How much fertilizer does a lawn need?
Lawns aren't dosed by "bags per yard" — they're dosed by pounds of actual nitrogen per 1,000 square feet, and the bag's N-P-K label tells you how much nitrogen is really in there. The formula every university extension service teaches is:
Product (lb) = (nitrogen rate ÷ %N as a decimal) × area ÷ 1,000
Example: you want the standard 1 lb of nitrogen per 1,000 sq ft and your bag reads 30-0-4 (30% nitrogen). 1 ÷ 0.30 = 3.3 lb of product per 1,000 sq ft. On a 5,000 sq ft lawn that's about 17 pounds per application — roughly one 15 lb bag plus a little. The calculator does exactly this, then converts to bags and cost.
Reading the bag: what N-P-K means
The three numbers are percentages by weight:
| Number | Nutrient | What it does |
|---|---|---|
| First — N | Nitrogen | Blade growth and deep green color; the number lawns are dosed by |
| Second — P | Phosphorus | Root development; mainly for seeding and sod ("starter" fertilizer) |
| Third — K | Potassium | Drought, heat, cold, and disease tolerance |
So a 15 lb bag of 30-0-4 contains 4.5 lb of nitrogen, no phosphorus, and 0.6 lb of potassium. Note the zero in the middle: many states restrict phosphorus on established lawns to protect waterways, which is why most maintenance blends are P-free. Use a starter (with P) only when seeding or sodding — ideally after a soil test says you need it.
How much product at common rates
Pounds of fertilizer per 1,000 sq ft to deliver 1 lb of nitrogen, by the bag's first number:
| Bag analysis | Product per 1,000 sq ft | For a 5,000 sq ft lawn |
|---|---|---|
| 10-10-10 (balanced) | 10 lb | 50 lb |
| 16-4-8 | 6.3 lb | 31 lb |
| 24-0-6 | 4.2 lb | 21 lb |
| 30-0-4 | 3.3 lb | 17 lb |
| 46-0-0 (urea) | 2.2 lb | 11 lb |
Higher-analysis bags look pricier but often cost less per pound of nitrogen — this table is how you compare them fairly.
When and how often to feed
Most home lawns want 2–4 lb of nitrogen per 1,000 sq ft per year, split into 2–4 applications — never all at once. Cool-season grasses (fescue, bluegrass, rye) feed mainly in fall with a lighter spring pass; warm-season grasses (bermuda, zoysia, St. Augustine) feed through late spring and summer while they're actively growing. Skip feeding cool-season lawns in mid-summer heat — at most a half-rate (0.5) slow-release pass. If you use a 4-step program, each step is simply this calculator run at about 0.75–1 lb N with that box's analysis.
Spreader tips for an even feed
- Weigh, don't guess. The spreader-setting chart on the bag is approximate. Put your calculated pounds in the hopper for a known area — when it's empty, you've applied the right rate.
- Split the pass. Set the spreader to half rate and cover the lawn twice at right angles — stripes come from single heavy passes.
- Water it in (unless the label says otherwise) and keep granules off driveways and out of storm drains — sweep them back onto the lawn.
- Don't round up the rate. Extra nitrogen doesn't make grass greener faster; it burns it and feeds the compost bin via clippings. Round bags up, rates down.
- Test the soil every few years. A $15–$25 test tells you whether you need P or K at all, and your real pH — often the cheapest lawn upgrade there is.