How to work out how much fertilizer to spread
Extension services give lawn advice in pounds of nitrogen per 1,000 square feet, not pounds of product. The bag tells you how much nitrogen it holds, so the math is a short conversion. Here it is with the values the calculator opens with: a 5,000 sq ft Kentucky bluegrass lawn, moderate care and a 40 lb bag of 16-4-8.
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1
Pick the yearly nitrogen
Purdue's standard program for bluegrass is about 3 lb of nitrogen (N) per 1,000 sq ft a year. Low care is about 1 lb and the greenest lawns get about 4.
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2
Split it into feedings
Apply no more than about 1 lb of quick-release N per 1,000 sq ft at a time. 3 lb ÷ 1 lb = 3 feedings of 1 lb each.
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3
Convert nitrogen to product
The first number on the bag is the percent nitrogen. Divide the nitrogen you want by that number as a decimal: 1 ÷ 0.16 = 6.25 lb of 16-4-8 per 1,000 sq ft.
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4
Scale to your lawn
6.25 lb x 5 (for 5,000 sq ft) = 31.25 lb per feeding. Three feedings use 93.75 lb a year.
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5
Count bags
A 40 lb bag of 16-4-8 holds 6.4 lb of N, enough for 6,400 sq ft at 1 lb. 93.75 ÷ 40 = 2.3, so buy 3 bags. At $35 a bag that is $105 for the year.
The same formula works in reverse to check a bag's label. If a bag claims to cover 5,000 sq ft, divide its nitrogen by 5. A 12.5 lb bag of 32-0-4 holds 4 lb of N, so it feeds 5,000 sq ft at 0.8 lb per 1,000.
Pounds of product per 1,000 sq ft
For 1 lb of nitrogen per 1,000 sq ft. Halve the numbers for a half-pound feeding.
| Bag analysis | Lb per 1,000 sq ft | For 5,000 sq ft |
|---|---|---|
| 46-0-0 (urea) | 2.2 | 10.9 lb |
| 32-0-4 | 3.1 | 15.6 lb |
| 21-0-0 (ammonium sulfate) | 4.8 | 23.8 lb |
| 16-4-8 | 6.25 | 31.3 lb |
| 10-10-10 | 10 | 50 lb |
| 5-3-0 (organic) | 20 | 100 lb |
Low-analysis organic products are not weaker per pound of nitrogen, just bulkier. You carry and spread four times as much 5-3-0 as 21-0-0 for the same feeding.
How much nitrogen each grass wants
Pounds of N per 1,000 sq ft per year. Cool-season figures follow Purdue and Virginia Tech; warm-season figures follow Texas A&M AgriLife. Drier, shadier and unwatered lawns sit at the low end.
| Grass | Low | Moderate | High |
|---|---|---|---|
| Kentucky bluegrass, ryegrass | 1 | 3–3.5 | 3.75–4.25 |
| Tall fescue | 1–1.5 | 2–2.5 | 3–3.5 |
| Fine fescue | 1–2 at most | ||
| Bermudagrass, common | 2 | 2–3 | 4–5 |
| Bermudagrass, hybrid | 2 | 3–4 | 5–6 |
| St. Augustine, sun | 2 | 2–3 | 3–4 |
| St. Augustine, shade | 1 | 1–2 | not advised |
| Zoysia | 1–2 | 2–3 | 3–4 |
| Centipede | 0–1 | 1–2 | not advised |
| Buffalograss | 0–1 | 2 | not advised |
The calculator uses one number from each range: 1, 3 and 4 for bluegrass and ryegrass; 1, 2 and 3 for tall fescue; 1, 1.5 and 2 for fine fescue; and the middle of each warm-season range, capped where extension says more is not advised. Centipede is the grass most often hurt by too much feeding: it turns yellow and declines. Returning clippings to the lawn feeds it too; bag them and you need more fertilizer.
When to fertilize
Cool-season grass (bluegrass, ryegrass, fescue) grows best in spring and fall. Purdue ranks the feedings in this order: early September, then early November, then late May, with a light 0.75 lb feeding in late July only for the high program. Dates are for central Indiana; move them about two weeks earlier in fall farther north. Heavy spring feeding drives fast top growth, more mowing and more summer disease.
Warm-season grass (bermuda, zoysia, St. Augustine, centipede, buffalo) grows in summer. Texas A&M advises the first feeding after the second spring mowing of the grass (not the weeds), about 6 weeks after the last frost. Space later feedings at least 45 to 60 days apart, and make the last one at least 6 weeks before the first expected fall frost.
Quick-release or slow-release?
