You buy chemical as a concentrate but apply it as a diluted finished tank mix. This tool walks the five steps a generic job-cost calculator skips — so the per-gallon figure is the cost of mixed solution, not raw concentrate.

A  concentrate $/fl oz  = price ÷ 128         (per-gallon basis; per-oz used as-is; per-container ÷ its fl oz)
B  fl oz / gal of mix   = rate × 1.28         (% mode; oz/gal mode used as-is — ×1.28 is US-gallon-specific)
C  mix cost / gallon    = (B) × (A)           ← headline
D  gallons of mix       = area × coverage     (normalised to canonical sq ft)
E  material cost / job  = (C) × (D)           ← headline
   per-area cost        = (C) × coverage
   marked-up price      = (E) ÷ (1 − markup)  (optional, margin-on-price)

The common incumbent error is pricing the diluted volume at the concentrate price — for the worked example that returns $200 instead of the correct $4.00, a 50× overstatement. We apply the dilution step first so that can't happen. The 2% → 2.56 fl oz/gal and 2.5% → 3.2 fl oz/gal conversions follow the cited US-gallon ×1.28 shortcut.

The concentration band (low under 1, moderate 1–4, high over 4 fl oz per gallon) is shown as information only. The product label always governs dilution and use.

Formula: mixCost/gal = (concentrate $/fl oz) × (fl oz/gal of mix); per-job = mixCost/gal × (area × coverage)

See Sources below