Spreader Calibration Calculator
A dial number is calibrated against one manufacturer’s own hopper with one granule size, which is why the setting printed on the bag rarely matches the spreader in your shed. Walk one measured strip, weigh what came out, and this turns that into the rate you really applied and the setting that hits the rate you wanted.
Effective swath is roughly 60–75% of the widest throw — the outer edge lands too thin to count, which is why you overlap wheel tracks.
Throw one pass across a clean drive and measure where coverage genuinely stops.
Walk it at the pace you will really use. Pace meters product as much as the dial does.
Weigh the loaded hopper before and after on a bathroom scale, then subtract.
Optional — needed for a corrected number rather than just a direction.
Product weight, not nitrogen. Get it from the rate calculator.