Lawn fertilizer calculator

How the fertilizer calculator works

Lawn feeding rates are given in pounds of nitrogen (N) per 1,000 square feet, not pounds of product, because every fertilizer has a different strength. The first number on the bag is the percentage of nitrogen by weight. A 29-0-4 fertilizer is 29 percent nitrogen, so 1 pound of nitrogen takes 1 ÷ 0.29 = 3.45 pounds of that product.

The calculator multiplies that by the size of your lawn in thousands of square feet. It also shows how many pounds of phosphate (the middle number) and potash (the last number) you are putting down, since many lawns need little or no phosphorus and some states restrict it.

• Product per 1,000 sq ft = lb of N wanted ÷ (N% ÷ 100).
• Total product = product per 1,000 sq ft × (area ÷ 1,000).
• Bags = total product ÷ bag weight, rounded up.

Worked example

A 5,000 square foot lawn, fed at 1 pound of nitrogen per 1,000 square feet with a 29-0-4 fertilizer: 1 ÷ 0.29 = 3.45 pounds per 1,000 square feet, times 5 = 17.2 pounds of product. A typical 15 pound bag covers about 4,350 square feet at that rate, so you need 2 bags. You are applying 5 pounds of nitrogen, 0 of phosphate and 0.7 of potash.

How much nitrogen a lawn needs per year

Most lawns do well on 2 to 4 pounds of nitrogen per 1,000 square feet per year, split into feedings of no more than 1 pound each. More than that at once can burn the grass and washes into storm drains.

Cool-season grasses (Kentucky bluegrass, fescue, ryegrass) are fed mostly in fall, with the biggest feeding in September and a lighter one in late spring. Warm-season grasses (Bermuda, zoysia, St. Augustine, centipede) are fed from late spring through summer while they grow actively. Centipede and older lawns that get grass clippings left on them need less, often 1 to 2 pounds a year.

• Cool-season lawns: 2 to 4 lb N per 1,000 sq ft per year, mainly September to November.
• Bermuda grass: 3 to 5 lb, May to August.
• Zoysia and St. Augustine: 2 to 3 lb, late spring and summer.
• Centipede grass: 1 to 2 lb, early summer.

Applying fertilizer evenly

Set the spreader to the rate on the bag, or a little lower, then go over the lawn twice at half rate, the second pass at a right angle to the first. That avoids the green and yellow stripes of a single pass. Fill and empty the spreader on a driveway or tarp, sweep any granules off hard surfaces back onto the grass, and water lightly afterward so the nitrogen moves into the soil. Skip feeding when the lawn is dormant from drought or heat.

Frequently asked questions