency after seedlings establish.
- Wait before applying selective weed-control products.
Frequently Asked Questions
What does overseeding mean?
Overseeding spreads new seed across an existing lawn. It improves density without removing the established turf. Healthy seedlings gradually fill thin or damaged areas.
How much seed should I buy?
Seed quantity depends on area and application rate. Germination and loss allowances increase the purchase amount. The calculator rounds purchases to complete seed bags.
Should bare soil use an overseeding rate?
Large bare areas often need establishment-level seeding rates. They may also need soil preparation and grading. Compare overseeding results against new-lawn label guidance.
Why adjust for germination?
Not every seed develops into a healthy plant. Germination percentages estimate the successful share of seed. Lower percentages require more seed for equal establishment.
Should I aerate before overseeding?
Aeration helps seed reach exposed soil and moisture. It also reduces compaction around existing grass roots. Compacted lawns usually benefit most from aeration first.
How many spreader passes work best?
Two crossing passes usually improve distribution across lawns. Divide the total seed equally between each pass. Avoid repeated overlap along borders and turning areas.
How often should new seed receive water?
New seed needs frequent light moisture during germination. Hot or windy conditions may increase watering frequency. Avoid puddles that move seed into uneven clusters.
When can I mow new seedlings?
Wait until seedlings reach a safe mowing height. Blades should resist gentle pulling before the cut. Use sharp blades and avoid aggressive turning movements.
Can I include fertiliser and compost?
The calculator estimates both supporting material quantities. Enter local rates, depths, and material prices. Soil testing should guide actual fertiliser nutrient choices.
Why might supplier rates differ?
Seed blends contain different species and seed counts. Coatings and purity also change effective seed weight. Follow the product label when it conflicts materially.
Is this estimate suitable for professional contracts?
The result supports planning and preliminary cost comparisons. Site conditions can change labour and material quantities. Verify measurements before final bids or purchases.
Important Disclaimer
This calculator provides planning estimates using entered assumptions. Climate, soil, cultivar, and equipment can change results. Consult local turf guidance before major lawn renovation.