How to use the lawn fertilizer calculator
- Measure your lawn (length × width) or enter the area. Leave out the house, drive and flower beds.
- Enter the three N-P-K numbers from the front of the bag, for example 24-0-6 or 10-10-10.
- Choose how much nitrogen to apply. 1 lb per 1,000 ft² is a full feeding; 0.5–0.75 lb is a light one.
- Optionally enter the bag size and price to see how many bags to buy.
The formula
Product per 1,000 ft² = Nitrogen rate ÷ (N% ÷ 100) Total product = Product per 1,000 ft² × Area ÷ 1,000 Phosphate applied = Product × P% ÷ 100 Potash applied = Product × K% ÷ 100
Worked example
A 5,000 ft² lawn, 24-0-6 fertilizer, 1 lb nitrogen per 1,000 ft²:
- Product per 1,000 ft² = 1 ÷ 0.24 = 4.17 lb
- Total = 4.17 × 5 = 20.8 lb — less than half of a typical 45–50 lb bag
- Potash = 4.17 × 0.06 = 0.25 lb K₂O per 1,000 ft²
Product needed for 1 lb N per 1,000 ft²
| Fertilizer | Per 1,000 ft² | Per 5,000 ft² | Metric (per 100 m²) |
|---|---|---|---|
| 46-0-0 (urea) | 2.2 lb | 10.9 lb | 1.06 kg |
| 32-0-4 | 3.1 lb | 15.6 lb | 1.53 kg |
| 24-0-6 | 4.2 lb | 20.8 lb | 2.03 kg |
| 16-4-8 | 6.3 lb | 31.3 lb | 3.05 kg |
| 10-10-10 | 10 lb | 50 lb | 4.88 kg |
| Milorganite 6-4-0 | 16.7 lb | 83.3 lb | 8.14 kg |
When to fertilize
- Cool-season grass (Kentucky bluegrass, tall fescue, ryegrass): the most important feeding is in early fall, with another in late fall and a light one in late spring.
- Warm-season grass (Bermuda, Zoysia, St. Augustine, centipede): feed from late spring through summer while it is actively growing. Avoid feeding when the grass is dormant.
- Don’t fertilize before heavy rain, on frozen ground or onto hard surfaces — sweep granules off paths and drives back onto the lawn.
Tips for even spreading
- Use a broadcast or drop spreader set from the bag’s chart, and calibrate it on a measured area.
- Spread half the product in one direction and the other half at right angles to avoid stripes.
- Water lightly after applying quick-release products unless the label says otherwise.
- A soil test every 3–4 years tells you whether you need phosphate, potash or lime at all.
Quick-release vs slow-release nitrogen
Quick-release sources such as urea and ammonium sulfate green the lawn within days but can burn it if overapplied and are used up in a few weeks. Slow-release sources (labelled as polymer-coated, sulfur-coated, methylene urea or natural organics) feed for 6–10 weeks and are safer at higher rates. Check the bag’s guaranteed analysis: the more slow-release nitrogen it lists, the more forgiving the product is in hot weather, and the longer you can wait before the next feeding.