How to calculate lawn fertilizer
Lawn fertilizer rates are based on nitrogen, the nutrient grass uses most. Extension services and fertilizer labels give rates in pounds of actual nitrogen per 1,000 sq ft. To turn that into pounds of product, divide by the nitrogen percentage, which is the first number of the N-P-K analysis on the bag.
Total product (lb) = product per 1,000 sq ft × lawn area (sq ft) ÷ 1,000
Bags = total product ÷ bag weight, rounded up
Worked example: 5,000 sq ft with a 24-0-6 fertilizer
- Target: 1.0 lb of nitrogen per 1,000 sq ft.
- Product per 1,000 sq ft: 1.0 ÷ 0.24 = 4.17 lb.
- Total for the lawn: 4.17 × 5 = 20.8 lb of product.
- Bags: 20.8 ÷ 15 = 1.39, so buy 2 bags of 15 lb and keep the rest for next time.
- Nutrients applied: 5 lb of nitrogen in total, no phosphate, and 20.8 × 0.06 = 1.25 lb of potash (0.25 lb per 1,000 sq ft).
The calculator also shows how many square feet one bag covers at your rate, which is a quick way to check it against the coverage printed on the bag.
What the three numbers on fertilizer mean
The N-P-K numbers are percentages by weight. A 50 lb bag of 24-0-6 contains 12 lb of nitrogen (N), no phosphate (P₂O₅) and 3 lb of potash (K₂O). The rest of the weight is the other elements in the fertilizer compounds, plus carriers and filler. Phosphorus and potassium are listed as the oxides P₂O₅ and K₂O, which is also how soil test reports usually express them, so you can compare directly.
Pounds of product per 1,000 sq ft
| Nitrogen (first number) | 0.5 lb N per 1,000 sq ft | 0.75 lb N per 1,000 sq ft | 1.0 lb N per 1,000 sq ft |
|---|---|---|---|
| 5% (many organic products) | 10 lb | 15 lb | 20 lb |
| 10% | 5 lb | 7.5 lb | 10 lb |
| 16% | 3.13 lb | 4.69 lb | 6.25 lb |
| 20% | 2.5 lb | 3.75 lb | 5 lb |
| 24% | 2.08 lb | 3.13 lb | 4.17 lb |
| 29% | 1.72 lb | 2.59 lb | 3.45 lb |
| 32% | 1.56 lb | 2.34 lb | 3.13 lb |
| 46% (urea) | 1.09 lb | 1.63 lb | 2.17 lb |
How much nitrogen per application?
Most extension guidance for home lawns falls between about 0.5 and 1.0 lb of nitrogen per 1,000 sq ft per application. Quick-release (water-soluble) nitrogen is the one to be careful with: much more than about 1 lb at once can burn the grass, push soft top growth and wash into waterways. Products with a high share of slow-release nitrogen, listed on the label, can go toward the upper end of that range.
Over a full season, cool-season lawns typically get about 2–4 lb of nitrogen per 1,000 sq ft, and fine fescues and other low-input lawns about 1–2 lb. Warm-season needs vary widely: centipedegrass is usually kept to about 1–2 lb a year, while bermudagrass and St. Augustinegrass often get more. Your state extension service publishes rates for your grass and region, and those beat any general rule.
When to fertilize
- Cool-season lawns (tall fescue, bluegrass, ryegrass): the most useful feeding is in early fall, often followed by a second one later in fall where that is allowed. Feed lightly, if at all, in late spring, and skip midsummer heat.
- Warm-season lawns (bermudagrass, zoysia, St. Augustine, centipede): start after the lawn has fully greened up in late spring, feed through summer, and stop about six to eight weeks before your average first fall frost.
- New lawns: a starter fertilizer at seeding or sodding helps roots establish. This is one of the situations where many phosphorus restrictions make an exception. See the grass seed calculator for seeding rates.
Follow the label and your local rules
The fertilizer label is the legal and practical guide for that product. Several states, including Minnesota, Michigan, New York, Wisconsin, Maryland and New Jersey, restrict phosphorus on established lawns unless a soil test shows a need. Some states and localities also limit nitrogen per application, set winter blackout dates, or ban summer applications, as several Florida counties and cities do. Check your state department of agriculture or extension service, and get a soil test before adding phosphorus or potassium.
Spreader settings and application tips
- Use the label's spreader setting. Settings are product-specific because granule size and weight differ. The same dial number on your spreader can put down very different amounts of two products.
- Calibrate if your spreader isn't listed. Weigh some product into the hopper, spread it over a measured area such as 1,000 sq ft, and weigh what is left. Adjust until you put down the amount per 1,000 sq ft shown above.
- Apply in two directions. Set the spreader for half the rate and make two passes at right angles to avoid stripes.
- Keep it on the grass. Sweep granules off hard surfaces, leave an unfertilized buffer next to ponds and streams, and don't apply to frozen ground or right before heavy rain.
Frequently asked questions
What do the three numbers on fertilizer mean?
They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 20 lb bag of 24-0-6 holds 4.8 lb of nitrogen, no phosphate and 1.2 lb of potash.
How many pounds of fertilizer do I need per 1,000 square feet?
Divide your target nitrogen rate by the nitrogen percentage. To apply 1 lb of nitrogen per 1,000 sq ft with a 24% product, use 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft. With a 32% product, it is 3.13 lb.
How do I tell how much nitrogen a bag puts down?
Multiply the bag weight by the nitrogen percentage, then divide by the coverage in thousands of square feet. A 12.5 lb bag of a 32% product labeled for 5,000 sq ft holds 12.5 × 0.32 = 4 lb of nitrogen, or 0.8 lb per 1,000 sq ft.
How much nitrogen does a lawn need per year?
Most cool-season lawns do well with about 2–4 lb of nitrogen per 1,000 sq ft per year, split into several applications. Low-input grasses such as fine fescue and centipedegrass need less, and some warm-season lawns get more. Check your state extension service for your grass type.
Can you put too much fertilizer on a lawn?
Yes. Too much quick-release nitrogen can burn grass, leaving yellow or brown streaks, and excess nutrients can wash into storm drains and waterways. Stay at or below the label rate, usually 1 lb of nitrogen per 1,000 sq ft or less per application.
What spreader setting should I use?
Use the setting printed on the fertilizer label for your spreader model. Settings are product-specific, so the number from one bag does not carry over to another. If your spreader is not listed, calibrate it by spreading a weighed amount over a measured area.
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