Water Quality Index Calculator
Free Water quality index Calculator for ecology & environmental. Enter variables to compute results with formulas and detailed steps.
Formula
WQI = SUM(Qi x Wi)
The NSF Water Quality Index is calculated by converting each parameter measurement to a Q-value (0-100) using standard curves, then multiplying by the parameter weight (Wi). Weights reflect relative importance: DO (0.17), pH (0.11), BOD (0.11), Temperature (0.10), Turbidity (0.08). The weighted Q-values are summed to produce the final WQI score from 0-100.
Worked Examples
Example 1: Clean River Assessment
Problem: A river has DO=9.5 mg/L, pH=7.4, BOD=1.5 mg/L, Temp=18C, Turbidity=5 NTU. Calculate the WQI.
Solution: DO saturation at 18C = 9.54 mg/L, %sat = 99.6%\nQ(DO) = 99.6 (near optimal)\nQ(pH) = 96.0 (near-neutral, excellent)\nQ(BOD) = 92.5 (very low organic load)\nQ(Temp) = 90.0 (close to reference 20C)\nQ(Turb) = 90.0 (clear water)\nWQI = 99.6(0.298) + 96(0.193) + 92.5(0.193) + 90(0.175) + 90(0.140)\nWQI = 29.7 + 18.5 + 17.9 + 15.8 + 12.6 = 94.5
Result: WQI = 94.5 (Excellent) | Suitable for drinking, swimming, and aquatic life
Example 2: Polluted Urban Stream
Problem: An urban stream: DO=3.0 mg/L, pH=6.2, BOD=15 mg/L, Temp=28C, Turbidity=60 NTU. Calculate WQI.
Solution: DO saturation at 28C = 7.83 mg/L, %sat = 38.3%\nQ(DO) = 38.3 (critically low)\nQ(pH) = 66.0 (acidic)\nQ(BOD) = 11.5 (heavy organic load)\nQ(Temp) = 48.0 (elevated temperature)\nQ(Turb) = 50.0 (murky water)\nWQI = 38.3(0.298) + 66(0.193) + 11.5(0.193) + 48(0.175) + 50(0.140)\nWQI = 11.4 + 12.7 + 2.2 + 8.4 + 7.0 = 41.7
Result: WQI = 41.7 (Bad) | Unsuitable for recreation; aquatic life impaired
Frequently Asked Questions
What is the Water Quality Index (WQI)?
The Water Quality Index is a single numerical value that summarizes multiple water quality parameters into an overall assessment of water condition. Developed by the National Sanitation Foundation (NSF) in 1970, the WQI considers up to nine parameters including dissolved oxygen, pH, BOD, temperature change, total phosphates, nitrates, turbidity, total solids, and fecal coliform. Each parameter is converted to a quality score (Q-value) from 0-100 using standardized curves, then multiplied by a weight reflecting its importance. The weighted scores are summed to produce the final WQI. Scores range from 0 (worst) to 100 (best quality water).
How does temperature affect water quality?
Temperature profoundly affects water quality through multiple mechanisms. Warmer water holds less dissolved oxygen (solubility decreases roughly 1-2% per degree Celsius increase), stressing aquatic organisms. Temperature directly affects metabolic rates of aquatic life, with most species having narrow optimal ranges. Elevated temperatures from thermal pollution (power plant discharge) or climate change can disrupt spawning, migration, and growth cycles. Temperature also affects the rate of chemical reactions and microbial decomposition. Rapid temperature changes are especially harmful; most aquatic organisms can tolerate gradual changes but not sudden shifts of more than 2-3 degrees.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.
How accurate are the results from Water Quality Index Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
What formula does Water Quality Index Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.