Lawn fertilizer calculator

The three numbers on a fertilizer bag, called the N-P-K analysis, are the percentages by weight of nitrogen (N), phosphate (P) and potash (K). Lawn rates are based on the nitrogen, not on the weight of the product: divide the nitrogen you want by the nitrogen percentage. 1 lb of N from a 24-0-6 product takes about 4.2 lb of fertilizer per 1,000 sq ft.

The fertilizer formula

Lawn recommendations from extension services are written as pounds of nitrogen per 1,000 sq ft, usually 0.5–1 lb per application; University of Minnesota Extension advises no more than 1 lb per application. The bag, on the other hand, is mostly not nitrogen: a 24-0-6 product is 24% nitrogen by weight. So you divide.

lb of product per 1,000 sq ft = lb of N wanted ÷ (N% ÷ 100)
total product = that × (lawn area ÷ 1,000)
example: 1 ÷ 0.24 = 4.17 lb per 1,000 sq ft
per acre: lb per 1,000 sq ft × 43.56

Two of these are worked through in full: how much 24-0-6 fertilizer for 5,000 sq ft and how much 10-10-10 fertilizer per 1,000 sq ft.

Slow-release nitrogen and bigger feedings

The guaranteed analysis on the back of the bag splits the nitrogen into forms. Anything listed as "slowly available", water-insoluble or coated (sulfur-coated or polymer-coated urea, methylene urea) feeds over weeks; the rest is quick-release and hits the grass within days. Purdue Extension advises no more than 1 lb of water-soluble N per 1,000 sq ft in any single application, and allows up to 2 lb of total N when more than half of it is slow-release. This calculator warns above 1 lb for mostly quick-release products and, more conservatively than Purdue, above 1.5 lb for products that are at least half slow-release.

To find the slow-release share, divide the "slowly available" line by total N. A 24% N bag that lists 12% slowly available nitrogen is 50% slow-release.

Solving for phosphate or potash

A soil test sometimes asks for phosphate or potash rather than nitrogen, and new seedings are often given a starter feeding measured in phosphate. Pick Phosphate or Potash under More options and enter the amount wanted: the product rate is that amount divided by the middle (or last) number, and the calculator shows how much nitrogen rides along with it so you can check it against the limits above.

product = P₂O₅ wanted ÷ (P% ÷ 100)
nitrogen that comes along = product × (N% ÷ 100)
example: 1 lb P₂O₅ from an 18-24-12 starter = 4.17 lb of product, carrying 0.75 lb N

What the P and K numbers mean

The second and third numbers are phosphate (P₂O₅) and potash (K₂O) as percentages of the bag weight. The working under the result shows how much of each you are putting down, which is useful if a soil test says you have plenty of phosphorus. University of Minnesota Extension notes that phosphorus is high in many Midwestern soils and that established lawns show little response to it, so it recommends phosphorus only when a soil test shows a need.

Several states restrict phosphorus on lawns. A 2012 Connecticut legislative research summary lists Illinois, Maine, Maryland, Michigan, Minnesota, New Jersey, New York, Vermont, Virginia, Washington and Wisconsin; most allow it when a soil test shows a deficiency or you are establishing new turf, and Illinois’ rule applies to hired applicators. Rules change, so check your state’s agriculture department before buying a bag with a middle number above zero.

How much nitrogen in a year?

Annual totals depend on the grass and how the lawn is kept. NC State Extension’s Gardener Handbook gives roughly 1.5–3.5 lb N per 1,000 sq ft a year for cool-season lawns and 0.75–4 lb for warm-season lawns, depending on species; University of Minnesota Extension ranges from 0.5–1 lb a year for unwatered, low-maintenance lawns to 2.5–4 lb for watered, high-maintenance ones. Pick A whole season under More options, enter the yearly total and the number of feedings, and the calculator splits it into feedings of equal size and tells you how many bags to buy for the year.

Practical guidance

• Get a soil test before a big fertilizer program; it tells you whether you need P, K or lime at all. The lawn lime calculator turns a lime recommendation into bags, and the soil sulfur calculator handles the opposite problem.
• Don’t fertilize dormant or drought-stressed lawns, and keep granules off driveways and sidewalks where they wash into storm drains.
• Cool-season lawns usually get most of their nitrogen in fall; warm-season lawns in late spring and summer. Your extension office publishes a local calendar.
• Weed-and-feed and crabgrass-preventer products are dosed by the herbicide, not the nitrogen — use the pre-emergent calculator and the label rate for those.
• Water in after applying unless the label says otherwise.

Walk through a full example in how to read a fertilizer bag.

N-P-K, slow-release nitrogen and the other terms on the bag are also in the glossary of yard and garden words.