Formula
The three numbers are percents by weight: nitrogen, available phosphate expressed as P₂O₅, and soluble potash expressed as K₂O. Product per 1,000 square feet is the nutrient rate divided by that percent. Total product scales with the area. An acre is 43.56 times 1,000 square feet. A garden bed is the 1,000-square-foot rate divided by 10.
lb product / 1,000 = rate ÷ (percent ÷ 100)
total = that × area ÷ 1,000
per acre = per 1,000 × 43.56
elemental P = P₂O₅ × 0.4364
elemental K = K₂O × 0.8301
Quick-release nitrogen, such as urea, is available at once and can burn turf if the application is heavy. Slow-release nitrogen is spread over more weeks; the label says how much of the nitrogen is water-insoluble. Phosphorus and potassium are better chosen from a soil test than from a guess. A spreader’s setting is not this weight. Catch a known area and weigh it if the setting has to match the pounds.
Typical guidance, not a rate for your lawn: University of Arkansas Cooperative Extension (FSA-2114) says not to apply more than 1.0 pound of nitrogen per 1,000 square feet at once unless at least half of it is slow-release, and it lists yearly ranges by grass. Purdue Extension PPP-155 says the same 1 pound ceiling for one application on cool-season lawns, with about 1 to 4 pounds of nitrogen per 1,000 square feet for the year depending on how the lawn is kept. UC IPM describes about four applications of 0.5 to 1 pound. Your grass, your soil test, and the bag can all disagree with those ranges.
Worked examples
24-0-11, 1 lb N per 1,000 sq ft, 5,000 sq ft. 4.17 lb of product per 1,000, 20.83 lb total. A 50 lb bag at $40 is 0.42 bag and $16.67, $3.33 per pound of nitrogen. Potash delivered is 2.29 lb.
10-10-10, 1 lb N per 1,000, 2,500 sq ft. 10 lb of product per 1,000, 25 lb total, 2.5 lb each of N, P₂O₅, and K₂O.
46-0-0 urea, 0.75 lb N per 1,000, 1 acre. 1.63 lb of product per 1,000, 71.02 lb per acre.
Reference
Yearly nitrogen ranges quoted from extension guides. They are not the rate this calculator applies. The rate field is yours.
| Source | What it says about one application |
|---|---|
| Arkansas FSA-2114 | Not more than 1.0 lb N / 1,000 ft² unless half or more is slow-release |
| Purdue PPP-155 | Generally not more than 1 lb N / 1,000 ft² in one application on cool-season lawns |
| UC IPM Healthy Lawns | About 0.5 to 1 lb N / 1,000 ft², four times a year as a general split |
Questions
How much fertilizer per 1,000 sq ft?
Divide the pounds of nutrient you want by the percent of that nutrient on the bag. One pound of nitrogen from a 24-0-11 product is 1 ÷ 0.24, about 4.17 pounds of product per 1,000 square feet.
What does 10-10-10 mean?
Ten percent nitrogen, 10 percent available phosphate as P₂O₅, and 10 percent soluble potash as K₂O, by weight. The rest of the bag is carrier. Those are the guaranteed-analysis terms used on fertilizer labels.
How do I calculate pounds of nitrogen?
Multiply the pounds of product by the first number divided by 100. Twenty-five pounds of 10-10-10 delivers 2.5 pounds of nitrogen, and the same weight of phosphate and potash.
How many bags of fertilizer do I need?
Divide the total product weight by the bag weight, then round up if you have to buy whole bags. The 5,000 square foot example of 24-0-11 is 20.83 pounds, 0.42 of a 50 pound bag, so one bag covers it with product left over.
How much fertilizer per acre?
An acre is 43,560 square feet, 43.56 times 1,000. Urea at 46-0-0, supplying 0.75 pound of nitrogen per 1,000 square feet, is about 1.63 pounds of product per 1,000 and about 71.02 pounds per acre.
How often should I fertilize my lawn?
University extension guides treat about 1 pound of nitrogen per 1,000 square feet as a typical ceiling for one application of quick-release nitrogen, and they split the year’s nitrogen by grass type. Cool-season lawns are fed mainly in fall. Warm-season lawns are fed while they are growing. The bag and your local blackout dates decide the calendar.
Can I use too much fertilizer?
Yes. Extra nitrogen can burn turf, push soft growth, and wash off the lawn. Phosphorus is often restricted unless a soil test asks for it. Follow the rate on the label.
Sources
- University of Arkansas Cooperative Extension, Fertilizing Your Lawn, FSA-2114. Yearly nitrogen by grass, and the 1.0 pound single-application ceiling unless half the nitrogen is slow-release.
- Purdue Extension, A Simple Procedure for Fertilizing Cool-Season Lawns, PPP-155. About 1 to 4 pounds of nitrogen per 1,000 square feet per year, and generally not more than 1 pound in one application.
- University of California IPM, Healthy Lawns, fertilization amount and frequency. About 0.5 to 1 pound of actual nitrogen per 1,000 square feet per application.
- Association of American Plant Food Control Officials. The guaranteed analysis states total nitrogen, available phosphate (P₂O₅), and soluble potash (K₂O).
- Elemental conversions used here: P = P₂O₅ × 0.4364 and K = K₂O × 0.8301, from the atomic weights (P 30.974, K 39.098, P₂O₅ 141.94, K₂O 94.20). An acre is 43,560 square feet.