WaterZen Grading Methodology

WaterZen.com has assigned a letter grade for the water quality of nearly 1,500 water providers across the US. Here's our methodology.

Dave Ericksen

WaterZen.com has compiled lead and copper data for the water supplies of nearly 1,500 water providers who serve around 255 million Americans (about 77% of the nation). Using this data, we assigned each water provider a letter grade and would like to explain our methodology.

Methodology in Three Steps

First, we looked at each water provider’s lead level and copper level detected at the 90th percentile, which means the maximum amounts of lead and copper found in 90% of the samples taken. By definition, the remaining 10% of samples had amounts that were higher than the numbers we used. If any water providers failed to provide data for the 90th percentile, we excluded their data from our analysis.

Second, we defined the best possible scores for lead and copper as equal to the EPA's Maximum Contaminant Level Goal (MCLG). Any amount of lead or copper below this MCLG number poses no known or expected health risk to people drinking the water. Any amount higher than this number means there is a health risk.

For lead, the best possible score or MCLG is 0.0 milligrams per liter (mg/L) of water, which means that any amount of lead in drinking water is a health risk. For copper, the MCLG is 1.3 mg/L, so any amount higher than that number is a health risk.

To simplify this example a bit, we'll look only at lead and not copper. To calculate a grade for lead amounts, we took the number difference between the MCLG (0.0 mg/L) and the legal limit allowed by the EPA, which is 15.0 mg/L. The difference is, of course, 15.0 mg/L.

We then divided that number into 10 equal sections (also called deciles) of 1.5 mg/L each and assigned a letter grade to each of the deciles. The closer to zero, the higher the grade. For instance, the first decile representing detected lead amounts from 0.0 mg/L to 1.5 mg/L was assigned a letter A grade. Here's a table showing the grade for each decile:

Grades & Ranges for Lead Detection

  • A → 0.0 – 1.5 mg/L
  • A- → 1.5 – 3.0 mg/L
  • B+ → 3.0 – 4.5 mg/L
  • B → 4.5 – 6.0 mg/L
  • B- → 6.0 – 7.5 mg/L
  • C+ → 7.5 – 9.0 mg/L
  • C → 9.0 – 10.5 mg/L
  • C- → 10.5 – 12.0 mg/L
  • D+ → 12.0 – 13.5 mg/L
  • D → 13.5 – 15.0 mg/L
  • F → >15.0 mg/L (EPA's legal limit)

Third, we combined the scores for lead and copper to come up with an overall grade for each water provider. For example, if a water provider scored a C grade for lead and an A grade for copper, the combined score would likely be a B.

Letter Grades vs. Data Averages

We decided not to consider relative performance (comparing averages for water providers against each other) in our grading system even though we continue to provide this information in our reports. If we use averages, water providers whose levels may be average or a bit below average, but are also far under the legal limit, get unfairly penalized.

For example, let’s say that the lead level of your city’s water provider is 4.0 mg/L. Our data has shown that the national lead average is 3.3 mg/L. This means your city’s lead level is barely below the national average, but far under the legal limit.

If we had decided to grade water providers relative to each other, your city would’ve received a C- or even a D+ because, like the old grading system in high school, a perfectly average provider would get a C. We feel this grade is unfair and would give an inaccurate picture of your city’s water quality.

However, since we decided to grade water providers relative to the legal limit, your city’s water provider would instead get a B+ because the lead level is so far under the legal limit. (See the table above.) This grade is a much more accurate picture of your city’s water quality, although your water provider should still work to improve that number to zero.

Remember, WaterZen.com’s grade for each water provider is based on an average of their lead level and copper level, not just lead alone. So, if your city scored a B+ on lead and an A on copper, the overall grade would likely be an A-. Conversely, if your city scored a B+ on lead and a C on copper, the overall grade would likely be a B-.

Tell us What You Think

We hope this article explains WaterZen.com’s grading methodology. If you have any other questions or comments, please contact us.


WaterZen exists to improve your family's health by making information about the water you drink more accessible, straightforward, and easy-to-understand. We do this by providing clear, simple water-quality information for your area based on data from your local water provider, unbiased comparisons, health standards, and expert opinions.

The way we see it, when you're informed about the quality of your water and then empowered to protect your family (and your community), you've achieved "water zen".