Fertilizer Calculator
An 18-0-10 bag at 0.5 lb of nitrogen per 1,000 ft² puts 15.2778 lb of product on 5,500 ft². That weight carries 2.7500 lb of nitrogen and 1.5278 lb of potash.
Figures checked . Area opens at 5,500 ft², the rate at 0.5, and the bag at 18-0-10, which is the Purdue example.
A fertilizer calculator and lawn fertilizer calculator. The nitrogen calculator step is the rate you want divided by the nitrogen fraction on the bag, then scaled to the lawn.
Seed instead of fertilizer: grass seed calculator. A compost layer: compost calculator.
Pounds of lawn fertilizer
Opening numbers are Purdue AY-22-W’s example: 5,500 ft², 0.5 lb of nitrogen per 1,000 ft², and an 18-0-10 bag. Iowa State’s lawn range is 0.50 to 1 lb of actual nitrogen per 1,000 ft². Type the rate you were given. Phosphate and potash here are only what that product weight contains. A soil test sets whether you need them.
Results
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The division the three extensions print
Purdue Extension AY-22-W, Fertilizing Established Cool-season Lawns, walks through an 18-0-10 product: convert 18 percent to 0.18, divide the 0.5 pound target by 0.18, divide 5,500 by 1,000, and multiply. The older turf.purdue.edu article URL now returns the university homepage, so the steps used here are the ones in AY-22-W. The authors are Cale A. Bigelow, Jim J. Camberato, and Aaron J. Patton.
Rutgers Cooperative Extension FS839, How to Calculate the Amount of Fertilizer Needed for Your Lawn, by Joseph R. Heckman and Susan Lance-Scibilia, uses the same division. One pound of nitrogen and a 20 percent bag is 5 pounds of product per 1,000 square feet. On 25,000 square feet that is 125 pounds. Table 1 below is their published one-decimal chart for a 1 pound nitrogen target. This calculator divides exactly and shows four decimals, so 3 percent is 33.3333 pounds here and 33.3 in the table.
Iowa State University Extension, How much fertilizer should I apply to my lawn?, says rates range from 0.50 to 1 pound of actual nitrogen per 1,000 square feet. Its worked line is 0.75 ÷ 0.20 = 3.75 pounds of 20-5-10 on 1,000 square feet. The sentence that says “0.75 lbs/sq.ft.” is the page’s own wording; the arithmetic it shows is per 1,000 square feet.
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Rutgers Table 1, pounds of product for 1 lb of nitrogen
| % nitrogen on the bag | Pounds per 1,000 sq ft |
|---|---|
| 1 | 100.0 |
| 2 | 50.0 |
| 3 | 33.3 |
| 4 | 25.0 |
| 5 | 20.0 |
| 6 | 16.7 |
| 7 | 14.3 |
| 8 | 12.5 |
| 9 | 11.1 |
| 10 | 10.0 |
| 11 | 9.1 |
| 12 | 8.3 |
| 13 | 7.7 |
| 14 | 7.1 |
| 15 | 6.7 |
| 16 | 6.3 |
| 17 | 5.9 |
| 18 | 5.6 |
| 19 | 5.3 |
| 20 | 5.0 |
| 21 | 4.8 |
| 22 | 4.5 |
| 23 | 4.3 |
| 24 | 4.2 |
| 25 | 4.0 |
| 26 | 3.8 |
| 27 | 3.7 |
| 28 | 3.6 |
| 29 | 3.4 |
| 30 | 3.3 |
| 31 | 3.2 |
| 32 | 3.1 |
| 33 | 3.0 |
| 34 | 2.9 |
| 35 | 2.9 |
| 36 | 2.8 |
| 37 | 2.7 |
| 38 | 2.6 |
| 39 | 2.6 |
| 40 | 2.5 |
| 41 | 2.4 |
| 42 | 2.4 |
| 43 | 2.3 |
| 44 | 2.3 |
| 45 | 2.2 |
| 46 | 2.2 |
Worked products
- Purdue inputs, unrounded: 2.7778 lb per 1,000 ft² and 15.2778 lb on the lawn. Potash is 1.5278 lb. Purdue’s rounded path prints 2.78 and 15.29.
- Iowa inputs on 1,000 ft²: 3.7500 lb of 20-5-10, with 0.1875 lb of phosphate and 0.3750 lb of potash in that weight.
- Rutgers 20 percent at 1 lb of nitrogen on 25,000 ft²: 125.0000 lb.
Fertilizer questions
How many pounds is Purdue’s 5,500 square foot example?
Leave area at 5500 ft², nitrogen wanted at 0.5, N at 18, P at 0, and K at 10. Product per 1,000 ft² reads 2.7778 lb. Product for the lawn reads 15.2778 lb. Nitrogen in that weight reads 2.7500 lb. Phosphate in that weight reads 0.0000 lb. Potash in that weight reads 1.5278 lb. Product, kilograms reads 6.9299 kg.
Why does Purdue print 15.29 pounds for that same example?
AY-22-W rounds 0.5 ÷ 0.18 to 2.78 pounds per 1,000 ft², then multiplies 2.78 × 5.5 = 15.29. This panel keeps the division unrounded: 0.5 ÷ 0.18 = 2.777…, times 5.5 is 15.277…, shown to four decimals as 15.2778. Both paths start from the same inputs. The four-decimal line is the one this page uses.
What does Iowa State’s 20-5-10 example return on 1,000 square feet?
Set area to 1000, nitrogen wanted to 0.75, N to 20, P to 5, and K to 10. Product per 1,000 ft² reads 3.7500 lb. Product for the lawn reads 3.7500 lb. Nitrogen reads 0.7500 lb. Phosphate reads 0.1875 lb. Potash reads 0.3750 lb. The Iowa page states 0.75 ÷ 0.20 = 3.75 pounds of 20-5-10 for 0.75 pounds of nitrogen on 1,000 square feet.
What is Rutgers example of 20 percent nitrogen on 25,000 square feet?
Set area to 25000, nitrogen wanted to 1, and N to 20. P and K can stay at 0. Product per 1,000 ft² reads 5.0000 lb. Product for the lawn reads 125.0000 lb. That is 1 ÷ 0.20 = 5 pounds per 1,000, times 25, which is the 125 pounds in FS839 Example 1.
Does the phosphate line tell me to apply phosphorus?
No. Purdue says the middle number is phosphate and the last number is potash, and it says phosphorus and potassium needs come from a soil test. The two lines multiply the product weight by those percents so you can see what that nitrogen-based purchase contains. They are not an application rate.
Which nitrogen rate should the box start from?
The box opens at 0.5 because that is the AY-22-W worked rate. Iowa State says lawn rates range from 0.50 to 1 pound of actual nitrogen per 1,000 square feet, depending on timing and how dense a lawn you want. Rutgers says a typical soluble inorganic application is one pound of nitrogen per 1,000 square feet, and that slow-release products may be used at higher rates. Type the rate from the source you are following.