How to use the lawn fertilizer calculator
- Enter the lawn area in square feet. If you need to measure it, use the square footage calculator first.
- Enter the nutrient rate in pounds of nutrient per 1,000 square feet, taken from your product label or lawn care plan.
- Enter the nutrient percent in the product (the first number of the N-P-K on the bag is nitrogen) and the bag weight.
- Read the pounds of product, the number of bags, the area one bag covers and, if you set it, the total for several applications.
Formula
Nutrient needed (lb) = area / 1,000 x rate per 1,000 sq ft
Product needed (lb) = nutrient needed / (nutrient percent / 100)
Bags = product needed / bag weight, rounded up
Coverage per bag = bag weight x percent / 100 / rate x 1,000 sq ft
The tool converts a rate into pounds of product. It does not choose the rate.
A 5,000 square foot lawn
Take 5,000 square feet at 1 pound of nitrogen per 1,000 square feet, with a 20% nitrogen product in 15 pound bags. The nutrient needed is 5 x 1 = 5 pounds. The product is 5 / 0.20 = 25 pounds, which is 2 bags of 15 pounds when rounded up.
One bag covers 15 x 0.20 / 1 x 1,000 = 3,000 square feet. A lawn of exactly 3,000 square feet needs 15 pounds, which is one bag with nothing left over. At four applications a year, the 25 pound figure becomes 100 pounds in total.
Rates come from you
How much nutrient a lawn should get, and when, depends on the grass, the soil, the season and local rules. This tool does not decide that. The 1 pound rate, the 20% product, the 15 pound bag and the single application are example values so the form shows a working result. Replace each one with the numbers on your label and your plan.
Some products are labeled by the area they cover rather than the nutrient rate. If you have that figure, compare it with the coverage line in the result. Follow the label for spreader settings, safe handling and the weather to avoid.
Planning around bag sizes
Bags are rounded up, so the result may leave some product over. If the leftover is large, check whether a smaller bag size gets closer to your need. For seed, sod and soil quantities, see the grass seed calculator and the sod calculator.
Assumptions and limits
- The nutrient rate, the nutrient percent, the bag weight and the number of applications are example values you replace.
- Area is measured in square feet and is assumed to be evenly treated.
- Only one nutrient is calculated at a time. Run the tool again for another percent on the bag.
- No advice on timing, grass type or local rules is given.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
They are the percent by weight of nitrogen, phosphate and potash, in that order. This tool uses one of them at a time, so enter the number for the nutrient whose rate you are following.
How many bags of fertilizer do I need for my lawn?
Divide the pounds of product needed by the bag weight and round up. The result shows both the pounds and the number of bags.
Why does the tool round bags up?
You cannot buy part of a bag, and rounding down would leave part of the lawn short. A figure that lands exactly on a whole number is not rounded.
Can I use kilograms or square meters?
This tool works in pounds and square feet. Convert your measurements first with the unit converter, then enter them in those units.