How to Calculate Fertilizer Rates for Lawn and Garden

Frequently, gardeners are given fertilizer recommendations as pounds of a nutrient (frequently nitrogen) per 1,000 or 100 square feet.  This recommendation tells you how much of the nutrient is needed, but it doesn't outline how much fertilizer is needed.  A few simple calculations are needed to determine how much fertilizer to apply.  

To begin, you need to collect some information, including the recommended fertilizer rate (example 0.5 lbs of N per 1,000 square feet), the grade or analysis of the fertilizer you plan to use (e.g., 24-8-16), and the size of the area being fertilized.  

Background Information  |  Steps to Calculating Fertilizer Rates  |  Calculating Rates for Nutrients Other Than Nitrogen  |  More Information

Background Information

Fertilizer Recommendation

Often, resources provide general fertilizer recommendations for growing specific plants or crops.  If you've conducted a soil test, many soil laboratories will provide a recommended rate of fertilizer to apply based on the test results. 

This recommended rate tells you how much nutrient to supply, but it doesn't tell you how much fertilizer to apply.  That depends on the grade or analysis of the fertilizer you use.

Fertilizer Analysis

The series of three numbers on a fertilizer package indicates the amounts or percentages of nitrogen, phosphate (P₂O₅), and potash (K₂O) in the fertilizer. 

The first number refers to the percentage of the fertilizer's weight in nitrogen.  The second (middle) number gives the percentage of phosphate (which contains phosphorous), and the third refers to the amount of potash (which contains potassium). 

For example, fertilizer analysis of 10-6-4 contains approximately 10% nitrogen, 6% phosphate, and 4% potash. You will notice that these percentages do not add up to 100 (10 + 6 + 4 = 20).  This is because fertilizers contain more than just these three elements.  Most fertilizers also have secondary macronutrients (sulfur, calcium, magnesium, etc.) as well as micronutrients (iron, boron, manganese, etc.).  Additionally, the fertilizer has a filler or carrier.  This inert substance aids in the safe and uniform distribution of the mineral elements.

Calculating Fertilizer Rates

To calculate the amount of fertilizer you need to apply, follow these steps: 

1. Select the fertilizer you plan to use by matching the ratio of nutrients recommended to the fertilizer grade of the product available.
2. Find the area of the garden or lawn to be fertilized in square feet.
3. Determine the amount of fertilizer to apply at the recommended rate.
4. Adjust the amount of fertilizer based on the size of the area to be fertilized.

Step 1: Select the Appropriate Fertilizer 

In many cases, a fertilizer recommendation is given in terms of the amount of nitrogen to apply. This is the simplest way to calculate fertilizer amounts (and the reason resources often give fertilizer recommendations in terms of nitrogen).  The first number in the analysis is the percentage of nitrogen in the bag.  For example, a 10-2-6 fertilizer would have 10% nitrogen. 

Selecting Fertilizer for Recommendations Using a General, All-Purpose Fertilizer

Fertilizer recommendations may be general or more specific.  Many resources giving general fertilizer recommendations will specify the use of a general or all-purpose fertilizer.  An all-purpose fertilizer typically has a fertilizer analysis with a ratio of nutrients that most plants need.  This is often 3-1-2, 5-1-2, and 5-1-3.  In these situations, the fertilizer recommendation is most often given in terms of the amount of nitrogen to apply, making the recommendation relatively simple, straightforward, and easy for the gardener to implement.  

For example, a resource advises applying an all-purpose fertilizer at the rate of 0.15 lbs of N per 100 ft².  Provided the fertilizer has a typical ratio in its analysis, you are not likely to over- or underapply other nutrients, such as phosphorous and potassium.

Selecting Fertilizer for Recommendations Using a Specific Fertilizer

In other situations, the recommendation will be more precise, and it will tell you the fertilizer analysis to use. For example, a resource advises applying a 5-1-3 fertilizer at the rate of 0.10 lbs of N per 100 ft². This fertilizer recommendation is more clear and precise, but it requires the purchase of the correct fertilizer (not allowing you to use that general fertilizer you may already have).  

