Statistics Canada
Symbol of the Government of Canada

Site navigation menu

Canadian Council of Ministers of the Environment Water Quality Index

Warning View the most recent version.

Archived Content

Information identified as archived is provided for reference, research or recordkeeping purposes. It is not subject to the Government of Canada Web Standards and has not been altered or updated since it was archived. Please "contact us" to request a format other than those available.

WQI presentation
WQI calculation

WQI presentation

The Canadian Council of Ministers of the Environment (CCME)1 Water Quality Index (WQI) allows to evaluate surface water quality for the purpose of protecting aquatic life (for example: fish, shellfish and plants) with the help of specific guidelines. The water body to which the index applies can be defined as a station (for example: a monitored location in clearly specified reach) or by a certain number of stations (for example: sites distributed across a lake). In 2005, 345 sampling stations located mostly in Southern Canada (see Appendix) that met the data quality standards were included in the publication.

The number of parameters to be measured is determined by water quality experts. These parameters correspond to various measurements taken at the stations (for example: pH, phosphorus, nitrate, etc.) and vary from one station to the other.

The sampling protocol requires at least four parameters, sampled at least four times. No maximum number of parameters or samples has been set. The type and number of parameters and samples used for the WQI calculation is left to the specialist's discretion. However, the specialist must use his professional judgment to ensure that the findings reflect the water quality in a given water area as accurately as possible. For the publication, data from three consecutive years were used to calculate the index value representing a given reference year, that is, the middle year of the interval.

For each parameter, water quality experts establish one or more guidelines that should not be exceeded. These water quality guidelines are numeric values that define physical, chemical, radiological or biological characteristics of the water that cannot be exceeded without causing harmful effects.

WQI calculation2

CCME Water Quality Index formula

Formula 1

Calculation of the index is based on three terms: scope (F1) – number of parameters that are not compliant with the water quality guidelines, frequency (F2) – number of times that the guidelines are not respected and amplitude (F3) – the difference between non-compliant measurements and the corresponding guidelines.

Division of these terms by 1.732 is based on the fact that each of the three factors contributing to the index can reach the value of 100. The maximal length is, therefore, expressed as:

Formula 2

Division by 1.732 reduces the maximal length to 100. The index produces a value from 0 to 100. The higher the number, the better is the water quality. 

Explanation of each term of the index

First of all, the term F1 (scope) expresses the percentage of parameters for which at least one measurement did not comply with the corresponding guideline during the period under study:

Formula 3

The term F2 (frequency) represents the percentage of analytical results that do not comply with the guidelines.

Formula 4

Finally, the term F3 (amplitude) represents the difference between the non-compliant analytical results and the guidelines to which they refer. The term F3 is an asymptotic function, representing the normalized sum of excursions (nse) in relation to guidelines within the range of values from 0 to 100.

Formula 5

To calculate the overall degree of non-compliance, we add the excursions of non-compliant analytical results and divide the sum by the total number of analytical results. This variable is called the normalized sum of excursions (nse).

Formula 6

There are three possible ways of determining the excursion:

  • If the finding must not exceed the guideline:
    Formula 7

  • If the finding must not be lower than the guideline:
    Formula 8

  • If the guideline is zero (equal to zero):
    Formula 9

The appendix contains a specific example of explaining the index calculation.

Index value categorization

Once the index has been calculated, we obtain a value of 0 to 100. The higher the index value, the better the water quality. The index is then placed in one of the following water quality categories:

Excellent: (CCME WQI value from 95.0 to 100.0) Water quality is intact. Conditions are very close to natural or desired levels. These index values can only be obtained if all measurements comply with the guidelines almost all the time.

Good: (CCME WQI value from 80.0 to 94.9) Water quality is intact and only one minor threat or deterioration is observed; conditions rarely differ from the natural or desirable levels.

Fair: (CCME WQI value from 65.0 to 79.9) Water quality is usually intact, but occasionally endangered or deteriorated; conditions sometimes deviate from the natural or desirable levels.

Marginal: (CCME WQI value of 45.0 to 64.9) Water quality is frequently endangered or deteriorated; conditions often deviate from the natural or desirable levels.

Poor: (CCME WQI value from 0.0 to 44.9) Water quality is almost always endangered or deteriorated; conditions usually deviate from natural or desirable levels.


  1. Canadian Council of Ministers of the Environment, 2001, "Canadian Water Quality Guidelines for the Protection of Aquatic Life: CCME Water Quality Index 1.0, user's manual", in Canadian Environmental Quality Guidelines, 1999, Winnipeg.
  2. Canadian Council of Ministers of the Environment, 2001, "Canadian Water Quality Guidelines for the Protection of Aquatic Life: CCME Water Quality Index 1.0, technical report", in Canadian Environmental Quality Guidelines, 1999, Winnipeg.