Urea, ammonium sulfate and ammonium nitrate dissolve fast and green the lawn within days, but wash out more easily on sandy soil. Slow-release sources, such as methylene ureas, sulfur-coated urea and natural organics, feed for weeks. The label lists the slow part as "water insoluble nitrogen" (WIN). Divide WIN by total N: if half or more is slow-release, Virginia Tech allows feedings of up to 1.5 lb N per 1,000 sq ft. That is what the calculator's 1.5 lb button is for.
What it costs
At the calculator's defaults, a year of feeding costs about $105 for 5,000 sq ft, about 2 cents per square foot. Your price depends on the analysis and the brand, so check the price per pound of nitrogen, not per bag: a $35 bag of 16-4-8 holds 6.4 lb of N, or about $5.47 per pound of N. Divide any bag's price by its pounds of nitrogen to compare. Lawn care companies usually bundle feeding with weed and insect control, so compare a quote with what you would spend on fertilizer and weed control together.
Tips from the trade
- Test the soil first. Phosphorus and potassium needs depend on your soil, and a soil test from your state's extension lab tells you. Without one, many experts suggest a fertilizer with little or no phosphate. Several states limit phosphorus on established lawns unless a soil test shows a need.
- Calibrate the spreader. Bag settings are a starting point. Purdue's method: weigh some product, spread it over a measured strip, weigh what is left, and work out pounds per 1,000 sq ft.
- Go half and half. Set the spreader to deliver half the rate and cover the lawn twice, the second pass at a right angle. It prevents stripes.
- Keep it off pavement. Sweep or blow stray granules from driveways, sidewalks and streets back onto the grass. On a hard surface they wash straight into storm drains.
- Mind the weather. Do not feed before a heavy rain or on frozen or dormant grass. Do not feed cool-season grass in midsummer heat.
- Read the whole label, especially for weed-and-feed products, which carry herbicide rules on timing, seeding and watering.
Common questions
How much fertilizer do I need for 5,000 square feet?
To apply 1 pound of nitrogen per 1,000 square feet, divide 1 by the first number on the bag, then multiply by 5. With a 16-4-8 fertilizer that is 6.25 pounds per 1,000 square feet, or about 31 pounds for 5,000 square feet. With a 32-0-4 fertilizer it is about 16 pounds.
What do the three numbers on a fertilizer bag mean?
They are the percent by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). A 40 pound bag of 16-4-8 holds 6.4 pounds of nitrogen, 1.6 pounds of phosphate and 3.2 pounds of potash. The rest is filler and carrier that help it spread evenly.
How often should I fertilize my lawn?
Most lawns do well with 2 to 4 feedings a year. Split the yearly nitrogen into doses of no more than about 1 pound per 1,000 square feet. Cool-season grasses get most of it in early fall and late fall. Warm-season grasses get it from late spring through summer, spaced 45 to 60 days apart.
When is the best time to fertilize a lawn?
Fertilize when the grass is actively growing. For bluegrass, ryegrass and fescue, September is the most important feeding. For bermudagrass, zoysia, St. Augustine and centipede, start after the second spring mowing, about 6 weeks after the last frost, and stop about 6 weeks before the first fall frost.
Can you put too much fertilizer on a lawn?
Yes. Too much quick-release nitrogen at once can burn grass, drive weak top growth that invites disease and wash into storm drains and streams. Stay within about 1 pound of nitrogen per 1,000 square feet per feeding, or 1.5 pounds with mostly slow-release fertilizer, and sweep granules off pavement.
Should I water after fertilizing?
Usually yes. About a quarter inch of water moves granules off the leaves and into the soil, which lowers the risk of burn. Skip it if heavy rain is forecast, since a downpour right after feeding can wash fertilizer away. Follow the label for weed-and-feed products, which often need dry soil and wet leaves.
Sources and assumptions
Rates come from land-grant university extension programs. They are general guides: your state's extension service may tune them for local soils and climate.
- Cool-season rates and dates (minimum, standard and maximum programs; September, November, late May and late July feedings; the product formula) come from Purdue Extension's AY-22, Fertilizing Established Lawns.
- Tall and fine fescue, slow-release limits and WIN come from Virginia Cooperative Extension's publication 430-011, Lawn Fertilization in Virginia, which also puts centipede and mature zoysia at 1 to 2 lb a year.
- Warm-season rates and timing (Table 1, the 1 lb cap, 45 to 60 day spacing, the 6-week frost rules and the 40 lb bag of 16-4-8 example) come from Texas A&M AgriLife Extension's Fertilizing Texas Lawns.
- Prices: bag prices vary widely by brand and analysis. The $35 default for a 40 lb bag is our estimate; retail listings in 2026 ran from about $25 for small 5,000 sq ft bags to about $70 for 15,000 sq ft bags. The spreader ($45) and soil test ($20) are also estimates; many state labs charge less for a basic test. Edit the bag price to match your store.
- Metric: 1 lb per 1,000 sq ft equals 4.9 g per square meter.