Any fertilizer with the same ratio could be used. So in this situation, fertilizers with analyses of 10-2-6 or 20-4-12 could also be used and deliver the nutrients in the recommended ratio. If the exact ratio cannot be matched, in many situations, a fertilizer that is close, such as 15-3-10, could be used.

Selecting Fertilizer for Recommendations from a Soil Test

When a soil test is conducted, the amount of nitrogen, phosphorous, and potassium recommended to apply will be given, and you will want to find a fertilizer with an analysis that closely matches that ratio.  If that ratio cannot be found, then separate fertilizers would need to be used to provide each deficient nutrient.  

For example, a soil test tells you to apply nitrogen at the rate of 0.15 lbs of N per 100 ft² and potassium at the rate of 0.15 lbs of K₂O per 100 ft² (a ratio of 1-0-1). Any fertilizer with that same ratio (or nearly so) could be used. 

Step 2: Find the Area of the Garden or Lawn to be Fertilized

Simple measurements can be made to determine the size of the area by multiplying the length and the width.  For example, a vegetable garden that measures 15 feet by 4 feet would be: 15 ft × 4 ft = 60 ft². 

Often, garden and lawn areas are more complex shapes than a simple rectangle.  Use this article to learn how to calculate the area of your planting bed or lawn, regardless of its shape: How do I determine the size of my garden bed or lawn in square feet?

Websites and phone apps are also available to determine the area of your garden bed or yard using online maps.  One example is this site: Measure My Lawn.

Step 3: Calculate the Amount of Fertilizer

To calculate how much fertilizer to apply based on a nitrogen rate, take the rate of nitrogen divided by the percentage of nitrogen in the bag.  

For example, the amount of 10-2-6 fertilizer needed to apply at a rate of 0.15 lbs of N per 100 ft² would be: 0.15 ÷ 0.10 = 1.5.  So, to apply at this rate, you would need to apply 1.5 pounds of the 10-2-6 fertilizer over 100 ft².

Lawns use fertilizer on a different scale, but the calculation is the same.  For example, the amount of 32-0-4 fertilizer needed to apply at a rate of 1 lbs of N per 1,000ft² would be: 1 ÷ 0.32 = 3.125.  So, to apply at this rate, you would need to apply 3.125 pounds of the 32-0-4 fertilizer over 1,000 ft². 

Reference tables have been provided to simplify this step.  Use these tables to determine the amount of fertilizer needed based on the fertilizer's analysis and the recommended rate.  Once known, use that value to move to step 4.

Step 4: Determine the Amount of Fertilizer Needed for the Area Being Fertilized

Of course, your garden or lawn is rarely exactly 100 ft² or 1,000  ft². To determine how much fertilizer is needed for your garden area or lawn, take the fertilizer amount in lbs per 100 ft² or 1,000 ft², multiply it by the size of your lawn, then divide by 100 ft² or 1,000 ft² (whichever is appropriate). 

For example, in the example above, 1.5 pounds of 10-2-6 fertilizer is needed for 100 ft^2, so the amount needed for a 60 ft² garden would be: (1.5 × 60) / 100 = 0.9 lbs.  This means 0.9 pounds of 10-2-6 fertilizer can be applied to the garden area to provide the recommended rate of fertilizer.

In the lawn example above, 3.125 pounds of 32-0-4 fertilizer is needed for 1,000 ft^2, so the amount needed for a 5,625 ft² lawn would be: (3.125 × 5,625) / 1,000 = 17.58 lbs.  This means 17.58 pounds of 32-0-4 fertilizer can be applied to the entire lawn to provide the recommended rate of fertilizer.

Quick Reference Tables: Amount of Fertilizer To Apply

If you want to skip some of the calculations, use the tables below to determine how much fertilizer to apply based on a recommended rate of nitrogen (replace step 3 above). 

Once you know the amount of fertilizer needed for 100 or 1,000 ft², you can determine the amount needed for the garden area or lawn. Just take the fertilizer amount from the table in lbs per 100 ft² or 1,000 ft², multiply it by the size of your garden or lawn, then divide by 100 ft² or 1,000 ft² (whichever is appropriate).  

