Fertilizer calculator for lawns and gardens

How to read N-P-K on a fertilizer bag

By law every fertilizer label lists three numbers, the guaranteed analysis. They are percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O).¹ A 40-lb bag of 16-4-8 therefore holds 40 × 0.16 = 6.4 lb of nitrogen, 1.6 lb of phosphate and 3.2 lb of potash.⁹ The rest is carrier and other ingredients.

• Nitrogen drives green color and leaf growth. It has the most effect on an established lawn, and there is no reliable soil test for how much a lawn needs, so lawn rates are set as pounds of N.¹
• Phosphorus and potassium matter most for root growth and new lawns. Apply them when a soil test shows a need.¹ Florida's lawn rule allows at most 0.25 lb P₂O₅ per 1,000 sq ft per application and 0.5 lb a year, and none beyond that unless a soil test shows a deficiency.¹¹
• Quick- vs slow-release. Urea and ammonium sulfate are quick-release; coated urea, methylene urea and natural organics release slowly. The label's guaranteed analysis lists which sources are in the bag.¹

How the calculator does the math

Lawn rates are given in pounds of nitrogen per 1,000 sq ft. To turn that into pounds of product:

lb of product = (target lb N per 1,000 sq ft ÷ (N% ÷ 100)) × (lawn area ÷ 1,000)

This is the method in Purdue's lawn fertilizer publication.¹ For a liquid, the label also gives the product's weight per gallon; pounds of product ÷ lb per gallon = gallons, and × 128 = fluid ounces. The garden mode works the same way per 100 sq ft.

Worked example

A 5,000 sq ft lawn, a target of 1 lb N per 1,000 sq ft and a 24-0-6 lawn fertilizer:

1. 24% nitrogen = 0.24.
2. 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft.
3. 5,000 ÷ 1,000 = 5.
4. 4.17 × 5 = 20.8 lb of product. It also supplies 20.8 × 0.06 = 1.25 lb of potash and no phosphorus.

A 40-lb bag covers about 9,600 sq ft at this rate, so one bag lasts almost two applications. Purdue's own example: 5,500 sq ft at 0.5 lb N with 18-0-10 needs 15.3 lb.¹

How much nitrogen per application, and per year

• Per application: normally 0.5–1.0 lb N per 1,000 sq ft. Purdue advises no more than 1 lb of water-soluble N in one application; products with more than 50% slow-release N can go up to 2 lb.¹ Florida's rule allows at most 0.7 lb of quick-release N per application.¹¹
• Per year, cool-season: Kentucky bluegrass and perennial ryegrass 2–4 lb, tall fescue 2–3 lb, fine fescues 1–2 lb (Penn State).³ Purdue puts fine fescue at 0.5–2 lb and warns against going over 2.²
• Per year, warm-season: bermudagrass 2–5 lb, zoysiagrass 2–3, St. Augustinegrass 2–5, centipedegrass 1–2 (UGA).⁴ Clemson's South Carolina schedules use 3–5 for bermuda and zoysia, 1–4 for St. Augustine and bahiagrass, and 1–2 for centipede (½–2 on the coast).⁶ UF/IFAS gives Florida ranges by region, from 1–3 lb for bahiagrass in North Florida up to 5–7 lb for bermudagrass in South Florida.¹¹

When to fertilize: cool-season vs warm-season grass

Cool-season lawns (bluegrass, ryegrass, fescues) benefit most when most of the year's nitrogen goes on from late summer through fall. Feed lightly in spring with slow-release N, and little or none in summer. From early October to early December, Purdue recommends water-soluble N at 0.5–0.75 lb about every three weeks.¹ Fine fescues need less, mostly in September–October.² The calendar above shifts Purdue's Indiana windows by the difference between your median frost dates and Indianapolis's (last frost April 20, first frost October 21, NOAA 1991–2020). That shift is our approximation. Purdue's dates are for Indiana.

Warm-season lawns (bermuda, zoysia, St. Augustine, centipede, bahia) are fed while actively growing. Texas A&M says to start after the lawn has greened up and needed mowing two or three times, usually about 6 weeks after the average last spring frost. The last application should go on no later than 6 weeks before the expected first fall frost.⁸¹⁰ The Southeast uses fixed dates: UGA says no nitrogen after September 1;⁵ Clemson says not after August 15, or September 1 on the coast, and not before full green-up.⁷

Vegetable gardens and flower beds

Garden rates are given per 100 sq ft, and the extension services differ a little. UF/IFAS suggests 2 lb of 10-10-10 per 100 sq ft broadcast before planting, or 4 oz per 10 ft of row banded beside the row. During the season it suggests 2–3 more applications at half the banded rate, placed just beyond the outer leaves.¹² Texas A&M suggests 2–3 lb of 10-20-10 per 100 sq ft for untested soil, 2 lb on sand and 3 lb on clay. It adds a side-dressing of about ½ cup of garden fertilizer per 10 ft of row.¹³ For annual flowers, Illinois Extension suggests 1–2 lb of a general-purpose fertilizer per 100 sq ft before planting. If plants need more, add ¼–½ of that rate 6–8 weeks later.¹⁵

Get a soil test first

A soil test from your state's land-grant university lab is the only way to know whether you need phosphorus, potassium or lime, and in what amounts. Purdue suggests testing sandy soils every year and clay soils every two to three years.¹ If the report gives a rate in pounds of N, P₂O₅ or K₂O, enter it in the calculator as a custom rate.

These calculations follow published land-grant university Extension guidance, but they are general guidance, not a prescription for your lawn. Always read and follow the product label, sweep granules off pavement back onto the lawn, and don't fertilize dormant, drought-stressed or frozen turf.¹ US Grow Zones is independent and not affiliated with the USDA or any university. Frost dates come from NOAA; zones come from the PRISM/OSU ZIP listings.