Lawn Fertilizer Calculator
Enter your lawn size, the nitrogen rate and the fertilizer's N-P-K to get how much product to spread and how many bags to buy.
How to use the lawn fertilizer calculator
- In Lawn area, type the size of the lawn and choose ft², m² or acre from the unit menu.
- In Fertilizer grade (N-P-K), pick a grade from the list, or choose Other (type the grade). If you choose that option, type the three numbers from your bag into Grade (N-P-K), written as % N – % P₂O₅ – % K₂O.
- In Nitrogen rate, type the rate from the product label and choose lb N/1,000 ft² or g N/m². The label sets the rate. Many lawn labels give 1 lb N per 1,000 ft² per application, which is about 4.9 g N/m².
- In Bag size, type the weight of one bag and choose lb or kg.
- Read the result rows: Fertilizer to spread, Total fertilizer for the lawn, Bags needed and Area one bag covers. Phosphorus applied and Potassium applied appear only when the grade contains phosphate or potash.
- Read any warning that appears, then open How it was calculated to check each step against your own numbers.
The example buttons above the form fill in sample values, such as 5,000 ft² with 29-0-4 or 10,000 ft² with 10-10-10. Copy result copies the results, and Copy link copies a page address that reopens the same inputs. Print / save as PDF prints the inputs, results, calculation steps and date. Reset restores the default values.
How much fertilizer do I need for my lawn?
Divide the nitrogen rate by the nitrogen share of your product to get pounds of product per 1,000 ft², then multiply by the lawn area in thousands of square feet. Round the number of bags up to the next whole bag, because you cannot buy part of one.
The product needed per 1,000 ft² is the nitrogen rate divided by the nitrogen fraction of the product: product = N rate ÷ (N % ÷ 100). For a 29-0-4 product and a rate of 1 lb N per 1,000 ft², that is 1 ÷ 0.29 = 3.448 lb per 1,000 ft².
For a 5,000 ft² lawn, multiply by 5: 17.24 lb of product. With 20 lb bags, 17.24 ÷ 20 is 0.86, which rounds up to one bag. One bag covers 20 ÷ 3.448 × 1,000 = 5,800 ft², which is the figure the tool shows as the area one bag covers.
How do you calculate pounds of nitrogen per 1,000 square feet?
The nitrogen rate is the pounds of nitrogen (N) you apply to each 1,000 square feet, and the label usually states it. To find how much nitrogen a product holds, multiply its weight by the nitrogen percentage, which is the first number in the N-P-K grade.
The first number is the percent of nitrogen by weight. A 29-0-4 product is 29% nitrogen, so 100 lb of product holds 29 lb of N, and a 20 lb bag holds 20 × 0.29 = 5.8 lb of N. The second and third numbers give phosphorus and potassium as P₂O₅ and K₂O. The tool converts them to elemental phosphorus with P = P₂O₅ × 0.4364 and potassium with K = K₂O × 0.8302.
Metric labels use grams of nitrogen per square meter. Because 1 lb is 453.59 g and 1,000 ft² is 92.903 m², 1 lb N per 1,000 ft² equals 4.882 g N/m². The tool converts units before it calculates, so you can enter either system.
Sources
- UConn Soil Nutrient Analysis Laboratory, “Suggested Fertilizer Practices for Lawns” — no more than 1 lb water-soluble N per 1,000 ft² at a time; product = 100 ÷ % N
- CIAAW standard atomic weights — oxide to element factors
- NIST Handbook 44, Appendix C and NIST SP 811 — exact unit definitions (1 acre = 43,560 ft² = 4,046.8564224 m², 1 lb = 0.45359237 kg, 1 US gal = 231 in³)
Worked examples
| Input | Result |
|---|---|
| 5000 ft², 1 lb N/1,000 ft², 29-0-4 | 3.448 lb per 1,000 ft² (16.84 g/m²); 1 × 20 lb bags |
| 10000 ft², 1 lb N/1,000 ft², 10-10-10 | 10 lb per 1,000 ft² (48.82 g/m²); 3 × 40 lb bags |
| 300 m², 5 g N/m², 24-0-11 | 4.267 lb per 1,000 ft² (20.83 g/m²); 1 × 15 kg bags |
| 0.5 acre, 1 lb N/1,000 ft², 32-0-4 | 3.125 lb per 1,000 ft² (15.26 g/m²); 2 × 50 lb bags |
Calculated by the tool above with these values.
Frequently asked questions
Is the lawn fertilizer calculator free, and is anything I type uploaded?
Yes, it is free. It runs entirely in your browser and needs no sign-up. Nothing you type is uploaded.
Can I use the results in commercial work such as quotes and bids?
Yes. Results, steps and printouts may be used in any work, including nutrient plans, spray records, quotes and invoices. No attribution is needed.
Does the calculator tell me which rate to use?
No. The calculator does not recommend rates. The product label, a soil test, an agronomist or the extension service gives the actual recommendation. The page notes that results are planning estimates.
What does the high nitrogen rate warning mean?
The tool shows “That is a high nitrogen rate for one lawn application. Check the unit and the rate on the product label.” Check the unit first, because a rate entered in the wrong unit can look far too high.
Can I enter my lawn in square meters or acres?
Yes. The lawn area accepts ft², m² or acre, and the tool converts units before calculating. Results appear in both unit systems where useful.
Why does the number of bags round up?
You cannot buy part of a bag, so the tool rounds up to the next whole bag. For example, a total of 17.24 lb with 20 lb bags needs one bag.
Related tools
- Fertilizer calculator: use it when you need fertilizer amounts outside this lawn-specific form.
- Lime calculator: use it when your soil needs lime and you want an amount for that.
- Square foot to acre converter: use it when your area is in square feet and you need acres.
- Hectare to acre converter: use it when your lawn or field is measured in hectares.
- Acreage calculator: use it when you need an area in acres.
- Acre per hour calculator: use it when you want to estimate how many acres you can cover in an hour.
- Seeding rate calculator: use it when you are planning seed rather than fertilizer.
- All calculators: use it when you want to browse every tool on the site.