How much fertilizer do I need? The math
Every lawn fertilizer recommendation — from your extension office, a soil-test report or the planner on this site — is written in pounds of actual nitrogen per 1,000 square feet. The bag is labeled in percent. Converting between them is one line:
One pound of N per 1,000 sq ft from a 24-0-6 bag: 1 × 100 ÷ 24 = 4.17 lb per 1,000 sq ft. On a 5,000 sq ft lawn that's 20.8 lb — about 2 of the usual 15 lb bags, with some left over. The calculator does the same thing and then keeps going: bags, cost, and what else the bag delivered.
Pounds of product per 1,000 sq ft, by bag
| Bag analysis | 0.5 lb N | 0.75 lb N | 1 lb N |
|---|---|---|---|
| 10-10-10 | 5.0 lb | 7.5 lb | 10.0 lb |
| 16-4-8 | 3.1 lb | 4.7 lb | 6.3 lb |
| 20-0-8 | 2.5 lb | 3.8 lb | 5.0 lb |
| 24-0-6 | 2.1 lb | 3.1 lb | 4.2 lb |
| 29-0-4 | 1.7 lb | 2.6 lb | 3.4 lb |
| 32-0-10 | 1.6 lb | 2.3 lb | 3.1 lb |
| Urea 46-0-0 | 1.1 lb | 1.6 lb | 2.2 lb |
| Milorganite 6-4-0 | 8.3 lb | 12.5 lb | 16.7 lb |
Product per 1,000 sq ft for each nitrogen target. This is also your spreader calibration number — see below.
What's a "pound of nitrogen," and why one per 1,000?
Turf responds to nitrogen more than to anything else you can buy, and it responds in proportion to the amount. A pound of N per 1,000 sq ft per application is the long-standing extension standard because it's enough to feed a lawn for six to eight weeks and little enough that a healthy lawn takes it up before it leaches or burns. Over a year, cool-season lawns want roughly 2–4 lb per 1,000 depending on species and how much you irrigate (the planner's template sums to 4 including the optional feedings); bermuda takes 4 or more, centipede barely 1 (the warm template sums to 5.25 for bermuda). Spoon-feeding — ¼ to ½ lb every few weeks — is the same yearly total in smaller bites, and is the enthusiast's way of avoiding growth surges.
A 12.5 lb bag of 29-0-4 labeled for 5,000 sq ft is putting down 0.73 lb N per 1,000 — which is fine, but the next brand's "5,000 sq ft" bag might be 0.6 or 1.0. Coverage claims aren't comparable; pounds of N are. Do the math once and the marketing goes quiet.
How many bags for my lawn?
| Lawn | 1 lb N as 24-0-6 | 15 lb bags | 1 lb N as urea 46-0-0 |
|---|---|---|---|
| 2,500 sq ft | 10.4 lb | 1 | 5.4 lb |
| 5,000 sq ft | 20.8 lb | 2 | 10.9 lb |
| 7,500 sq ft | 31.3 lb | 3 | 16.3 lb |
| 10,000 sq ft | 41.7 lb | 3 | 21.7 lb |
One application at 1 lb N per 1,000 sq ft. Bags round up; the calculator shows the leftover.
What does nitrogen actually cost? Urea vs "lawn food"
Because the nutrient is the same molecule whichever bag it's in, the honest way to compare products is dollars per pound of nitrogen. Plain urea from a farm store is almost always the cheapest N you can buy; branded "lawn food" in small bags costs several times as much per pound of N — what you're paying for is slow-release coating, a bit of potassium, and a spreader setting printed on the bag. Sometimes that's worth it (slow-release is genuinely gentler and lasts longer); sometimes it's a $25 bag of mostly filler. Example prices below are ballpark — enter your own in the calculator.
| Product (example price) | $ per lb of N | 1 lb N on 5,000 sq ft |
|---|---|---|
| Urea 46-0-0 — 50 lb, $40 | $1.74 | $8.70 |
| "Lawn food" 29-0-4 — 12.5 lb, $25 | $6.90 | $34.48 |
| Slow-release 24-0-6 — 15 lb, $30 | $8.33 | $41.67 |
| Weed & feed 28-0-3 — 14 lb, $35 | $8.93 | $44.64 |
| All-purpose 10-10-10 — 40 lb, $25 | $6.25 | $31.25 |
| Milorganite 6-4-0 — 32 lb, $20 | $10.42 | $52.08 |
Illustrative big-box prices; the $/lb N and per-application columns are computed from them. Milorganite and other organics are slow, low-burn and carry iron — the premium buys something, but know what.
Slow-release vs quick-release — and the 1 lb rule
Quick-release nitrogen (urea, ammonium sulfate, most cheap blends) is available to the plant immediately, greens up in days, and burns if overdone: keep it at 1 lb N per 1,000 sq ft or less per application, and water it in. Slow-release products (coated urea, methylene urea, organics) meter nitrogen out over weeks; labels typically allow up to 1.5 lb per application because the plant can't get it all at once. The bag says what fraction is slow — look for "X% slowly available nitrogen" in the fine print. Tick the slow-release box above and the calculator's guard rail moves accordingly.
What about phosphorus and potassium?
The calculator prints the P₂O₅ and K₂O your application delivers, because you're putting them down whether you meant to or not. Most established lawns don't need phosphorus — it accumulates in soil, it's the nutrient that feeds algae blooms downstream, and a dozen states now ban or restrict it on turf unless a soil test shows a deficiency or you're seeding. Potassium is more often short, especially on sandy soils, and a fall application helps winter hardiness. The only way to know either is a soil test; until you have one, a nitrogen-only or low-P product is the safe default.
Spreader settings: why the bag's number is only a start
The setting printed on the bag was measured with one spreader model walked at one speed. Yours differs, and the difference is routinely ±30%. The fix is cheap: the calculator gives you pounds per 1,000 sq ft, so weigh what the spreader puts down over a measured 1,000 sq ft strip and compare. The spreader calibration calculator does the rest — it aims the next setting from your own test runs (the dial isn't linear, so "a notch lower" is a guess) and measures how far apart your passes should be. Two passes at half rate in a crosshatch hides any unevenness.
Liquid fertilizer math
Liquid labels give nitrogen as a percentage by weight, so you need the product's weight per gallon (on the label or SDS; most are 9–11 lb/gal) to turn a target into ounces. The liquid mode above does it: pounds of N needed ÷ N% ÷ lb-per-gallon = gallons, × 128 = fluid ounces. How much water you mix it with is a sprayer question, not a nutrient one — enough to get even coverage, usually 1–2 gallons of carrier per 1,000 sq ft.
When to fertilize
Timing is the other half and it depends on your grass: fall-heavy for cool-season lawns, summer for warm-season, never when dormant. The Lawn Care Planner lays the applications out by date for your zone and, if you save this bag, prints each one in pounds of your product.
Sources: UNH Extension, Calculating Lawn Fertilizer Rates; Rutgers NJAES FS839, How to Calculate the Amount of Fertilizer Needed for Your Lawn; Penn State Extension, Calculations Used to Determine the Amount of Fertilizer Needed to Treat Turf. Slow- and quick-release limits per those publications and typical product labels.