How the fertilizer calculator works
Lawn feeding is planned around nitrogen, the nutrient grass uses most. The first number on the bag is the percentage of nitrogen by weight. To find how much product gives a set amount of nitrogen, divide the target by that percentage.
product per 1,000 sq ft (lb) = target N (lb) ÷ (N % ÷ 100)
total product (lb) = lawn area ÷ 1,000 × product per 1,000 sq ft
Worked example
A 5,000 sq ft lawn and a bag labeled 24-0-6. To apply 1 lb of nitrogen per 1,000 sq ft: 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft. For the whole lawn, 5 × 4.17 ≈ 20.8 lb, or 9.45 kg. That spreads 5 lb of nitrogen, no phosphate and 1.25 lb of potash. In metric, it is about 20.3 g of product per m².
Product needed for 1 lb of nitrogen
| First number on bag (N %) | Product per 1,000 sq ft |
|---|---|
| 10 | 10 lb |
| 16 | 6.25 lb |
| 20 | 5 lb |
| 24 | 4.17 lb |
| 29 | 3.45 lb |
| 32 | 3.13 lb |
| 46 (urea) | 2.17 lb |
The higher the first number, the less product you spread. That is why a bag of 10-10-10 covers far less lawn than the same weight of a high-nitrogen lawn fertilizer.
How much nitrogen per application?
A widely used rule of thumb is no more than about 1 lb of nitrogen per 1,000 sq ft in a single application, and less with quick-release products in hot weather. Lighter feedings of 0.5 to 0.75 lb suit new lawns, sandy soil and grasses that need little feeding. Yearly totals depend on grass type, climate and whether clippings are left on the lawn; your local extension service publishes schedules for your area.
| Target | Per 1,000 sq ft | Per m² |
|---|---|---|
| Light | 0.5 lb N | 2.4 g N |
| Moderate | 0.75 lb N | 3.7 g N |
| Common maximum per feeding | 1 lb N | 4.9 g N |
Quick-release and slow-release nitrogen
Two bags with the same numbers can behave differently. Quick-release nitrogen, such as urea and ammonium sources, greens the lawn fast but can burn grass if over-applied and is more easily lost in heavy rain. Slow-release forms, often listed on the label as coated urea, methylene urea or water-insoluble nitrogen, feed over several weeks. Many lawn products combine both. The label shows what share of the nitrogen is slow-release.
The calculator works the same way for both, because it is based on total nitrogen. Products with a large slow-release share are often labeled for slightly higher single applications; the label rate is the one to follow.
Granular or liquid?
Granular fertilizer is spread with a broadcast or drop spreader and is easy to measure by weight. Liquid fertilizer is mixed in a sprayer or hose-end applicator and dosed by volume according to the label, so this calculator applies only when the label gives the product's N percentage and you are measuring by weight.
Fall feeding
For cool-season lawns such as tall fescue and Kentucky bluegrass, many extension programs treat early to mid fall as the most valuable time to fertilize, with a final feeding while the grass is still green but growth is slowing. Warm-season lawns such as Bermuda are normally fed during their summer growth, not as they go dormant. Do not apply fertilizer to frozen or snow-covered ground.
Safety and the environment
- Read and follow the label. It sets the legal maximum rate and tells you about pets, children and re-entry.
- Sweep or blow granules off driveways, sidewalks and streets back onto the lawn. Fertilizer washed into drains pollutes waterways.
- Do not fertilize just before heavy rain, or near streams, ponds and wells.
- Some states and towns restrict phosphorus in lawn fertilizer or set blackout dates. Check local rules.
- Calibrate your spreader. Settings on the bag are a starting point; walk at a steady pace and overlap passes as the spreader maker advises.
Seeding at the same time? The grass seed calculator covers how much seed to buy.
Frequently asked questions
How much fertilizer do I need for 1,000 square feet?
Divide your nitrogen target by the first number on the bag as a decimal. For 1 lb of nitrogen with a 29 percent N product, 1 ÷ 0.29 = 3.45 lb of product per 1,000 sq ft.
How much 10-10-10 do I need for 5,000 square feet?
To apply 1 lb of nitrogen per 1,000 sq ft, spread 10 lb of 10-10-10 per 1,000 sq ft, so 50 lb for 5,000 sq ft. That also adds 5 lb each of phosphate and potash.
Is it better to apply less fertilizer more often?
Lighter, more frequent feedings reduce the risk of burning grass and losing nitrogen to runoff. Many lawns do well with 0.5 to 1 lb of nitrogen per 1,000 sq ft per application.
Why does the calculator only use the first number?
Lawn rates are set by nitrogen. The second and third numbers are shown so you can see how much phosphate and potash come along with it.