Sprayer Calibration Formulas: How to Calculate GPM & GPA

Precision is key in agricultural and turf spraying, and the right sprayer volume per acre, spray nozzle size, pressure, and spraying speed can make all the difference. But how do you determine these variables for your sprayer? Fear not, because we’ve got you covered. In this post, we’ll walk you through the formulas and tools you need to calculate sprayer flow rate, gallons per acre, and more. Whether you’re a seasoned vet or a newbie in the field, you’ll find this guide helpful in calculating the optimal settings for your sprayer. 

Sprayer GPM Formula

Before spraying, it’s important to calculate the flow rate needed to apply the correct amount of liquid to your target area. Calculating this flow rate will tell you the exact gallons per minute you need per spray nozzle. With this information, you can select the right spray nozzle size to use. 

This can be calculated using the following formula:

Flow rate (GPM) = (Gallons per acre x Speed (MPH) x Nozzle width) / 5940

To use this formula, you will need to input the following data:

• Gallons per acre: This refers to the amount of liquid you want to apply to a single acre of land. Depending on what you’re spraying, this can vary. Every pesticide or herbicide product will have a recommended ratio of chemical to water and a total amount of that spray mixture to apply per acre. To determine the appropriate volume for your situation, you may need to refer to information from your seed, pesticide, or fertilizer manufacturer, or consult with a local agronomist.
• Speed (MPH): This is the speed at which you will be driving your sprayer. Speed can be a hard thing to identify if your machine does not have a speedometer. You can use a GPS unit to measure your speed or time how long it takes you to travel a set distance at your desired speed to calculate your mph. (More on this below)
• Nozzle width: This is the width between the nozzles on your sprayer. This is measured from center to center of each spray nozzle. You can usually find this information in your sprayer’s manual or by measuring the distance yourself. Nozzle spacing is key. The wider the nozzle spacing the lower the Gallons per acre applied if all other factors remain constant.

Now let’s look at an example. Let’s say you have a 5-acre field that you need to spray with a herbicide. Your herbicide label instructs you to apply your chemical mixed with water at a rate of 10 gallons per acre. You find that your average speed will be about 6 miles per hour and your sprayer boom has nozzles spaced 30 inches apart. 

To calculate the flow rate of the sprayer nozzle that you need, you can use the formula above. Plugging in your values for GPA, speed, and spacing we get: (10 x 6 x 30) / 5940. This means your flow rate for each nozzle is = 0.303 GPM.

RELATED: How to Use Spray Nozzle Charts

So, you will need a set of nozzles on your sprayer that can deliver a flow rate of 0.303 gallons per minute each to achieve the desired rate of 10 gallons per acre.

To save time and allow you to experiment with different rates and speeds, You can use this calculator to input different variables:

Total Sprayer GPM

The GPM per nozzle multiplied by the total number of nozzles on your boom equals the total GPM of your sprayer.

Calculating Nozzle Size For Gallons Per 1000/Square Feet

Not all applications prescribe spray rates in gallons per acre. There are applications where you may need to determine your nozzle size based on a rate measured in gallons per 1000 square feet. 

In this case, you would need to use this formula to determine the flow rate needed from each sprayer nozzle:

GPM (per nozzle) = (GAL/1000 ft2 x MPH x W) / 136

Key:

• GPM: Gallons per minute
• GAL/1000 ft2: Gallons per 1000 square feet
• MPH: Miles per hour
• W: Nozzle spacing (in inches)

How to Use This Formula

Let’s say you’re preparing to spray herbicide on your 2,500-square-foot lawn. You have a small boom sprayer with nozzles that are spaced 20 inches apart. You want to travel at a speed of 4 miles per hour. You need to find what size nozzles you need to achieve your target application rate of 2 gallons of spray solution (chemical & water mixed) per 1000 square feet.

In this scenario, you would enter 2 for the application rate (GAL/1000 ft2), 4 for the sprayer speed (MPH), and 20 for the spray nozzle width (inches). The calculator would tell you that you need a flow of 0.37 GPM per nozzle to achieve your desired application rate.

Using this information, you can choose the appropriate nozzle size, based on a spray nozzle chart, that will produce your application rate. You can refer to this article on selecting spray nozzles for information on finding and using a nozzle chart.

You can also use this calculator instead of doing the calculation yourself:

What About Pressure?

Changing pressure certainly does affect the output of your sprayer. Spring at a higher pressure will result in a higher application rate if all of the things I kept constant, and a lower pressure lower application rate.

When can you get your results from punching your numbers into this formula or calculator you will get a gallon per minute flow needed from each nozzle in your room to achieve your desired application rate. 

To choose the right nozzle, you refer to the nozzle size chart and find the nozzle that will give you the needed flow rate at a pressure suitable for that type of droplet and that your sprayer can produce. 

Not sure what a nozzle chart is? No problem, this article covers where to find a nozzle chart, how to read it, and the different nozzle types.

