Water Quality Index Calculator
The Water Quality Index (WQI) is a composite score that combines multiple water quality measurements into a single number from 0 (very poor) to 100 (excellent), making it easy to communicate overall water quality status. This calculator implements a simplified version of the National Sanitation Foundation (NSF) WQI, using four parameters weighted by their relative importance to aquatic health and human safety: dissolved oxygen (0.31), nitrate (0.27), pH (0.22), and turbidity (0.20). Each parameter is scored 0-100 using standardized sub-index functions, and the WQI is the weighted sum.
WQI formula (weighted sub-index)
WQI = (DO_score * 0.31) + (NO3_score * 0.27) + (pH_score * 0.22) + (Turb_score * 0.20)
Sub-index scores are 0-100 based on parameter-specific scoring curves
Categories: 90-100 Excellent, 70-90 Good, 50-70 Medium, 25-50 Bad, 0-25 Very Bad
Sub-index scoring: DO uses a linear scale from 0 (0% sat.) to 100 (100% sat.); nitrate uses 100 at 0 mg/L declining to 0 at 100 mg/L; pH uses 100 at pH 7.0 declining to 0 at pH 2 or 12; turbidity uses 100 at 0 NTU declining to 0 at 100 NTU. All sub-index scores are clamped to the range 0-100.
Understanding water quality parameters
- Dissolved oxygen below 5 mg/L (roughly 60% saturation at 20 degrees C) is considered hypoxic and stresses most aquatic species.
- Nitrate above 10 mg/L N (the EPA drinking water standard) causes methemoglobinemia (blue baby syndrome) in infants.
- pH outside 6.0 to 9.0 is lethal to most fish species. Acid rain can lower lake pH below 5.0 in sensitive watersheds.
- High turbidity reduces light penetration, harming aquatic plants and providing habitat for pathogens.
- EPA's Clean Water Act requires states to set water quality standards for all designated water bodies, setting minimum sub-index thresholds for regulatory purposes.
Frequently asked questions
What is a Water Quality Index (WQI)?
A WQI is a composite score that reduces multiple water quality parameters into a single number, typically ranging from 0 (very poor) to 100 (excellent). It helps communicate overall water quality to non-technical audiences. Different organizations use different parameters and weighting methods.
What parameters are used in this WQI?
This calculator uses four key parameters from the NSF (National Sanitation Foundation) WQI method: pH (weight 0.22), dissolved oxygen as percent saturation (weight 0.31), turbidity in NTU (weight 0.20), and nitrate in mg/L as N (weight 0.27). Each parameter is scored 0-100 and multiplied by its weight.
What are good values for each parameter?
For drinking water and aquatic health: pH 6.5-8.5 is ideal (EPA MCL); dissolved oxygen above 90% saturation is excellent (80% is the typical minimum for healthy fish); turbidity below 1 NTU is excellent (4 NTU is the EPA MCL for drinking water); nitrate below 2 mg/L N is good (10 mg/L N is the EPA MCL).
How does the NSF WQI scoring work?
Each parameter is converted to a sub-index score (0-100) using lookup curves developed from expert opinion. The overall WQI is the sum of (sub-index * weight) for all parameters. A WQI above 90 is Excellent; 70-90 is Good; 50-70 is Medium; 25-50 is Bad; below 25 is Very Bad.
Where can I find water quality data?
EPA's Water Quality Portal (waterqualitydata.us) provides monitoring data from USGS, EPA, and state agencies. USGS National Water Quality Assessment (NAWQA) program publishes data on streams and groundwater across the US. Local utilities publish annual water quality reports under EPA's Consumer Confidence Rule.
Official sources
- EPA: EPA Water Quality Portal.
- USGS: USGS Water Resources.
- EPA: National Primary Drinking Water Regulations.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.