Lead & Copper 90th Percentile Calculator
For public water systems: paste your tap sampling results and get the LCR 90th percentile, a ranked list, and the action-level comparison — using the exact EPA method for 5-sample, small, and large sample sets.
90th Percentile Result
| Rank (ascending) | Result (mg/L) |
|---|
How the calculation works
90th percentile method — 40 CFR 141.80(c)(3)
1. Rank all tap results from lowest to highest. Let n = number of results.
2. If n < 5 → the 90th percentile is the highest result.
3. If n = 5 → the 90th percentile is the average of the two highest results.
4. If n ≥ 6 → rank position = n × 0.9; take the value at that rank (rounding per the selected method).
5. Compare the 90th percentile against the action level for the analyte and rule version.
1. Rank all tap results from lowest to highest. Let n = number of results.
2. If n < 5 → the 90th percentile is the highest result.
3. If n = 5 → the 90th percentile is the average of the two highest results.
4. If n ≥ 6 → rank position = n × 0.9; take the value at that rank (rounding per the selected method).
5. Compare the 90th percentile against the action level for the analyte and rule version.
| Analyte | Action level — current (LCR/LCRR) | Action level — LCRI (from 2027-11-01) |
|---|---|---|
| Lead (Pb) | 0.015 mg/L (15 µg/L) | 0.010 mg/L (10 µg/L) |
| Copper (Cu) | 1.3 mg/L (1300 µg/L) | 1.3 mg/L (1300 µg/L) |
Notes & limitations
Primacy-agency worksheets differ on how the n × 0.9 rank position is rounded — Iowa, for example, uses round-half-to-even. Always verify your result against your primacy agency’s worksheet before reporting.
- Method per 40 CFR 141.80(c)(3). The rounding selector affects only the n ≥ 6 rank-position step; the 5-sample average rule and the < 5-sample highest-result rule do not change.
- Non-detects: enter as 0, or follow your state’s specific instruction for handling non-detects.
- Exceeding the action level triggers treatment-technique, public-education, and (for lead) service-line requirements — it is not an MCL violation.
- LCRI general compliance date is 2027-11-01; rule versions and state implementation schedules vary. Confirm the applicable requirements with your primacy agency.
- This tool is for operational and compliance assistance. Verify critical submissions against the current federal rule and your primacy agency’s guidance.
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