Measure net seed area
Break irregular lawns into measurable sections and subtract areas that will not be seeded.
Use the same area basis as the published seeding rate.
Choose the rate for the seed type
Turf species and mixtures can have materially different pounds-per-1,000-square-feet recommendations.
Keep the assumption visible so it can be replaced when project or product data is available.
Multiply rate by area
Divide lawn area by 1,000 square feet when the rate is stated per 1,000 square feet, then multiply by pounds per unit area.
Keep the arithmetic rate separate from any contingency seed.
Round to bag sizes last
Convert required pounds to the actual package sizes available from the supplier.
Buying extra seed is a procurement decision, not a change to the agronomic rate.
Account for establishment conditions
Seedbed preparation, timing, irrigation, shade, and seed-to-soil contact affect establishment.
A quantity calculation alone does not guarantee germination or turf density.