How to Do Fertilizer Calculations

Calculating fertilizer is one way many property owners decide how much fertilizer to use on their lawns. By spreading the right amount of fertilizer, grass and plants are able to grow in optimal conditions. Fertilizer calculations can easily be done on your own, or through a professional lawn care analysis.

When purchasing fertilizer, it's important to check the bag for a guaranteed analysis. This is a list of nutrient percentages that each bag contains. They are usually measured in pounds per 1,000 square feet.

It should be noted that fertilizer isn't entirely composed of nutrients. You will have to check the percentage of nutrients in the bag and calculate how much you need to spread in order to reach the recommended amount of nutrients per 1,000 square feet.

This is why learning how to calculate fertilizer is so important – too much, and your lawn may not be able to handle it; too little, and the lawn may wither and fade.

In general, fertilizer calculations are done through the following:

  • Weight of fertilizer x percentage of nutrients in fertilizer = weight of nutrient in the fertilizer
  • Weight of fertilizer = (Weight of nutrient)/(% of nutrient)

Here is a sample calculation:

Problem: You need to spread 2 pounds of nitrogen per 1,000 square feet on your 5,000 sq. ft. yard.


  1. Calculate the amount of nitrogen you need, which is 10 pounds (2 pounds for every 1,000 square feet = 10 pounds for 5,000 square feet).
  2. Let's say you purchased a five-pound bag of fertilizer with a fertilizer analysis of 13-13-13. This means it contains 13% nitrogen.
  3. Calculate the weight of nitrogen in each bag by multiplying the weight by the nutrient percentage:

    5 pounds x .13 nitrogen = .65 pounds of nitrogen in each bag

  4. Calculate the number of bags needed to obtain 10 pounds of nitrogen

  5. 10 lbs / .65 lbs = 15.38 bags

  6. Round up to get the number of whole bags needed to fertilize your yard. In this case, 16 five-pound bags of fertilizer should be enough to cover your yard.

Why Calculate?

There are several reasons why you should calculate the amount of fertilizer before purchasing.

For one, it helps you avoid wasting money on excess fertilizer. Depending on the quality, it can rack up quite the expense. Avoid overspending by making sure you're only purchasing what you need. Also, you may need to fertilize again within a few months or by the next season, which makes it even more crucial to watch for high costs.

Secondly, fertilizer only needs to be used in moderation. Excess fertilizer will not necessarily make your lawn green or healthy. Fertilizer that isn't completely utilized by the soil may turn into runoff, which can cause pollution in the groundwater and surface environment. Having just the right amount of fertilizer can help it grow to its fullest potential and last longer through the seasons.

If you're finding difficulty in getting the accurate amount of fertilizer for your lawn, consider reaching out to a professional lawn care service for help. It may be easier for a professional to calculate the amount of bags you need, so that you make sure you're purchasing just the right amount.

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