How this calculator works
The first number on the bag is the percentage of nitrogen by weight. Divide your nitrogen target by that percentage (as a decimal) to get pounds of product per 1,000 sq ft, then scale to your lawn area.
Prefer to follow the bag? Switch to “the rate printed on the bag” and enter it as pounds per area — for example 12.5 lb per 5,000 sq ft. The calculator converts it to your lawn size, counts the bags and shows how much nitrogen that really applies.
Product per 1,000 sq ft = N target ÷ (N% ÷ 100)
Total product = Product per 1,000 × Area ÷ 1000
Example: 1 ÷ 0.32 = 3.125 lb of 32-0-4 per 1,000 sq ft
Assumptions and tips
- A soil test is the best guide to whether your lawn needs phosphorus or potassium.
- Spreader settings on the bag are for the manufacturer’s recommended rate — adjust if your target differs.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Rate to use | My nitrogen target (lb N per 1,000 sq ft) |
| Lawn area | 5,000 sq ft |
| Nitrogen target | 1 lb N/1,000 sq ft |
| N (first number on bag) | 32 % |
| P (second number) | 0 % |
| K (third number) | 4 % |
| Bag weight | 15 lb |
| Result | Value |
|---|---|
| Fertilizer to apply | 15.6 lb of product |
| Per 1,000 sq ft | 3.13 lb of product |
| 15 lb bags | 2 bags |
| Nitrogen applied | 5 lb N |
| Phosphate applied (P₂O₅) | 0 lb |
| Potash applied (K₂O) | 0.63 lb |
Step by step:
- Product per 1,000 sq ft = 1 ÷ 0.32 = 3.125 lb
- Total = 3.125 × 5,000 ÷ 1000 = 15.63 lb
- Bags = 15.63 ÷ 15 → 2
Pounds of product for 1 lb of nitrogen per 1,000 sq ft
| Fertilizer analysis | Per 1,000 sq ft |
|---|---|
| 10-10-10 | 10 lb of product |
| 16-4-8 | 6.25 lb of product |
| 24-0-11 | 4.17 lb of product |
| 29-0-4 | 3.45 lb of product |
| 32-0-4 | 3.13 lb of product |
| 46-0-0 (urea) | 2.17 lb of product |
Common mistakes to avoid
- Reading the bag weight as nitrogen — a 32-0-4 bag is only 32% nitrogen.
- Spreading before heavy rain, which washes fertilizer away.
Frequently asked questions
My lawn is 900 sq ft and the bag says 12.5 lb per 5,000 sq ft. How much do I use?
12.5 ÷ 5,000 × 900 = 2.25 lb of product.
How much 32-0-4 per 1,000 sq ft?
For 1 lb of nitrogen, about 3.1 lb of product per 1,000 sq ft.
What do the three numbers mean?
They are the percentages of nitrogen (N), phosphate (P₂O₅) and potash (K₂O) by weight.