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Water Quality Index Calculator

Work out water quality index instantly with clear inputs, formula shown and shareable results.

About Water Quality Index Calculator

A water quality index converts individual parameters into 0-100 sub-indices and combines them with weights reflecting their importance. The weighted arithmetic form is simple but can mask a single very poor parameter; the geometric form penalises poor sub-indices more heavily, which is why both are usually reported.

Formulas

Weighted index

WQI = sum of sub-index x weight

Geometric index

WQI = product of sub-index^weight

Worked example

With the default values below, Water Quality Index Calculator returns Water Quality Index of 74.850. Change any input above and every figure here is recalculated in your browser.

Inputs used

Dissolved Oxygen Sub-Indexaccepts 0–100
85
BOD Sub-Indexaccepts 0–100
70
pH Sub-Indexaccepts 0–100
90
Faecal Coliform Sub-Indexaccepts 0–100
55

Results

Water Quality Index
74.850
Geometric Mean Index
73.438
Lowest Sub-Index
55.0
Water Quality Class
Good

Step by step

  1. 1Values used
  2. 2Weighted index
  3. 3Geometric index
  4. 4Water Quality Index= 74.850
  5. 5Geometric Mean Index= 73.438
  6. 6Lowest Sub-Index= 55.0
  7. 7Water Quality Class= Good

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is the eclipsing problem?

A single catastrophic parameter can be masked by good scores elsewhere in an arithmetic index. Geometric or minimum-operator forms avoid it, which matters when the failing parameter is a pathogen indicator.

Are indices a substitute for standards?

No. Compliance is assessed parameter by parameter against standards. An index is a communication and trend tool, not a regulatory test.

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