Fertilizer Calculator
How much of the bag to put down for the rate your lawn needs, how many bags to buy, what the other nutrients come along at, and how to split it across applications.
- Lawn
- 5,000 sq ft, Tall fescue, zone 7a
- Recommendation
- 2.50 lb of Nitrogen (N) per 1,000 sq ft, in 3 applications
- Bag
- 24-0-6, 40 lb, 30% slow-release nitrogen
What goes down
for the whole rate, from this bag| Nutrient | Per 1,000 sq ft | Over the lawn | As the element |
|---|---|---|---|
| Nitrogen (N) | 2.50 lb | 12.5 lb | |
| Phosphate (P₂O₅) | 0.00 lb | 0.0 lb | 0.00 lb as P |
| Potash (K₂O) | 0.63 lb | 3.1 lb | 0.52 lb as K |
Applications
spread each one evenly over the whole lawn| Application | When | Product | Per 1,000 sq ft | Nitrogen per 1,000 |
|---|---|---|---|---|
| 1 | Feb | 17.4 lb | 3.47 lb | 0.83 lb |
| 2 | Sep | 17.4 lb | 3.47 lb | 0.83 lb |
| 3 | Nov | 17.4 lb | 3.47 lb | 0.83 lb |
How this is worked out
What a lawn wants in a year depends on the grass. Extension offices in every state publish the same table: pounds of nitrogen per 1,000 square feet per year by grass, and the months to put it down, which shift with the climate. The calculator carries that table for ten grasses, with the USDA hardiness zone standing in for the region: zones 5 and below take the northern calendar, 6 and 7 the transition zone's, 8 and up the south's. Type a ZIP and the zone comes from the 2023 map. Pick the grass and the year's nitrogen, the number of applications and their months fill in; change any of them if your extension office or a soil test says otherwise. A new seeding gets a starter fertilizer instead, at a pound of phosphate per 1,000 square feet, with the follow-up feeding described.
The three numbers on a bag are the percent by weight of nitrogen, phosphate and potash. A recommendation is pounds of one of those per 1,000 square feet. So the pounds of product per 1,000 square feet is the rate divided by the bag's percentage: in the example on this page, 2.50 lb of nitrogen from a 24-0-6 bag is 2.50 ÷0.24 = 10.42 lb. Multiply by the thousands of square feet for the total, and divide by the bag weight for bags. The other two nutrients follow from the same pounds of product and the bag's other two numbers.
Bags state phosphorus as phosphate (P₂O₅) and potassium as potash (K₂O), because the label law says so. Some soil tests give the element instead. If yours does, say so and the recommendation is converted before it meets the bag: phosphorus × 2.29, potassium × 1.2.
Nitrogen is the one that can do harm in a single dose. Extension guidance is not to put down more than a pound of quick-release nitrogen per 1,000 square feet at a time; what the grass cannot take up burns it or washes away. Slow-release nitrogen is released over weeks, so a bag that says how much of its nitrogen is slow-release can go higher in one go. The calculator warns when an application crosses that line and suggests splitting it.
How to use it
- Measure the lawn. Pace it out or use a satellite map; square feet or acres, either way.
- Pick the grass, type your ZIP code for the zone, and say whether the lawn is established or a new seeding. The season's recommendation fills in: how much, how many times, and when.
- If you have a soil test or your extension office says different, change the rate, the nutrient or the number of applications to match.
- Copy the bag: the three numbers, the weight, the slow-release share if it is printed, and the price if you want a cost.
- Read off pounds per 1,000 square feet and bags to buy. The bag's own spreader setting is for its own rate; scale it, or calibrate over a measured patch.
What it assumes
- The season program (how much nitrogen a year, in how many applications, in which months) is what extension offices publish for the grass in that part of the country, with the USDA hardiness zone standing in for the region: zones 5 and below take the northern calendar, 6 and 7 the transition zone, 8 and up the south. It is a starting point; a soil test or local extension advice overrides it.
- The three numbers on the bag are percent by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O), which is what the label law requires. A soil test that gives phosphorus or potassium as the element is converted to the oxide first.
- The rate is the amount of the chosen nutrient you want down over the whole area. If it is a season total, the applications split it; if it is a single application, leave applications at one.
- The quick-release ceiling of 1 lb of nitrogen per 1,000 sq ft per application is the common extension guidance for established turf. Slow-release products can go higher because the nitrogen is released over weeks; the bag says how much of its nitrogen is slow-release.
- Spreader settings are not worked out here. The bag gives a setting for its own rate; scale it, or calibrate the spreader over a measured area.
Sources
- How to Calculate the Amount of Fertilizer Needed for Your Lawn (FS839): the pounds-of-product arithmetic and the 1 lb soluble nitrogen per application guideline — Rutgers New Jersey Agricultural Experiment Station (checked 2026-09-28)
- Suggested Fertilizer Practices for Lawns: never more than 1 lb of water-soluble N per 1,000 sq ft at one time; slow-release may go higher — University of Connecticut Soil Nutrient Analysis Laboratory (checked 2026-09-28)
- Calculating Lawn Fertilizer Rates: the worked bag example, 0.9 lb per application and a 3.25 lb annual total under New Hampshire law, at least 20% slow-release — University of New Hampshire Extension (checked 2026-09-28)
- Understanding the Numbers on Your Soil Test Report (FSA-2118): P × 2.29 = P₂O₅ and K × 1.20 = K₂O; labels state the oxides — University of Arkansas Division of Agriculture (checked 2026-09-28)
- Fertilizing Lawns (HGIC 1201): pounds of nitrogen per year by grass, and the months to apply it in the Piedmont and the Coastal Plain — Clemson Cooperative Extension (checked 2026-09-29)
- Turfgrass Fertilization: A Basic Guide for Professional Turfgrass Managers: suggested annual nitrogen for Kentucky bluegrass and perennial ryegrass (2.0 to 3.5), fine fescues (1.0 to 2.0) and tall fescues (1.0 to 3.0) — Penn State Extension (checked 2026-09-29)
- Fertilizing Established Cool-season Lawns (AY-22-W): programs by maintenance level, fall-weighted, with no more than 1 lb of soluble nitrogen per application — Purdue Extension (checked 2026-09-29)
- Tall Fescue and Kentucky Bluegrass Home Lawn Calendar: 1 lb of nitrogen in mid-September, November and February — NC State Extension (checked 2026-09-29)
- Homeowner Best Management Practices for the Home Lawn (ENH979): nitrogen ranges for bahiagrass, centipedegrass, St. Augustinegrass and zoysiagrass in north, central and south Florida, and when to start and stop — UF/IFAS Extension (checked 2026-09-29)
- Management of Buffalograss Turf in Nebraska (G1947): 0.5 to 1 lb of nitrogen between mid-June and mid-July, never more than 2 lb a year — Nebraska Extension (checked 2026-09-29)
- Establishing Turfgrass Areas (AY-3-W): starter fertilizer at 1.0 to 1.25 lb of P₂O₅ per 1,000 sq ft before seeding, then 1 lb of nitrogen at four and seven weeks after germination — Purdue Extension (checked 2026-09-29)
- 2023 USDA Plant Hardiness Zone Map, zones by ZIP code (public domain). The table used here was checked against the map for 130 ZIP codes and agreed on all of them — USDA Agricultural Research Service and Oregon State University PRISM Climate Group (checked 2026-09-29)