Granular Spreader Calibration Calculator

The catch test is the source of truth. You walk a measured pass at your real pace, collect what the spreader actually delivers over that known area, and weigh it. Everything else is arithmetic on that one number.

(A) test area   = swath (ft) × distance (ft)
(B) actual rate = (caught lb ÷ test area) × 1000   → lb / 1,000 sq ft
    per acre    = actual rate × 43.56              (1 acre = 43,560 sq ft)
(C) deviation % = ((actual − target) ÷ target) × 100
    band: |dev| ≤ tolerance        → SAFE
          |dev| ≤ 1.5 × tolerance  → CAUTION
          |dev| >  1.5 × tolerance → DANGER
(D) target catch = (target ÷ 1000) × test area     → what you SHOULD collect
(E) ×10 shortcut: size the course to ~100 sq ft and applied rate ≈ caught × 10

Worked example 1. 12 ft swath × 100 ft = 1,200 sq ft. Catch 4.8 lb → (4.8 ÷ 1,200) × 1000 = 4.0 lb / 1,000 sq ft (174.2 lb/acre). Against a 4.0 target that is 0% — on target, no change.

Worked example 2. 3 ft swath × 33 ft = 99 sq ft. Catch 0.26 lb (≈120 g) → ~2.63 lb / 1,000 vs a 2.0 target = +31% — over-applying, so reduce the setting and re-test. At an exactly-100 sq ft course the ×10 shortcut gives the same answer: 0.26 × 10 = 2.6 lb / 1,000.

How to get the caught weight. Use a catch pan under a rotary spreader, sweep-and-weigh the swath on a drop spreader, or weigh the hopper before and after the pass and take the difference. Low-rate granular insecticides and fire-ant baits throw tiny catches — weigh those in ounces or grams.

Forward vs reverse. This tool is the reverse-verify half of the spreader workflow: caught weight → actual rate. Its forward counterpart, the granular insecticide rate calculator, goes the other way — label rate + treatment area → pounds, bags, and cost. They are two halves of one job; we cross-link rather than merge them.

Sources: TheYieldGrid — Fertilizer Spreader Calibration Calculator (test-area, back-calc, and ±5 / ±10 / ±15% banding); Pesticide Environmental Stewardship — Drop Spreaders (the 100-sq-ft course and ×10 shortcut, the 2.0 ± 10% acceptance band). The UMass / STMA ±5% professional tolerance and the iterate-to-tolerance loop back only the optional ±5% preset and the re-test advisory — no formula depends on it (its page 403s to automated fetch; cited with that hedge intact).

Formula: actual rate (lb/1,000) = (caught lb ÷ test area) × 1000; test area = swath × distance

See Sources below ↓