Lawn Fertilizer Calculator: Pounds of Product From the N-P-K Numbers

The formula

Lawn advice from extension services is written in pounds of actual nitrogen per 1,000 square feet, because nitrogen is the nutrient grass uses most and the one that does damage when overapplied. The bag tells you what share of its weight is nitrogen. So:

lb of product per 1,000 sq ft = lb of N wanted ÷ (N% ÷ 100)

Then multiply by your area in thousands of square feet. A handy shortcut for 1 lb N: divide 100 by the first number on the bag. For a 25-0-5 fertilizer, 100 ÷ 25 = 4 lb per 1,000 sq ft.

Worked examples

How much nitrogen at a time

Purdue's guide to established cool-season lawns suggests up to 1 lb N per 1,000 sq ft in late August–September, then 0.5–0.75 lb about every three weeks from early October until the ground freezes or turf is dormant. Its yearly totals range from 1–3 lb N per 1,000 sq ft for low-maintenance lawns to 3–5 lb for high-maintenance irrigated lawns. Warm-season grasses are fed during their summer growth instead. See our fertilizer schedule guide for timing.

The calculator warns when you enter more than 1 lb N per 1,000 sq ft for a single application. Heavy single doses raise the risk of burn, surge growth and nutrient runoff.

Phosphate and potash

The second and third numbers are phosphate (P₂O₅) and potash (K₂O). Many lawn fertilizers now contain little or no phosphate, and some states restrict phosphorus on established lawns unless a soil test shows a need. A soil test from your state extension lab tells you whether your lawn needs either one.

Spreading it evenly

• Set your spreader using the bag's setting chart as a starting point, then check it. Virginia Cooperative Extension describes a method that uses a measured area and a weighed amount of product rather than trusting the dial.
• Spread half the amount in one direction and half crosswise to avoid stripes.
• Sweep granules off driveways, sidewalks and streets back onto the lawn so they don't wash into storm drains.
• Water in if the label says to.

Common questions

What do the three numbers on fertilizer mean?

They are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A 50 lb bag of 20-0-5 contains 10 lb of nitrogen, no phosphate and 2.5 lb of potash.

How many square feet does a bag of fertilizer cover?

Divide the bag weight by the pounds of product per 1,000 sq ft. A 50 lb bag of 20-0-0 applied at 1 lb N (5 lb product) per 1,000 sq ft covers 10,000 sq ft.