Contact Information

Teton County Courthouse
150 Courthouse Drive - Room 107
Driggs, ID 83422

150 Courthouse Drive- Room 107
Driggs, ID 83422

208-821-0984 (phone)
208-354-8410 (fax)

Summer Hours: May vary. Usually Mon-Thu 8:00 to 6:00


How much chemical do you add to the tank?

An herbicide label specifies the amount to be applied per acre. What is added to your tank depends on the Gallons Per Acre (GPA) your sprayer puts out. GPA depends on:

  • The speed at which the pesticide is applied.
  • The output of your nozzles.
  • The pressure of the sprayer.
  • Product Mixing Rate (oz per gallon, lbs per gallon, etc.) = Application Rate (oz, fl oz, lbs) /GPA

Handheld or backpack sprayer calibration

  1. Measure and mark an area that is 18.5' by 18.5' or equivalent (area is 1/128th of an acre).
  2. Time the number of seconds required to spray the area uniformly using a steady sweeping motion and constant pressure.
  3. Spray into a container for the time calculated in step 2.
    • The fluid ounces collected is the GPA for your sprayer. Volume sprayed in fluid ounces = Gallons Per Acre

Nozzle Volume Method for ATVs, Tractors, etc. with booms or boomless nozzles. Use This Form

  1. Measure the spray width by measuring the wetted area covered by your boom or boomless nozzles.
  2. Collect nozzle output for 1 minute- ounces per minute (OPM) is equal to the volume of liquid collected in ounces.
  3. Use the equation given on the form to calculate GPA.

More Information


  • Calibrate with water, not mixed herbicide.
  • Repeat at least twice and average the values.
  • Ensure that nozzles are clean.
  • Maintain constant pressure in calibration and application.
    • Pumps for most rangeland applications should be between 20 and 30 psi.
    • Pressure control valves are available for backpack sprayers.
  • Use a realistic travel speed for field applications- usually below 5mph. 
  • The minimum total spray volume should be above 10 gallons per acre (10 GPA).
  • Yes. You should calibrate even if you are only spot spraying!
  • Teton County is here to help- contact us for assistance.

Here are helpful videos from our friends in Teton, WY:

Here are useful forms for calibration, and practice questions:

The practice questions are hard. If you can do them, you will do well on the applicator's exam and in real life.

Backpack Calibration Form
Boom/Boomless Calibration Form
Calibration Practice Questions

AH Digital FX Studios, Inc.