Ethan Carter, Regional Crop IPM Agent, and Kalyn Waters, Holmes County Agricultural Agent

Calibrating a a wide-swath boomless spray nozzle, requires different calculations than are normally used for typical sprayers.  Credit: J. Boyd – U. of Arkansas

Trees, fences, and uneven ground can make applying pesticides to pastures with a boom-type sprayer challenging. One way to avoid these hazards is to utilize a boomless sprayer. Like a traditional boom-type sprayer, a boomless sprayers is able to cover a wide swath of ground with each pass, but without the cumbersome, rigid boom. Boomless sprayers utilize only one or two centrally mounted nozzles (as seen in the photo above).

Like with any piece of spray equipment, accurate calibration is key to effective use of a boomless sprayer.  The 1/128th Acre Method, is a commonly used calibration protocol. While this method is ideal for typical boom-type sprayers, it is less effective for boomless sprayers. One reason for this is that the method relies on nozzle spacing to determine the course distance traveled during the calibration process (340 divided by nozzle spacing in feet). Obviously, with only one or two nozzles this method will not work. However, there are several other easy methods that can be used to calibrate boomless sprayers.

Before beginning the calibration process some preparation is necessary. Fill the clean sprayer 1/4 to 1/3 full of water. Collect all necessary supplies; pencil and paper, measuring tape and/or wheel, stop watch, 5 gallon bucket, smaller bucket or heavy duty garbage bag, and measuring pitcher. Pin flags are not absolutely necessary but they can be helpful. The calibration course can take several hundred feet, find a location with terrain similar to what you will typically be spraying and plenty of open space. Note: This process is a two person job. 

Method 1

    • Determine the width of the swath covered by the sprayer. Operate the sprayer at desired pressure and engine/PTO RPM (with the spray vehicle stationary). Measure the total width of the spray swath. The effective width of the spray swath is approximately 80% of the total width.
    • Based on the effective width of the spray swath, select the corresponding course distance from the table below. If your spray width is not shown, simply divide 5460 (1/8th of an acre) by the width of the spray swath, in feet.
    • Measure and mark the appropriate course length.
    • Determine the desired spray vehicle operating speed (gear). Remember, the engine RPM must remain the same as it was when the spray swath was measured. This may take some trial and error. Be sure you have settled on a comfortable speed/gear before moving on to the next step.
    • Drive the course, using the predetermined gear/RPM. Record the amount of time (seconds) it takes to travel the course. To insure the spray vehicle is at full operating speed for the entire course, a rolling start should be utilized. Drive the course and record the time at least three times. Calculate the average time.
    • With the spray vehicle stationary and the engine/PTO at the same predetermined RPM, engage the sprayer and catch/collect the entire spray output for the number of seconds it took to travel the course. Catching all of the spray output can be challenging. You can place a bucket or heavy duty garbage bag over the spray nozzle(s) and use it to direct the spray into the collection bucket. The exact technique will vary depending on your sprayer’s nozzle configuration. There will likely be several gallons of spray to be collected. It is important that the entire spray output is collected.
    • Measure the amount of water collected. The number of pints (16 ounces) collected equals the sprayers output in gallons per acre (GPA).

Spray Width (feet)

Course Length (feet)



Method 2

This method utilizes the Gallons Per Acre equation GPA = (5940*GPM)/(MPH*W). 5940 is a constant, used to for unit conversion. The constant is multiplied by your gallons per minute (GPM), which is found by collecting nozzle output for one minute and converting it to gallons.

  • With the spray vehicle stationary, collect the spray output for one minute. See step 6 above for a description of how to collect the spray output.
  • Measure the collected spray output. This measurement needs to be in gallons. Some conversion of units will likely be necessary (1 gallon = 128 ounces or 3786 milliliters). This number can now be entered into the GPA equation as “GPM”.
  • Determine your MPH (even if your equipment has a speedometer it is still good to confirm MPH). This can be done through the MPH formula: MPH = (distance (ft)*60) / ( time (sec)*88).  When using this equation, 100ft is a good distance to use. With the spray vehicle operating exactly like it would be while spraying (gear and engine/PTO RPM), travel the 100ft at least three times. Record the travel times (seconds), calculate the average. Complete the MPH formula. The result can be entered into the GPA equation as “MPH”.
  • Determine the width of the swath covered by the sprayer. Operate the sprayer at desired pressure and  predetermined engine/PTO RPM (with the spray vehicle stationary). Measure the total width of the spray swath. The effective width of the spray swath is approximately 80% of the total width. The spray swath must be measured in inches. That value can be entered into the GPA equation as “W”.
  • Using the GPM, MPH, and W figures you just determined, Complete the equation and solve for GPA: (5940*GPM)/(MPH*W).


Regardless of the method used, after determining your sprayer’s GPA output there are still more factors to consider.  Is your GPA acceptable? 15-20 GPA is generally the targeted range. Lower spray volumes can reduce pesticide effectiveness and excessively high spray volumes are impractical due to excessive stops to refill the spray tank. If your calculated spray volume is unacceptable it can be adjusted in a variety of ways. The type of spray nozzle can be changed; with boomless sprayers, the orifice size of the nozzle can have considerable effect on the spray volume. Spray pressure can be adjusted somewhat, slightly effecting spray volume. The simplest and most effective way to adjust spray volume is to change the speed of the spray vehicle. Increasing speed decreases GPA while decreasing speed increases GPA. Note: If spray vehicle speed is adjusted, the entire calibration process needs to be repeated to calculate the new GPA.

Why is knowing the sprayer’s GPA important? Knowing the sprayer’s gallons per acre output allows you to calculate how many acres you can cover per spray tank. GPA / Gallons (size of spray tank) = Acres covered per tank. Knowing the number of acres covered per tank allows for accurate amounts of pesticide to be used (Pesticide use rates are generally expressed on a “per acre” basis).

Example: Sprayer output = 20 GPA; Sprayer has a 100 gallon spray tank; Recommended herbicide rate is 24 oz./ac
100 gal / 20 GPA = 5 acres per spray tank.
5 acres * 24 oz/ac = 120 oz per spray tank.

All pesticide applicators should know their sprayer’s GPA. After you’ve gone through the calibration process, write down the sprayers GPA and gear/RPM.  Note: If the sprayer is used on a different spray vehicle the calibration process should be repeated.

The calibration process for any sprayer is very important and fairly involved. Do not hesitate to contact you county’s extension agriculture agent for assistance. We have all the necessary equipment and will be happy to assist you.


For more information or resources, see the following articles:


Kalyn Waters