Lawn fertilizer needed.

Worked example

A 5,000 sq ft lawn, a 40 lb bag labelled 20-5-10, and a target of 1 lb of nitrogen per 1,000 sq ft. Nitrogen needed: 1 × 5,000 / 1,000 = 5 lb. Product needed: 5 / 0.20 = 25 lb, so one 40 lb bag covers it with 15 lb left over. One bag would spread 40 × 0.20 / 1 × 1,000 = 8,000 sq ft at that rate.

How it works

Fertilizer rates are set by the nutrient, not the product. Nitrogen needed (lb) = rate × area / 1,000. Product (lb) = nitrogen needed / (N% / 100). Bags are rounded up. Coverage per bag is bag weight × N% / rate × 1,000 square feet. Square metres and kilograms are converted to square feet and pounds first.

What this is not. It does not tell you whether your lawn needs nitrogen, phosphorus or potassium; a soil test does. It also does not set the schedule or product type. Slow-release and quick-release products follow different rules, and the label rate and any local fertilizer bylaws or phosphorus restrictions take priority. Keep products off hard surfaces and away from storm drains, and water in as the label directs.

Sources and basis (checked 2026-10-06): Penn State Extension advises not going above about 1 pound of nitrogen per 1,000 square feet in a single application unless the product is mostly slow-release, with late spring rates of 0.75 to 1 pound (Turfgrass Fertilization: A Basic Guide for Professional Turfgrass Managers); the arithmetic is the standard percent-nutrient formula.

FAQ

How much fertilizer do I need per 1,000 square feet?

It depends on the N percentage. For 1 lb of nitrogen per 1,000 sq ft, divide 1 by the decimal N percentage: a 20 percent product takes 5 lb, a 10 percent product takes 10 lb, a 46 percent product takes about 2.2 lb.

How many bags of fertilizer do I need?

Divide pounds of product by the bag weight and round up. The calculator does that and also shows how many square feet one bag covers at your chosen rate.

Is more fertilizer better?

No. Too much quick-release nitrogen can burn grass and washes into waterways. The calculator warns above 1 lb of nitrogen per 1,000 sq ft in a single application. Follow the label.

What do the three numbers on a fertilizer bag mean?

Percent nitrogen (N), phosphate (P2O5) and potash (K2O) by weight, in that order. Only the first number is used here, because lawn rates are normally set by nitrogen.