Water Quality Index (WQI) Calculator

This calculator builds a single 0-100 composite score out of seven separate water quality measurements, following the same general approach as the NSF Water Quality Index: each parameter is first converted to its own 0-100 sub-index using a curve tailored to how that parameter actually behaves. Dissolved oxygen scores highest near 100% saturation and falls off in either direction; pH scores highest near neutral (7.0) and drops off sharply once it strays outside the 6.5-8.5 range that most aquatic life tolerates; BOD, nitrate, phosphate, and turbidity are all "lower is better" curves that hit zero once they cross a pollution threshold (30 mg/L, 50 mg/L, 10 mg/L, and 100 NTU respectively); and temperature change scores highest with no deviation from baseline. The seven sub-indices are then combined into the final score using weights adapted from the standard NSF weighting scheme, renormalized to sum to 1.0 since this calculator omits two of the original nine NSF parameters (fecal coliform and total dissolved solids).

Because those two parameters are missing, treat the result as an approximation of the real NSF WQI rather than a certified equivalent — it's most useful for tracking relative water quality trends at one site over time, or comparing sites measured the same way, rather than for regulatory reporting. The final score maps to a five-tier classification (Excellent, Good, Medium, Poor, Very Poor) and a suggested range of suitable uses, from drinking water at the top down to industrial cooling only at the bottom. The sub-index breakdown is often more actionable than the composite number itself: if your overall score is dragged down by one parameter (say, phosphate from agricultural runoff), the sub-indices make it easy to see exactly where the problem lies rather than guessing from the composite score alone.