Enter the measured lawn area, the nitrogen rate specified for this application, the product’s nitrogen percentage and the bag weight. The rate is an assumption to replace with the product label and appropriate local turf guidance; one pound per 1,000 square feet is not lawful or suitable everywhere.
Lawn Fertilizer: worked example
| Lawn area (sq ft) | 5,000 |
|---|---|
| Nitrogen to apply (lb per 1,000 sq ft) | 1 |
| Nitrogen in the bag (%) | 20.0% |
| Bag weight (lb) | 15 |
| Fertilizer needed (lb) | 25 |
| Bags needed | 2 |
Lawn Fertilizer: changing Lawn area (sq ft)
| Lawn area (sq ft) | Fertilizer needed (lb) | Bags needed |
|---|---|---|
| 4,000 | 20 | 2 |
| 4,500 | 22.5 | 2 |
| 5,000 (example) | 25 | 2 |
| 5,500 | 27.5 | 2 |
| 6,000 | 30 | 2 |
Lawn Fertilizer: compare input ranges
| Input | Tested from | To | Fertilizer needed (lb): low input | Fertilizer needed (lb): high input | Swing |
|---|---|---|---|---|---|
| Nitrogen in the bag (%) | 18.0% | 22.0% | 27.8 | 22.7 | 5.1 (20%) |
| Lawn area (sq ft) | 4,500 | 5,500 | 22.5 | 27.5 | 5 (20%) |
| Nitrogen to apply (lb per 1,000 sq ft) | 0.9 | 1.1 | 22.5 | 27.5 | 5 (20%) |
| Bag weight (lb) | 14 | 17 | 25 | 25 | 0 |
Lawn Fertilizer: individual input comparisons
| Input | Value | Fertilizer needed (lb) | Bags needed |
|---|---|---|---|
| Lawn area (sq ft) | 4,000 | 20 | 2 |
| 4,500 | 22.5 | 2 | |
| 5,000 | 25 | 2 | |
| 5,500 | 27.5 | 2 | |
| 6,000 | 30 | 2 | |
| Nitrogen to apply (lb per 1,000 sq ft) | 0.8 | 20 | 2 |
| 0.9 | 22.5 | 2 | |
| 1 | 25 | 2 | |
| 1.1 | 27.5 | 2 | |
| 1.2 | 30 | 2 | |
| Nitrogen in the bag (%) | 16.0% | 31.3 | 3 |
| 18.0% | 27.8 | 2 | |
| 20.0% | 25 | 2 | |
| 22.0% | 22.7 | 2 | |
| 24.0% | 20.8 | 2 | |
| Bag weight (lb) | 12 | 25 | 3 |
| 14 | 25 | 2 | |
| 15 | 25 | 2 | |
| 17 | 25 | 2 | |
| 18 | 25 | 2 |
Lawn Fertilizer: combined input changes
| Lawn area (sq ft) / Nitrogen to apply (lb per 1,000 sq ft) | 0.8 | 1 | 1.2 |
|---|---|---|---|
| 4,000 | 16 | 20 | 24 |
| 4,500 | 18 | 22.5 | 27 |
| 5,000 | 20 | 25 | 30 |
| 5,500 | 22 | 27.5 | 33 |
| 6,000 | 24 | 30 | 36 |
Lawn Fertilizer: which assumptions move the result?
Lawn Fertilizer: values and definitions
| Quantity and definition | Value used |
|---|---|
| Lawn area (sq ft) | 5,000 |
| Nitrogen to apply (lb per 1,000 sq ft) — Use the actual product label and applicable local nutrient restrictions. | 1 |
| Nitrogen in the bag (%) — The first number on the bag, such as 20 in a 20-5-10 fertilizer. | 20.0% |
| Bag weight (lb) | 15 |
| Fertilizer needed (lb) | 25 |
| Bags needed | 2 |
Lawn Fertilizer: half and double Nitrogen in the bag (%)
| Nitrogen in the bag (%) | Fertilizer needed (lb) |
|---|---|
| 10.0% | 50 |
| 20.0% | 25 |
| 40.0% | 12.5 |
A soil test and applicable local restrictions determine nutrient needs and permitted rates. The N-P-K grade lists nitrogen, phosphate expressed as P₂O₅ and potash expressed as K₂O, rather than elemental P and K percentages. Calibrate the spreader and keep fertilizer off pavement; follow the label for watering.
Lawn Fertilizer: formula
NIST Handbook 44: Units and Systems of Measurement · EPA WaterSense: Outdoor Water Use · University of Minnesota Extension: Fertilizing lawns
Common error: Doubling the dose to speed things up. Too much nitrogen burns grass and washes into storm drains, so stay within the label rate and keep it off pavement.
Lawn & Garden calculators: Sod Calculator, Lawn Watering Calculator, Grass Seed Calculator.
Fertilizer is measured by the nitrogen it carries, not by the bag. A 20-5-10 bag is 20 percent nitrogen, so a 5,000 square foot lawn that needs 1 pound of nitrogen per 1,000 square feet takes 25 pounds of product, which is two 15 pound bags.
The first fertilizer analysis number is its nitrogen percentage. A stronger bag needs fewer pounds for the same nitrogen, so the amount changes when you switch products.
Estimate seed on the grass seed calculator, watering on the lawn watering calculator and sod on the sod calculator. The topsoil and mulch pages help with beds, and the area and percentage calculators support the arithmetic.
Should I apply every bag I buy?
No. The example buys 30 pounds in two bags but calculates an application of 25 pounds. Keep the surplus rather than increasing the entered rate.
Are the second and third grade numbers elemental phosphorus and potassium?
No. They are label conventions expressed as phosphate P₂O₅ and potash K₂O. The first number is nitrogen by mass.
How much nitrogen does a lawn need?
Use a soil-test-based recommendation and the product label for the site and grass type. The calculator converts an entered nutrient target into product weight; it does not establish the agronomic target. Seasonal limits and local restrictions can also affect an application.
What do the three numbers on a fertilizer bag mean?
The fertilizer grade states percentage by weight of nitrogen, available phosphate expressed as P₂O₅, and soluble potash expressed as K₂O. The second and third numbers are not simply elemental phosphorus and potassium percentages. Use the stated label basis when calculating product weight.
Can I put too much fertilizer on a lawn?
Yes, it burns the grass and runs off into waterways, so follow the rate and water it in.