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Granular applicator and spreader calibration

Quotable answer

How do I calibrate a granular pesticide spreader?

Weigh everything the spreader lays down across your swath width over a measured course length, then scale that weight up to a full acre (43,560 square feet): pounds per acre equals pounds collected times 43,560, divided by the course length times the swath width in square feet.

Backpack Boom Airblast Granular
Step 2, guided catch test

Run the weighed catch test

Run the spreader over a measured course at your normal setting and ground speed, catch (or sweep up) everything it puts down across a known swath width, weigh it, then scale that weight up to a full acre (43,560 square feet) to get pounds per acre.

Diagram of a granular spreader catch test: a hopper spreader casting granules across a measured plot marked with dashed boundary lines, next to a balance scale weighing the collected catch.
Catch everything spread across a measured plot and weigh it, then divide by the area.
  1. Measure your spreader's effective swath width in feet.
  2. Mark and drive a measured course length, at your normal ground speed and hopper setting.
  3. Catch or sweep up everything applied across the swath width for that course length and weigh it, in pounds.
  4. Output rate (pounds per acre) equals pounds collected times 43,560, divided by the area covered (course length times swath width, in square feet).

lb/acre = (lb collected x 43,560) / (course length x swath width, sq ft)

Step 3, check against the label

Compare to the label's target rate and load the hopper

Enter the label's target rate and the area you plan to cover. The workflow shows how far your calibrated rate is from target and how many pounds to load for that area.

The pesticide label is the legal controlling document. This workflow is a study and field-planning aid, never a substitute for label directions. Learn more about how output rate is calculated.

FAQ

Common questions

Frequently asked questions

Do I catch the granular product or just measure hopper weight change?
Either works, as long as you weigh what actually left the hopper over the measured course, either by catching it on a tarp across the swath width or by weighing the hopper before and after the pass. Catching on a tarp is usually more accurate for narrow swath widths; weighing the hopper is easier for wide broadcast spreaders.
What if my calibrated rate does not match the label's target rate?
Adjust your ground speed or the spreader's gate or dial setting, then recalibrate, never just apply for a different amount of time to compensate. The comparison tool below shows the percent difference and which direction to adjust.
Does wind affect granular calibration the way it affects sprays?
Less directly for the catch test itself, since granular material is heavier and less drift-prone than a fine spray droplet, but wind can still push lighter granules off a narrow swath during application. Calibrate on a calm day for the cleanest catch, then account for wind separately when you actually apply.