How to Calculate Liquid Fertilizer Rates

Proper food and water are essential for plants, and our lawn’s turf is no different. By now, I am sure you have an excellent watering system hydrating your yard but are you feeding it the right amount of food? Unfortunately, It’s not as easy as dumping a bag of compost on the lawn and calling it a day. 

Our turf needs fertilizer to grow thick and rich, and not all additives are equal. If you want the best results, you will need to calculate the correct fertilizer rate. How do you calculate a fertilizer rate? That’s what we will find out, as well as how to mix the ultimate grass juice to give you the lawn you’ve always wanted.

Calculating the Fertilizer Rate

The rate of fertilizer needed for grass is typically dependent on the nitrogen needs of your lawn. You usually water soil additives into your lawn, so you will need to add the correct amount of fertilizer to the correct amount of water to cover your entire yard. 

Packages of fertilizer will give you the percentage of nitrogen in the fertilizer solution, but to know how much product to add to the water, you will also need to look at pounds of nitrogen. Then you have to figure out how many gallons of the fertilizer and water solution are required to cover an acre of lawn with adequate growth booster. 

Unless your yard is the exact size that your fertilizer’s label’s premixed formula covers, the ratios, values, and rates will still need some adjusting. You will need to figure out how many acres or percentages of an acre your yard is and make enough solution to cover it all.

Math will be involved, but getting this formula down will save you money when fertilizing your lawn and allow you to prevent over and underfeeding-related disasters. First, what the heck are these numbers and letters? 

What Is NPK? 

18-3-6 Liquid Fertilizer (50% SRN & Micronutrients) (2.5 Gallons)

All fertilizers will prominently display 3 numbers that look like 0-0-0 and they represent the percentages of macronutrients in each sack. The numbers can be high like 16-16-8 for corn or low like 3-1-2 found in lawn fertilizers. Let’s look at what NPK and the corresponding numbers mean for your lawn. 

N – Nitrogen

This is the first number on a fertilizer packet and denotes the percentage of the N macronutrient. Nitrogen is responsible for keeping plants green and helping them grow lush and rich. This is typically the highest percentage in fertilizers as most plants need more nitrogen than any other nutrient to grow.

P – Phosphorous 

The second number found on a fertilizer label is Phosphorous. This number can be as high as nitrogen or as low as 0, depending on the type of plant and the stage of development. P helps seeds germinate and allows plants to build strong roots deep in the soil.   

K – Potassium 

Think of P as the immune system of your grass. While plants don’t usually need tons of this macronutrient, giving your lawn enough potassium will allow it to fight off stress, drought, and disease. It is especially important for cool-season grasses as it aids in turf surviving blazing hot summers and long exposure to the icy winter air.

Common Fertilizer Ratios

Don’t be surprised when you see a huge variety in NPK ratios on fertilizer packages. The variety of numbers covers a whole range of types of plants and feeding needs. But let’s focus on some good numbers for lawn fertilization. 

  • 4:2:1 – Considered the best ratio for lawn growth 
  • 3:1:2 – Ideal for healthy lawn maintenance 
  • 10:1:2 – A ratio intended for a newly planted or severely malnourished turf

Mixing Your Fertilizer 

Now that you understand what the letters and numbers on the label mean, it’s time to put all the information together and calculate the correct fertilizer rate for 1000 sqft and then an acre. After that, you’ll need to adjust the amount of solution in relation to how much larger or smaller your lawn is to an acre. 

Pounds of Nitrogen in Bag

The first thing you need to do is find out how many pounds of nitrogen are in the bag of fertilizer. The easiest way to do this is to find the total weight of the bag and then find your N %. 


  • 25 pounds of fertilizer
  • 4-2-1 Macronutrient Ratio

4 = 4% nitrogen in a 25lb pound bag,  so we multiply 25(weight) by .04(4% N) = 1

This means that one 25lb bag of fertilizer can apply 1lb of nitrogen to a 1,000 sqft area. 

Gallons of Liquid Fertilizer

Using a similar formula as above, we can calculate for liquid fertilizer rates. In this case, you need to find the weight in pounds per gallon and then find the amount of nitrogen per pound. After that, you dilute it to the proper ratio with water


  • 10 pounds of liquid fertilizer (approx 1 gallon)
  • 4-2-1 Macronutrient Ratio
  • 4-8 Gallons of Water

4= 4% nitrogen in a 10lb jug, so we multiply 10(weight) by .04(4% N) = .4 

This means one 10lb jug can apply .4lbs of nitrogen to a 1,000 sqft area. If you wanted to get the full 1lb nitrogen per 1,000 sqft, then you would need to use 2.5 gallons of product or choose a product with a higher N ratio, such as 8 or 16.