Lawn Fertilizer Calculator
Turn a nitrogen recommendation into the amount of fertilizer to buy. Enter your lawn size, the pounds of nitrogen per 1,000 square feet and the nitrogen percentage on the bag, and get the total weight of product, the bags needed, and how much one bag covers.
Lawn fertilizer calculator
How to use the lawn fertilizer calculator
- Measure the grass area. Enter the lawn size, leaving out paths, patios and beds, or type an area you already know.
- Enter the nitrogen rate. Use the rate from your soil test or lawn guide, in pounds of nitrogen per 1,000 ft² (grams per m² in metric). One pound is a common single application.
- Enter the bag analysis. Type the first number of the N-P-K label, for example 20 for a 20-5-10 fertilizer.
- Add the bag weight. Optionally enter the bag size to see how many bags to buy.
- Read and calibrate. Use the product-per-1,000 figure to set your spreader, and follow the label for settings and watering in.
Lawn fertilizer formula
Product per 1,000 ft² = nitrogen rate ÷ (N% ÷ 100)
Total product = product per 1,000 ft² × lawn area ÷ 1,000
Fertilizer guidance is written in pounds of nitrogen per 1,000 square feet, but bags are sold by weight and analysis, so the two steps above convert one to the other. This is the method used by Rutgers, Iowa State and Purdue. In metric, 1 lb per 1,000 ft² is about 4.9 g per m², and the calculator converts for you.
Worked examples
A typical lawn. 4,000 ft² at 1 lb N with a 20% fertilizer: 5 lb per 1,000 ft², so 20 lb, or 2 bags of 15 lb (1.33 needed).
The Iowa State example. 5,625 ft² with a 32-0-4 fertilizer at 1 lb N: 3.125 lb per 1,000 ft² gives 17.6 lb.
A lighter dose. 0.75 lb N from a 20-5-10 fertilizer is 3.75 lb per 1,000 ft², and on 8,000 ft² that is 30 lb.
A metric lawn. 300 m² at 5 g N per m² with a 10% fertilizer uses 1.5 kg N, so 15 kg of product.
Pounds of product for 1 lb of nitrogen
| N on the bag | Product per 1,000 ft² |
|---|---|
| 10% | 10 lb |
| 15% | 6.7 lb |
| 20% | 5 lb |
| 25% | 4 lb |
| 32% | 3.1 lb |
| 46% | 2.2 lb |
Applying it safely
Iowa State suggests 0.5 to 1 lb N per 1,000 ft² a time, and Rutgers notes about 1 lb for one application of quick-release fertilizer, with slow-release products allowing more. The tool refuses rates above 4 lb N per 1,000 ft² as a safety check. Include only the grass area, calibrate your spreader, sweep granules off paving, and avoid applying before heavy rain or on frozen ground. Always read the label.
Features
- Rectangle, circle or known-area input in imperial or metric.
- Product weight, nitrogen applied, bags and coverage per bag.
- Guard against implausible nitrogen rates.
- Copy result, reset and a printable report in five themes on A4 or Letter.
Plan the rest of the lawn
Get the area right with the lawn area calculator, seed thin patches using the grass seed calculator, and work out irrigation with the lawn watering calculator. The compost calculator covers soil improvement.
Frequently asked questions
How do I calculate how much fertilizer to put on my lawn?
Divide the pounds of nitrogen you want per 1,000 ft² by the nitrogen percentage as a decimal, then multiply by the lawn area in thousands of square feet. One pound of N from a 20% fertilizer needs 5 lb of product per 1,000 ft².
How much fertilizer do I need for 5,000 square feet?
With 1 lb of N per 1,000 ft² and a 20% fertilizer, you need 5 lb × 5 = 25 lb. With a 32% fertilizer it is 3.125 × 5 = 15.6 lb.
What do the three numbers on a fertilizer bag mean?
They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Nitrogen is always the first number, and this calculator uses it.
How much nitrogen does a lawn need?
Iowa State Extension gives 0.5 to 1 lb of actual nitrogen per 1,000 ft² per application, depending on timing and the lawn quality you want. Rutgers advises about 1 lb for a single quick-release application.
How many square feet does a bag of fertilizer cover?
Coverage is bag weight ÷ product per 1,000 ft² × 1,000. A 15 lb bag of 20% fertilizer at 1 lb N covers 3,000 ft². Check the bag, which is calculated for its own recommended rate.
Do I need phosphorus or potassium?
Not always. Lawn advice is written around nitrogen, and phosphorus and potassium are best added only when a soil test shows a need. Follow local rules, since some areas restrict phosphorus fertilizer.
How do I use a spreader with this figure?
Set the spreader to the label setting for your product, then calibrate: the product-per-1,000 figure tells you how much should come out over a test area. Kentucky Extension explains how to calibrate spreaders.
Related calculators
Grass Seed Calculator
Pounds of grass seed for a new lawn or overseeding, by grass type.
Lawn Area Calculator
Square feet and acres of an irregular lawn, with quick seed and fertilizer amounts.
Lawn Watering Calculator
Gallons per week and sprinkler run time for your lawn.
Compost Calculator
Cubic yards and bags of compost for lawns, beds and new planting areas.
Sources and references
- Rutgers Cooperative Extension – FS839: How to calculate the amount of fertilizer needed for your lawn
- Iowa State University Extension – How to calculate fertilizer rates for the lawn
- Iowa State University Extension – How much fertilizer should I apply to my lawn?
- Purdue Turf – Calculating the pounds of fertilizer to apply
- University of Kentucky – Calibrating fertilizer spreaders for the home lawn
About this calculator
Written and maintained by Rashid Mehmood, founder of CalcTen. The formulas are unit-tested against hand-worked examples, and the content is reviewed whenever product sizes or guidance change. Last updated 14 November 2026. See our editorial policy.
Results are estimates for planning and ordering. Coverage, soil and plant needs vary with site and product, so check labels and local advice, and follow the manufacturer's instructions for any chemical or fertiliser. Read the disclaimer.