The P and K numbers are oxides: phosphate is about 44% elemental phosphorus and potash is about 83% elemental potassium.
How the fertilizer calculator works
FertilizerCalc reads the guaranteed analysis on the bag as percentages by weight: nitrogen (N), phosphate (P2O5) and potash (K2O). The pounds of product per 1,000 square feet are your nutrient target divided by that nutrient's percentage as a decimal. That rate times your area in thousands of square feet is the total, rounded up to whole bags; the rate times 43.56 is the amount per acre. The same rate also tells you how much of the other two nutrients you are applying, and how far one bag goes.
Yearly nitrogen by grass type
| Grass | lb N per 1,000 sq ft per year | Type |
|---|---|---|
| Tall fescue | 2–4 | cool-season |
| Kentucky bluegrass | 3–5 | cool-season |
| Perennial ryegrass | 2–4 | cool-season |
| Fine fescue | 1–2 | cool-season |
| Bermudagrass | 3–5 | warm-season |
| Zoysiagrass | 2–3 | warm-season |
| St. Augustinegrass | 2–4 | warm-season |
| Centipedegrass | 1–2 | warm-season |
| Buffalograss | 1–2 | warm-season |
Pounds of product for 1 lb of nitrogen per 1,000 sq ft
| Fertilizer | Per 1,000 sq ft | Per acre |
|---|---|---|
| 46% N (e.g. 46-0-0) | 2.2 lb | 95 lb |
| 32% N (e.g. 32-0-4) | 3.1 lb | 136 lb |
| 29% N (e.g. 29-0-4) | 3.4 lb | 150 lb |
| 24% N (e.g. 24-0-4) | 4.2 lb | 182 lb |
| 16% N (e.g. 16-0-4) | 6.3 lb | 272 lb |
| 10% N (e.g. 10-0-10) | 10 lb | 436 lb |
Ranges are typical US university extension guidance; your local extension office and a soil test give the best numbers for your lawn. Follow the product label and local rules on phosphorus and timing. Results are estimates.
Frequently asked questions
What is the best N-P-K ratio for lawns?
Established lawns mostly need nitrogen, so high-N products such as 24-0-11 or 32-0-4 are common. Add phosphorus only if a soil test or new seeding calls for it.
What does 10-10-10 fertilizer mean?
It contains 10% nitrogen, 10% phosphate and 10% potash by weight, so a 40 lb bag has 4 lb of each.
Why is the middle number zero on many lawn fertilizers?
Many soils already have enough phosphorus, and several US states restrict phosphorus on lawns to protect lakes and streams.
Is FertilizerCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.