Gallons Per Acre Calibration Formula 

What if you have a sprayer and want to know how many gallons per acre (GPA) you can apply at different speeds, or with different nozzle sizes?

This can be calculated using the following formula:

Gallons per acre = (5940 x GPM per nozzle) / (MPH x Nozzle spacing)

This calculation is similar to the one above. We need to know speed and nozzle spacing but we will also need to know the gallons per minute per nozzle. This refers to the flow rate of each spray nozzle on your sprayer. To determine this value, you can consult your sprayer’s manual or do a “catch test”. 

A catch test involves Using a container to “catch” water as it comes out of one of the nozzles on your sprayer.

You would park your sprayer and run it at your desired operating pressure, collecting water from one nozzle for 60 seconds. The volume of water that you collect is the GPM you would use in your calculations.

If you have several nozzles on a large spray boom, you would want to collect from multiple nozzles on each end of your boom and at the center and then average them. 

There is a much easier way to determine the flow rate of your nozzles. Especially, if you have a conventional boom with flat fan nozzles. There is a standard color code that indicates the flow rate of a sprayer nozzle, you can find all the details in this article about sprayer nozzles. 

This universal numbering system allows you to know the expected flow rate of any nozzle at 40 PSI. For example, if you have yellow-colored nozzles they are designed to spray 0.2 gallons per minute at 40 psi. A catch test is still a good way to confirm this especially if you have old nozzles that may be worn. 

It’s valuable to know and understand this formula so you can Gather the information you need and make these calculations yourself. but to save time and make it easier to see what nozzle size or speeds will get you a different application rate you can use this Calculator.

This calculator allows you to quickly change the variables to see what happens to your sprayer output at various speeds and nozzle sizes. 

Gallons Per 1000/Sq Ft Calibration Formula

Gallons per acre is the most common way to measure application rates for large fields, but if you are working on a small area or lawn and want to calculate your output. You can determine the gallons applied per 1000 square feet. 

Here is the formula:

GAL/1000 ft2 = (136 x GPM per nozzle)/(MPH x W)

As with GPA, you will need to know your nozzle GPM, speed, and nozzle spacing

• GPM: Gallons per minute per nozzle
• GAL/1000 Sq ft: Gallons per 1000 square feet
• MPH: Miles per hour
• W: Nozzle spacing (in inches)

You can also use this calculator if you don’t want to calculate the math yourself:

Using Gallon/1000 Sq Ft Formula

Let’s say you have a yard 50 x 70 ft. You want to know how many gallons of liquid your sprayer is applying. You have a sprayer with three nozzles that each spray 0.5 gallons per minute (GPM). You will be driving the sprayer at a speed of 4 miles per hour (MPH) and the nozzle spacing is 20 inches.

To calculate the gallons of pesticide needed per 1000 square feet, follow these steps:

1. Enter the GPM per nozzle, in this case 0.5, then speed (4 mph), and nozzle spacing (20 inches) into the appropriate spots in the calculator.
2. Click the “Calculate” button.
3. The calculator will display the result in gallons per 1000 square feet.
4. Your area, 50 x 70 ft = 3500 square feet. So take your result x 3.5 to get the total amount you will apply. 
5. You can compare this amount to how much liquid is left in your tank vs when you started spraying. This will provide you guidance on calibrating your sprayer and help you determine if you need to change nozzles or adjust pressure, or speed. 

Calibrating Your Sprayer

Using these formulas will provide you with the information you need to apply the proper amount of liquid to your land. However, it is still necessary to physically calibrate your sprayer. 

These formulas and calculators are based on the assumption that all other variables are consistent. The fact is, your sprayer’s performance will probably not match the results you get from these calculators. There are just too many factors at play. For instance, you might set your pressure at 40 psi, but the actual pressure at the nozzle is 38 psi. 

For these reasons, you must calibrate your sprayer to ensure accuracy. If you have not done this before, you can find more guidance in this post about calibrating an ATV sprayer. 

Calibrating involves spraying water over a measured area of land at a constant speed. The actual volume of liquid sprayed is then measured and compared to the results you get from these calculators. This allows you to make adjustments to your pressure, speed, etc., to ensure you hit your target rate. 

Calculating the Travel Speed of a Sprayer 

Speed is an important factor for calculating your GPM or how many gallons per acre you are applying. If you spray with a machine that has no speedometer, like a mower, old tractor, ATV, etc. You can still figure out your speed.

You will need to drive your machine over a measured distance, usually 100 or 200 ft. Run in the gear and at the throttle position that you will spray at. Time how long in seconds, it takes to cover the distance.

You can use this formula to calculate your speed:

Speed (in mph) = (Distance (in feet) x 60) / (Time (in seconds) x 88)

The math is simple, but you can use this calculator to make it easy: 

Conclusion

Accuracy when spraying is important to limit waste and keep from damaging your lawn or crops. If you want to ensure that you are accurately applying liquid with your sprayer you should know these formulas or at least have these calculators handy!

*Note: Calculators on this page were built using help from https://chat.openai.com/