How do I calculate the 90th percentile?

If you are required to collect more than five samples:

1. Place lead results in ascending order (from lowest to highest value).
2. Assign each sample a number, 1 for lowest value.
3. Multiply the total number of samples by 0.9.
4. Compare the 90th percentile level to the action level of 0.015 mg/L (can also be expressed as 15 parts per billion (ppb)). If your 90th percentile value is higher than 0.015 mg/L, you have an exceedance.

Repeat this procedure for copper sample results, except compare the 90th percentile copper level against its action level of 1.3 mg/L. If your 90th percentile value is greater than 1.3 mg/L, you have an exceedance.

If you are required to collect five samples:

1. Place lead or copper results in ascending order.
2. Take the average of the 4th and 5th highest sample. This is your 90th percentile level.
3. Compare the 90th percentile level against the lead action level of 0.015 mg/L or copper action level of 1.3 mg/L.

You can also use this Excel spreadsheet (XLSX), which will automatically calculate your 90th percentile once you enter your Lead and Copper results in the respective cells.