Lawn Fertilizer Calculator
Enter your lawn size and how much nitrogen you want to apply, plus the nitrogen number on your fertilizer bag, and get exactly how much product to spread — no more, no less.
Runs entirely in your browser — your numbers never leave this page.
How it works
Fertilizer math is really nitrogen math. Grass mostly wants nitrogen (N) — the first of the three numbers on every bag (N-P-K). The calculation is two steps:
Total N needed (lbs) = Lawn area (sq ft) ÷ 1000 × Rate (lbs N per 1000 sq ft)
Product to spread (lbs) = Total N ÷ (N% ÷ 100)
- Example: 5,000 sq ft at 1 lb N per 1,000 sq ft = 5 lbs of nitrogen. With a 24-0-0 bag (24% N), that's 5 ÷ 0.24 ≈ 20.8 lbs of product.
- Rate: 0.5–1 lb N per 1,000 sq ft per feeding is the safe standard. Slow-release products can go a touch higher; quick-release should stay at the low end.
- Spreaders: use a broadcast spreader and walk at a steady pace — overlapping passes double-dose stripes of lawn.
- Water it in: a light watering after spreading moves granules off the blades and prevents burn.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
They are the N-P-K percentages by weight: nitrogen (N) for leaf growth, phosphorus (P) for roots, potassium (K) for overall health. A 24-0-0 bag is 24% nitrogen and no phosphorus or potassium.
How often should I fertilize my lawn?
Typically 2–4 times per year. Cool-season grasses (fescue, bluegrass) want most of their feeding in fall; warm-season grasses (bermuda, zoysia) are fed from late spring through summer. Never fertilize dormant or stressed grass.
Can too much fertilizer kill my grass?
Yes — it is called nitrogen burn. Excess nitrogen draws water out of grass blades, leaving brown streaks or dead patches. Stick to 1 lb N per 1,000 sq ft max per feeding, spread evenly, and water it in.
Granular vs. liquid fertilizer — which is better?
Granular is cheaper, easier to measure, and usually slow-release (feeds for weeks). Liquid acts fast and is great for quick green-up, but washes through quickly and needs more frequent application. For homeowners, granular slow-release wins on value.
When is the best time of year to fertilize?
For cool-season lawns, the single most important feeding is early fall (September). For warm-season lawns, start after full green-up in late spring. Avoid fertilizing in extreme heat or right before a heavy storm.
Should I get a soil test before fertilizing?
Yes — it is cheap (often under $20 through your state extension service) and tells you pH and nutrient levels. Many lawns need lime for pH more than they need fertilizer, and a test stops you from buying nutrients you already have.
What is the best time of day to spread fertilizer?
Morning, when grass is dry and temperatures are mild. Water it in lightly afterward. Avoid midday heat (burn risk) and late evening (granules sitting wet overnight invite fungus).
Why do some bags have zero phosphorus (like 24-0-0)?
Many states now restrict phosphorus on established lawns because runoff feeds algae blooms in lakes and rivers. Unless a soil test shows a phosphorus deficiency, a zero-P fertilizer is the responsible default.
How soon can kids and pets use the lawn after fertilizing?
Once the granules have been watered in and the lawn is dry to the touch — usually a few hours. Always follow the specific product label, which is the legal instruction.
Does this calculator account for slow-release nitrogen?
The math is the same, but slow-release products are more forgiving — the nitrogen releases over 6–10 weeks, so burn risk is lower. If your bag says "slow-release" or "controlled-release," you can safely use the upper end of the rate range.