How A–F Lake Water Quality Grades Are Calculated

Every one of the 7,664 lakes on LakeQuality carries a single A-through-F grade, but that letter is the end of a repeatable, entirely rules-based calculation, not a subjective judgment. This guide documents the exact method: where the numbers come from, the thresholds each metric is measured against, and how three water-chemistry readings collapse into one letter. Every figure here is derived from the site's own graded dataset, last refreshed 2026-07-05.

Step 1 — Pull the measurements

The raw data comes from the EPA Water Quality Portal, which aggregates state, federal, and tribal monitoring records into one standardized feed. For grading we use three parameters: Secchi disk depth (clarity), total phosphorus (the nutrient that fuels algae), and chlorophyll-a (a direct proxy for algae concentration). Only summer measurements (June through September) are used, because that is when recreational conditions and algae growth peak; if a lake has no summer record, all available readings are used instead. For each parameter we take the median of the summer readings so a single anomalous sample cannot swing the grade.

Step 2 — Grade each metric on published thresholds

Each median is graded independently against the Metropolitan Council threshold set — a standard used for Twin Cities lake monitoring since the 1980s and applied here to all northern-tier states:

Clarity thresholds are defined in meters (an A is above 4.5 m) and converted to feet for display. Higher clarity is better; lower phosphorus and chlorophyll-a are better.

Step 3 — Average the metric grades

Each metric grade becomes a number (A=4, B=3, C=2, D=1, F=0). Those numbers are averaged with equal weight — no metric counts more than another — and the mean is mapped back to a letter: 3.5+ is an A, 2.5 an B, 1.5 a C, 0.5 a D, and anything lower an F. A lake with an A for clarity, a B for phosphorus, and a B for chlorophyll-a averages to 3.33, which rounds to a B overall. When only one parameter is available the lake still gets a grade, but it is flagged as limited data. In the current dataset 5,306 lakes (69%) are graded on two or three metrics.

Step 4 — Classify trophic state separately

Independently of the letter grade, the same medians feed the Carlson Trophic State Index, a logarithmic scale that places a lake on a productivity spectrum: oligotrophic (TSI under 40, clear and nutrient-poor), mesotrophic (40–50), eutrophic (50–70, nutrient-rich and algae-prone), or hypereutrophic (over 70). TSI is reported for context but never alters the A-through-F result.

How the grades come out across the dataset

Applied to all 7,664 lakes across 13 states, the method produces the distribution below. It is not engineered to a curve — it is simply where measured water quality falls against fixed thresholds:

Because the thresholds are fixed, a grade is comparable across states and years: a phosphorus reading that earns a C in Minnesota earns a C in Ohio. The one deliberate exception is Florida, where subtropical lakes are graded on Florida LAKEWATCH breakpoints so that naturally tannic, productive waters are not unfairly flunked by northern cutoffs.

Frequently Asked Questions

Sources: EPA Water Quality Portal (measurements); Metropolitan Council lake-grading thresholds; R.E. Carlson, “A trophic state index for lakes,” Limnology & Oceanography (1977); Florida LAKEWATCH (subtropical breakpoints). Grades are a classification of measured data, not a safety certification.