The fertilizer formula
Product per 1,000 sq ft = nutrient rate ÷ (nutrient % ÷ 100)
Total product = product per 1,000 sq ft × lawn area ÷ 1,000
The rate is the nutrient you want to apply, usually nitrogen. The percentage is the matching number on the bag.
Worked example
A 50 × 40 ft lawn (2,000 sq ft), a 16-4-8 fertilizer, and a target of 1 lb of nitrogen per 1,000 sq ft:
- 1 ÷ 0.16 = 6.25 lb of product per 1,000 sq ft
- 6.25 × 2 = 12.5 lb for the lawn
- That also puts down 12.5 × 0.04 = 0.5 lb of phosphate and 12.5 × 0.08 = 1 lb of potash.
That’s what the calculator shows with its regular-feeding preset. Your spreader’s setting chart, on the bag or from the spreader maker, turns pounds per 1,000 sq ft into a dial setting.
Reading N-P-K
| Number | Nutrient | What it does in a lawn |
|---|---|---|
| First (N) | Nitrogen | Leaf growth and green color; the main lawn nutrient |
| Second (P) | Phosphate, P₂O₅ | Root development, most useful when establishing new grass |
| Third (K) | Potash, K₂O | Stress, drought and wear tolerance |
A 40 lb bag of 24-0-6 holds 40 × 0.24 = 9.6 lb of nitrogen, no phosphate and 40 × 0.06 = 2.4 lb of potash. The rest of the bag is other nutrients and carrier.
Pounds of product for 1 lb of nitrogen
| Nitrogen % on the bag | Product per 1,000 sq ft | For 5,000 sq ft | For 10,000 sq ft |
|---|---|---|---|
| 10% | 10 lb | 50 lb | 100 lb |
| 16% | 6.25 lb | 31.3 lb | 62.5 lb |
| 20% | 5 lb | 25 lb | 50 lb |
| 24% | 4.17 lb | 20.8 lb | 41.7 lb |
| 29% | 3.45 lb | 17.2 lb | 34.5 lb |
| 32% | 3.13 lb | 15.6 lb | 31.3 lb |
For half a pound of nitrogen, halve the figures.
Starter fertilizer for new sod and seed
New lawns are short on roots, and phosphate helps them get established. That’s why starter fertilizers have a high middle number. To use the calculator for a starter product:
- Pick “Starter for new sod or seed” in the first box. It sets the nutrient to phosphate at 1 lb per 1,000 sq ft and loads an example 10-20-10 analysis.
- Replace the N-P-K with the numbers on your bag.
- Check the bag’s own rate and adjust if it differs.
Example: 2,000 sq ft of new sod with a 10-20-10 starter at 1 lb of phosphate per 1,000 sq ft: 1 ÷ 0.20 = 5 lb per 1,000 sq ft, so 10 lb for the lawn, which also supplies 1 lb of nitrogen in total.
When: spread it on the prepared soil just before laying sod or sowing seed, then rake it in lightly. Avoid a second heavy feeding until the new grass has rooted and been mowed a few times; follow the product label for timing.
Rules on phosphorus. A number of states and towns restrict phosphorus on established lawns to protect lakes and rivers, usually with an exception for new lawns or where a soil test shows a need. Check your local rules before buying.
How much nitrogen in a year
Lawns differ a lot: cool-season grasses such as tall fescue and bluegrass are often fed mostly in fall, and warm-season grasses such as Bermuda and zoysia in late spring and summer while they’re growing. Total yearly nitrogen depends on the grass, your climate, whether clippings are left on the lawn, and how green you want it. Your local extension office publishes calendars for your area; a soil test tells you about phosphate, potash and pH.
Spreading tips
- Calibrate or check your spreader setting before you start.
- Spread half the product in one direction and half across it to avoid stripes.
- Keep granules off driveways, walks and the street; sweep any back onto the lawn.
- Water lightly after spreading if the label says to.
- Don’t fertilize dormant or drought-stressed grass, or right before heavy rain.
Fertilizer rates here are a starting point for the math. The product label is the legal and practical guide for how much to apply and when.
Questions people ask
- How do I calculate how much fertilizer to put on my lawn?
- Divide the pounds of nutrient you want per 1,000 sq ft by the nutrient's percentage on the bag (as a decimal). That gives pounds of product per 1,000 sq ft; multiply by your lawn area in thousands. For 1 lb of nitrogen from a 20-5-10 fertilizer: 1 ÷ 0.20 = 5 lb per 1,000 sq ft.
- What do the three numbers on fertilizer mean?
- They are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A 50 lb bag of 20-5-10 contains 10 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash.
- How much fertilizer per 1,000 square feet?
- Many lawn feeding programs apply about 0.5 to 1 lb of nitrogen per 1,000 sq ft at a time. How many pounds of product that takes depends on the analysis: 5 lb of a 20 percent nitrogen product, or 10 lb of a 10 percent one, for 1 lb of nitrogen.
- How much starter fertilizer should I use on new sod or seed?
- Follow the bag. Starter products are high in phosphate, and new-lawn guidance often works out to around 1 lb of phosphate (P2O5) per 1,000 sq ft, ideally based on a soil test. Pick the starter preset in the calculator and enter your bag's numbers.
- Can I put down more fertilizer to get a greener lawn faster?
- No. Too much nitrogen at once can burn grass, especially in heat, and excess fertilizer washes into storm drains and waterways. Stay at or below the label rate.