For many granular fertilizers, 1 cup of fertilizer weighs about 0.5 pounds.

Amount of Fertilizer to Apply Based on Rates of Nitrogen (per 100 ft²)

Amount of Fertilizer to Apply Based on Rates of Nitrogen (per 1,000 ft²)

Calculating Fertilizer Rates Based on Nutrients Other Than Nitrogen

Occasionally, a soil test will indicate a deficiency of a specific nutrient and recommend applying that nutrient at a very specific rate.  For example, a soil test indicates a major deficiency in potassium, but other nutrients are not needed.  A fertilizer like potassium sulfate (0-0-50) can be used to provide potassium, but it will not supply other nutrients like nitrogen (which isn't a problem because additional nitrogen wasn't needed). In this situation, nitrogen cannot be used as the base of the calculations, as it typically is.

Other times, a nutrient may be needed at a level where the ratio of that nutrient to nitrogen cannot be found in a fertilizer. For example, a soil test indicates the need for nitrogen at the rate of 0.15 lbs of N per 100 ft² and potassium at the rate of 0.15 lbs of K₂O per 100 ft² (a ratio of 1-0-1), but indicates phosphorous is at high levels and should not be added.  However, none of the fertilizers available to you have a ratio of 1-0-1.  In this scenario, two different fertilizers could be used - one to supply nitrogen (like sulfur-coated urea, 36-0-0) and one to supply potassium (like potassium sulfate, 0-0-50).  Two calculations must be made, one for each fertilizer.

Calculations based on Phosphorous and Potassium

The first step to calculating a fertilizer amount based on phosphorous or potassium is to determine if the recommendation is given in pounds of phosphate or potash per 100 ft² or 1,000 ft², or if it is given in terms of actual P or K per 100 ft² or 1,000 ft².  

Recommendation Indicates the Amount of Phosphate or Potash

If given in terms of phosphate or potash per 100 ft² or 1,000 ft², then the calculations will look very similar to the steps described above. Find the percentage of phosphate (the second number in the fertilizer analysis) or the percentage of potash (the third number in the analysis) and walk through each of the four steps above to determine the right amount of fertilizer to use.  

Recommendation Indicates the Amount of Actual Phosphorous (P) or Potassium (K)

If the recommendation is given in terms of phosphorous (P) or potassium (K), then an extra step is added to the calculation.  The fertilizer analysis tells you the percentage of phosphate (P₂O₅) and potash (K₂O) in the bag, but these are not equivalent to the amount of actual P or actual K.  

To convert phosphate (P₂O₅) to elemental P, multiply the percentage of phosphate (as indicated by the second number in the analysis) by 0.44. To convert potash (K₂O) to elemental K, multiply the percentage of potash (as indicated by the third number in the analysis) by 0.83. 

For example, the recommendation calls for 0.10  lbs of K (not K₂O) per 100 ft². When calculating the amount of fertilizer needed using a 0-0-22 fertilizer, first multiply the percentage of potash (the third number in the analysis) by 0.83 to get the percentage of elemental K in the bag.  Then divide the rate by that number. So the calculation would be: 0.10 ÷ [0.22 × 0.83] = 0.55.  So to apply at this rate, you would need to apply 0.55 pounds of the 0-0-22 fertilizer over 100 ft².

Calculations Based on Other Nutrients

Overall, the steps for calculating a fertilizer amount based on a nutrient other than nitrogen, such as calcium, boron, or manganese, will look the same as described above.  Start by finding the percentage of that particular nutrient in the fertilizer, and then use that percentage to walk through each of the four steps above to determine the right amount of fertilizer to use.  

Remember, the percentages for nutrients other than nitrogen, phosphorous, and potassium are not indicated in the fertilizer analysis (the three numbers on the fertilizer bag).  Careful reading of the label may be needed to find the percentage.

More Information

• Fertilizing in the Home Garden
• Soil Testing Resources for Home Gardeners
• Managing Garden Soil

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