It is critical to know the output of a sprayer in order to add the proper amount of herbicide to the tank.  Applying too heavy can kill desirable species and wastes money.  Applying too light contributes to resistance, reduces effectiveness, and wastes money. 

Handgun or Backpack Sprayer

1. Measure a plot that is exactly 18.5 ft X 18.5 ft (1/28th of an acre).

2. Spray the plot uniformly with water, keeping the sprayer pressure constant. Be sure to record the length of time it takes you to spray the entire area using a stopwatch.

3. Spray into a bucket (or a graduated container such as a Nalgene) for the same number of seconds.  Keep the sprayer pressure constant!

4. Measure the number of ounces of water collected in the bucket or Nalgene.

5. The number of ounces of water measured from the bucket is equal to the number of gallons per acre (GPA) the sprayer is delivering.

6.   Add the proper amount of herbicide to your tank using the chart.

Boom or Boomless Sprayer

Volume  Method

1. On level ground, fill sprayer tank to a known level with water.

2. Turn on sprayer until all nozzles have output, shut off and refill tank to desired level.

3. Measure off ¼ of an acre, marking the start and end with a flag.

4. Drive the measured ¼ acre with the speed that you will be using to spray, turning on the sprayer at the starting mark and off at the ending flag.

5. Return to the level ground that you filled the sprayer at and carefully measure the amount of water it takes to refill to the known level from step 1.

6. Multiply the amount from above step by 4. This is GPA (Gallons Per Acre).

Stationary Method

1. On level ground, fill sprayer tank with water and drift agent (if using).

2. Turn on sprayer and measure spray pattern width in feet.

3. Collect liquid from each nozzle for 1 minute. Measure in ounces.

4. Divide ounces by 128 to determine GPM (Gallons Per Minute).

5. Determine speed (MPH) you will be using during spraying.

6. With the below formula calculate GPA (Gallons Per Acre).

      GPA = (GPM x 495)/(MPH x swath width in feet)

More info on sprayer calibration can be found at this link: Invasive Plant Management Guide (pg 17)