How to Calculate Fertilizer Rates for the Lawn

Frequently, gardeners are given lawn fertilizer recommendations as pounds of a nutrient (frequently nitrogen) per 1,000 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.  

The recommendations below are for lawns.  Information about calculating fertilizer rates for garden areas can be found in this article: How to Calculate Fertilizer Rates for the Garden.

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 indicates the percentage of the fertilizer's weight that is 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 16-4-8 contains approximately 16% nitrogen, 4% phosphate, and 8% potash. You will notice that these percentages do not add up to 100.  This is because fertilizers contain more than just these three elements.  Most fertilizers also contain secondary macronutrients (sulfur, calcium, magnesium, etc.) and micronutrients (iron, boron, manganese, etc.).  Additionally, the fertilizer has a filler or carrier.  This inert substance aids in the safe, uniform distribution of 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 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 25-0-4 fertilizer would have 25% nitrogen. 

Selecting Fertilizer for General Recommendations

Fertilizer recommendations may be general or more specific.  Many resources giving general fertilizer recommendations will specify the use of a lawn fertilizer.  A lawn fertilizer typically has a ratio with a high amount of nitrogen and a low amount of phosphorus.  

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

Occasionally, the recommendation will be more precise and specify the fertilizer analysis to use, because it is being used for new seeding or because a soil test indicates other nutrients are deficient.  For example, a resource advises applying an 18-12-14 fertilizer at the rate of 0.10 lbs of N per 100 ft². This fertilizer recommendation is clearer and more precise, but it requires purchasing the correct fertilizer (so you can't use the general one you may already have).  

Any fertilizer with the same ratio could be used. So in this situation, fertilizers with analyses of 9-6-7 or 27-18-21 could also be used and deliver the nutrients in the recommended ratio. If the exact ratio cannot be matched, in some situations a fertilizer close to it, such as 10-10-10, 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 0.75 lbs of N per 1,000 ft² , phosphorus at zero, and potassium at the rate of 0.5 lbs of K₂O per 1,000 ft² (a ratio of 3-0-2). Any fertilizer with that same ratio (or nearly so) could be used. Alternatively two sperate fertilizers could be applied such as urea (46-0-0) and muriate of potash (0-0-60)

Step 2: Find the Area of the 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, an lawn area that measures 75 feet by 25 feet would be: 75 ft × 25 ft = 1,875 ft². 

Often, 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 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 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 lawn is rarely exactly 1,000  ft². To determine how much fertilizer is needed for your lawn, take the fertilizer amount in lbs per 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, 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 1,000 ft², you can determine the amount needed for the lawn. Just take the fertilizer amount from the table in lbs per 1,000 ft², multiply it by the size of your lawn, then divide by 1,000 ft².  

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

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

More Information

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

Photo credits: 1:The Toidi/AdobeStock; 2: murdocksimages/AdobeStock