What to measure
How to use the fertilizer calculator
Measure the lawn as a rectangle or circle and type the three numbers from the bag (N-P-K). Set how much nitrogen you want per 1,000 sq ft: 1 lb is the usual ceiling for one application of quick-release fertilizer. Set the bag size you plan to buy to see how many bags and how much lawn one bag covers.
The formula
Pounds of fertilizer = area (sq ft) ÷ 1,000 × lb of nitrogen per 1,000 sq ft ÷ (N% ÷ 100)
N% is the first number on the bag. Phosphate and potash applied = pounds of fertilizer × their percentages.
Worked example: 50 × 40 ft lawn, 32-0-4, 1 lb of nitrogen
- 50 × 40 = 2,000 sq ft
- 1 ÷ 0.32 = 3.13 lb of fertilizer per 1,000 sq ft
- 2,000 ÷ 1,000 × 3.13 = 6.3 lb of fertilizer → 1 bag of 12.5 lb
- One 12.5 lb bag covers 12.5 ÷ 3.13 × 1,000 = 4,000 sq ft
- It puts down 2 lb of nitrogen and 0.25 lb of potash
Pounds per 1,000 sq ft by nitrogen percentage
For 1 lb of nitrogen per 1,000 sq ft. For 0.5 lb halve these; for 2 lb (slow-release only) double them.
| N on the bag | lb of fertilizer per 1,000 sq ft |
|---|---|
| 10% (10-10-10) | 10 |
| 16% | 6.25 |
| 20% | 5 |
| 24% | 4.17 |
| 29% | 3.45 |
| 32% | 3.13 |
| 46% (urea) | 2.17 |
Fertilizer by lawn size
At 1 lb of nitrogen per 1,000 sq ft.
| Lawn area | 32% N fertilizer | 10-10-10 |
|---|---|---|
| 1,000 sq ft | 3.1 lb | 10 lb |
| 2,500 sq ft | 7.8 lb | 25 lb |
| 5,000 sq ft | 15.6 lb | 50 lb |
| 10,000 sq ft | 31.3 lb | 100 lb |
| 1 acre (43,560 sq ft) | 136.1 lb | 435.6 lb |
Common mistakes
- Reading the bag weight as the nitrogen. A 12.5 lb bag of 32-0-4 holds 4 lb of nitrogen, not 12.5.
- Putting down too much quick-release nitrogen. More than 1 lb per 1,000 sq ft at once can burn the grass and washes off as runoff. Split it into two applications instead.
- Adding phosphate the lawn doesn’t need. Many lawns have enough. A soil test tells you whether to pick a fertilizer with a middle number of 0.
- Measuring the whole lot. Subtract the house, driveway and beds; only the turf you’ll fertilize counts.
- Spreading in one pass. Set the spreader to half the rate and go over the lawn twice, the second time at right angles, for even cover.
Frequently asked questions
How much fertilizer per 1,000 square feet?
Divide the nitrogen you want by the nitrogen percentage on the bag. For 1 lb of nitrogen per 1,000 sq ft, that's 5 lb of a 20% fertilizer, 3.13 lb of a 32% fertilizer or 10 lb of 10-10-10.
What do the three numbers on a fertilizer bag mean?
They're the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O), always in that order. A 50 lb bag of 20-5-10 holds 10 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash.
How much nitrogen should I put on my lawn at once?
With quick-release (soluble) fertilizer, no more than 1 lb of nitrogen per 1,000 sq ft in a single application. Slow-release products, such as natural organics and controlled-release fertilizers, can be applied at higher rates (Rutgers fact sheet FS839).
How much 10-10-10 do I need for 1,000 square feet?
10 lb gives 1 lb of nitrogen per 1,000 sq ft. It also puts down 1 lb of phosphate and 1 lb of potash.
How much fertilizer for an acre?
An acre is 43,560 sq ft. At 1 lb of nitrogen per 1,000 sq ft, that's 136.1 lb of a 32% fertilizer (11 bags of 12.5 lb) or 435.6 lb of 10-10-10.
How much fertilizer for 5,000 square feet?
At 1 lb of nitrogen per 1,000 sq ft, 15.6 lb of a 32% fertilizer, 25 lb of a 20% fertilizer or 50 lb of 10-10-10.
Why does the calculator use nitrogen and not the whole bag weight?
Lawn recommendations are written in pounds of nitrogen per 1,000 sq ft, because nitrogen is what the grass needs most. The bag weight changes with the analysis, so the calculator works backwards from the nitrogen.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.