How Many Bags of Fertilizer Do I Need?

Fertilizer Calculator
Lawn Area ft. by ft.
of Bag
(See Bag)
        % N
        % P
        % K
Amount of
N, P, or K Needed
(Soil Test)
lbs. N per 1000 sq.ft.
lbs. P per 1000 sq.ft.
lbs. K per 1000 sq.ft.

The number of bags of fertilizer you need depends on the size of the bag, the NPK percentages, and results of your soil test. A soil test tells you how many pounds of nitrogen (N), phosphorus (P), and potassium (K) you need per 1,000 square feet of lawn. For a typical lawn, you need about 1 pound of nitrogen per 1,000 square feet. However, depending on your soil conditions, you may need less or more.

Nitrogen, phosphorus, and potassium are known as macronutrients. They are the essential building blocks for plant cell structures and biochemical reactions within plant cells. Plants also need carbon, hydrogen, and oxygen, however, these three elements are readily available from the air and water. On the other hand, nitrogen, phosphorus, and potassium can only be obtained from the soil. A plant's growth is bounded by the amount of N, P, and K in the soil, so it is important for gardeners, farmers, and horticulturists to apply fertilizer.

To determine how many bags of fertilizer you need, the first step is to perform a soil analysis using a home soil test kit. You can also contact professional landscaping and gardening companies in your area, and they can analyze the soil for you. Some fertilizer companies will also perform soil analyses on mailed samples. The soil test results will tell you how many pounds of nitrogen, phosphorus, and potassium you need per 1,000 square feet of your lawn. (For reference and comparison, one acre = 43,560 square feet, and 1,000 square feet = 0.029957 of an acre.)

Next, find the value of four variables:

  • N, the lbs of nitrogen needed per sq. ft. E.g. N = 0.9
  • n, the percent of nitrogen as shown on the bag of fertilizer E.g. n = 28
  • w, the weight of the bag in pounds E.g. w = 3
  • A, the area of your lawn in sq. ft. E.g. A = 1300

With these variables, you now find the number of bags of fertilizer needed with this formula:

Bags = (AN)/(10wn)

Example Calculation: Suppose our soil test results indicate that we need to apply 0.9 lbs of nitrogen per 1,000 sq. ft., and suppose our lawn is 1,300 sq. ft. Also suppose we have a three-pound bag of fertilizer that is 28% nitrogen. So we have the values N = 0.9, A = 1300, w = 3, and n = 28. Then we obtain

Bags = (1300*0.9)/(10*3*28)
= 1170/840
= 1.393

So we need just a little less than one and a half bags of fertilizer for our lawn. You can perform the exact same calculations for the amounts of phosphorus and potassium needed. Be sure to buy the right kind of fertilizer for your soil's needs. Different fertilizers have varying ratios of N, P, and K. For example, potash is a kind of fertilizer that just contains potassium (K).

© Had2Know 2010