Lawn Fertilizer Calculator: How Much Fertilizer Do I Need?

How to use the fertilizer calculator

1. Measure the lawn. Count only the grass. Subtract the house, driveway, patio and planting beds.
2. Type the N-P-K from the bag, or tap a common analysis. The first number is the percent nitrogen, which drives the rate.
3. Pick your grass and program. The hint under the menu shows the yearly nitrogen range and the best timing for that grass.
4. Set the nitrogen for this feeding. Most extension guides work in pounds of actual nitrogen per 1,000 sq ft. One pound is a full feeding and half a pound is a light one.
5. Enter the bag weight to see how many bags to buy and how much lawn one bag covers.

If the bag uses a slow-release nitrogen source for more than half its N, turn on the switch. Purdue allows up to 2 lb of N per feeding in that case, and the calculator warns you if you go over the limit.

How much fertilizer do I need? The formula

The numbers on the bag are percentages by weight. Clemson Extension explains that a 100 lb bag of 16-4-8 holds 16 lb of nitrogen, 4 lb of phosphate and 8 lb of potash. To apply 1 lb of nitrogen per 1,000 sq ft with 16-4-8, divide 1 by 0.16: 6.25 lb of product per 1,000 sq ft.

Purdue’s guide works the same way. For a 5,500 sq ft lawn, a rate of 0.5 lb N per 1,000 sq ft and an 18-0-10 product: 0.5 ÷ 0.18 = 2.78 lb per 1,000 sq ft, and 2.78 × 5.5 = 15.3 lb for the lawn. The calculator gives the same answer.

Clemson’s table of straight nitrogen sources gives the same result in reverse. It takes about 5 lb of ammonium sulfate (21% N), 3 lb of ammonium sulfate with urea (34% N) or 2 lb of urea (45–47% N) to supply 1 lb of nitrogen per 1,000 sq ft.

How much nitrogen for cool-season lawns

Kentucky bluegrass, tall fescue, fine fescue and perennial ryegrass grow most in spring and fall. Purdue’s AY-22-W sets the yearly nitrogen by how much care and color you want:

All figures are pounds of actual N per 1,000 sq ft. Purdue recommends putting most of the year’s nitrogen down from late summer through autumn. Use less during the spring growth flush, preferably slow-release, and little or none in summer unless you water often. In general, no single application should go over 1 lb of N, and rates of 2 lb should be split into two feedings.

How much nitrogen for warm-season lawns

Bermuda, zoysia, St. Augustine, centipede and bahia grow in summer, so they are fed after spring green-up. Clemson Extension’s South Carolina schedules give these yearly totals:

Each feeding is ½ lb of N per 1,000 sq ft for a low-maintenance lawn or 1 lb for a high-maintenance one. Make the first nitrogen application once the grass has greened up and is growing. Centipede needs the least: Clemson warns that more than 2 lb of N a year often leads to “centipedegrass decline”, and that established centipede should not get extra phosphorus unless a soil test calls for it. For St. Augustine, a slow-release source in summer helps limit chinch bugs.

Get a soil test first

Nitrogen is set by the grass and how hard you push it. Phosphorus and potassium are different: both Purdue and Clemson say to apply them based on a soil test. If the test shows enough P and K, adding more does nothing for the lawn. Purdue also notes that nitrogen and phosphorus moving off lawns and other land has been linked to poor water quality, which is one more reason not to over-apply.

A soil test also reports pH and lime needs, which no fertilizer fixes. Your county extension office can tell you how to collect a sample and where to send it. Then use the calculator to turn the lab’s recommendation into pounds of product.

When to use more or less

• New lawns. Purdue says lawns in their first 12 to 18 months, whether seeded or sodded, may need up to 50% more nitrogen. If you are starting one, see the grass seed calculator or the sod calculator.
• Mature lawns fertilized for more than 10 years have built up organic nitrogen, so they need less.
• Shade. Cut the rate by half, and keep shaded turf under about 2 lb of N a year.
• Clippings bagged. Removing clippings may raise the need by 25 to 50%. Returning them recycles nutrients.
• Never fertilize dormant, drought-stressed or frozen turf. Sweep granules off sidewalks and driveways back onto the grass.

Worked example

A 5,000 sq ft tall fescue lawn gets its main fall feeding of 1 lb N per 1,000 sq ft from a 50 lb bag of 16-4-8. Per 1,000 sq ft that is 6.25 lb, so the lawn takes 31.25 lb, which is one bag with about 19 lb left over. One bag covers 8,000 sq ft at this rate. The feeding also puts down 1.25 lb of phosphate and 2.5 lb of potash. If your soil test says P is already high, a 15-0-15 or a straight nitrogen source would be a better choice.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Product per 1,000 sq ft = target lb of nitrogen ÷ (N% ÷ 100). Total = that × area ÷ 1,000. Bags are rounded up. This is the method in Purdue AY-22-W and Clemson HGIC 1201.
• Phosphate and potash applied = product weight × the P and K percentages on the bag (as P₂O₅ and K₂O, the way labels list them).
• The warning above 1 lb N per feeding (2 lb when over half the N is slow-release) follows Purdue AY-22-W Table 2, footnote b.
• Annual programs: cool-season from Purdue AY-22-W Table 2; warm-season from Clemson’s South Carolina schedules. Other states’ extension services may differ, and a soil test overrides all of them for P and K.

Sources

1. Purdue University Extension, Fertilizing Established Cool-season Lawns (AY-22-W). Table 2 annual N by maintenance level and season; ≤ 1 lb N per application; worked example; new, shaded and clippings-removed lawns.
2. Clemson Cooperative Extension, Home & Garden Information Center, Fertilizing Lawns (HGIC 1201). N-P-K labels; pounds of product per 1 lb N (Table 4); South Carolina schedules and yearly N by grass; centipede ≤ 2 lb N a year.

Last reviewed October 26, 2026. Product figures vary by brand; the label on what you buy wins.