How to use the fertilizer calculator
Enter the lawn's size and the three numbers from the fertilizer bag. Choose how much nitrogen you want to apply per 1,000 square feet; 1 lb is a common single application for many lawns. The calculator gives the pounds of product to spread, the bags to buy, and how much phosphate and potash come along with the nitrogen.
The formula
product per 1,000 ft² = nitrogen wanted ÷ (N % ÷ 100)
total product = product per 1,000 ft² × area ÷ 1,000
phosphate applied = total product × P %; potash = total × K %
Worked example: 5,000 ft² of lawn with 24-0-6
- Product per 1,000 ft²: 1 ÷ 0.24 = 4.17 lb.
- For 5,000 ft²: 4.17 × 5 = 20.8 lb of fertilizer.
- That's less than half of a 50 lb bag, which at this rate covers 12,000 ft².
- It also applies 20.8 × 0.06 = 1.25 lb of potash.
Reading a fertilizer bag
| Number | Nutrient | What it does |
|---|---|---|
| First | Nitrogen (N) | Leaf growth and green color |
| Second | Phosphate (P₂O₅) | Root growth; important for new seedings, often unneeded on established lawns |
| Third | Potash (K₂O) | Stress, drought and cold tolerance |
A soil test from your extension service tells you whether you need phosphorus or potassium at all. Many states restrict phosphorus on established lawns to protect waterways.
Spreading tips
- Calibrate by weight. Weigh out the amount for one area (say, 1,000 ft²), set the spreader low, and make two passes at right angles until it's used up.
- Keep it off hard surfaces. Sweep granules from driveways and sidewalks back onto the lawn so they don't wash into drains.
- Water it in if the label says so, but not before heavy rain.
- Time it to the grass. Cool-season lawns are fed mostly in fall; warm-season lawns from late spring through summer.
Questions people ask
How much fertilizer do I need per 1,000 square feet?
Divide the nitrogen you want by the nitrogen percentage on the bag. For 1 lb of nitrogen per 1,000 square feet with a 26-5-10 fertilizer: 1 ÷ 0.26 = 3.85 lb of product per 1,000 square feet. That's the method Penn State Extension teaches.
What do the three numbers on fertilizer mean?
They're the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 24-0-6 contains 12 lb of nitrogen, no phosphate and 3 lb of potash.
How much nitrogen should I put on my lawn at once?
University of Georgia Extension advises no more than 1 to 1.5 lb of nitrogen per 1,000 square feet in one application when it's a soluble (quick-release) source. Slow-release sources can go somewhat higher. Follow the label.
How much fertilizer for an acre?
Multiply the pounds per 1,000 square feet by 43.56. At 3.85 lb of 26-5-10 per 1,000, an acre takes about 168 lb.
Why does my bag say it covers 5,000 square feet?
The bag's coverage assumes the maker's chosen rate, often about ¾ to 1 lb of nitrogen per 1,000 square feet. If you want a different rate, this calculator gives the right amount.
Sources and assumptions
- Penn State Extension: calculations used to determine the amount of fertilizer needed to treat turf
- University of Georgia Extension, Bulletin 1533: no more than 1-1.5 lb of soluble nitrogen per 1,000 ft² per application
- Always follow the product label, and local rules on phosphorus and fertilizer near water.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.