How We Grade Beaches

BeachGrade summarizes years of government beach-water-quality monitoring into one plain A–F grade. Here’s exactly how — and what it can and can’t tell you.

The data

Every grade is built from real public data only — no estimates, no synthetic values:

Beach inventory & advisories come from the U.S. EPA BEACON program, which tracks the public beaches covered by the federal BEACH Act, their locations, and their advisory and closure history.
Bacteria samples come from the Water Quality Portal (WQX/STORET), the shared federal repository where state and local agencies upload beach-monitoring results.

The indicator: fecal bacteria

Beach programs test for fecal-indicator bacteria — the signal that runoff or sewage may have reached the water and that the risk of gastrointestinal and other illness is elevated:

Enterococcus at saltwater and Great Lakes beaches — Beach Action Value 104 CFU/100 mL.
Escherichia coli (E. coli) at other freshwater beaches — Beach Action Value 235 CFU/100 mL.

The Beach Action Value (BAV) is the single-sample threshold at which water managers typically post a swimming advisory. A sample above it is an “exceedance.”

The Beach Safety Grade

For each beach we gather the monitoring stations within a few kilometers, pool their recent samples (multiple years), and compute the exceedance rate — the share of samples that came back above the Beach Action Value. That rate maps to a letter grade:

Grade	Samples over the safe-swimming limit
A	Under 3% — almost never
B	3–7% — rarely
C	7–12% — occasionally
D	12–25% — frequently
F	Over 25% — very often

We only assign a letter grade when a beach has at least 20 samples in two or more distinct sampling years. Beaches with fewer samples are marked “not yet rated” rather than guessed at — we never treat missing data as a passing or failing grade.

How much evidence is behind a grade

Clearing the floor for a grade is not the same as having a deep record, so every graded beach also carries a confidence tier:

High confidence — at least 60 samples, from two or more monitoring stations, with results reported in the last three sampling years.
Moderate confidence — a real grade, but one of those is missing. The beach page names which: a single monitoring station, a small number of samples, or a record that stops a few seasons back.

Today, 1,182 graded beaches are high confidence and 1,818 are moderate.

The distinction matters more than it sounds. Just under half of all graded beaches are covered by exactly one monitoring station — one point on a shoreline rather than the whole beach — and the sample counts behind a grade range from 20 to more than 5,800. Two beaches with the same letter can rest on very different evidence. A moderate-confidence grade is a fair summary of what has been measured at that beach; it is not a fine-grained ranking against a beach one letter away. We show the sample count, the station count, and the years covered on every beach page so you can judge for yourself.

Grade vs. current advisory

The grade is a multi-year track record. A current advisory reflects today’s water. A well-graded beach can still be under a temporary advisory after a storm, because bacteria spike when rain flushes runoff into the water. We show both, and we always link to the official state or county advisory — check it the day you swim.

Limitations

Monitoring is seasonal and uneven. Some beaches are sampled weekly; others rarely. Grades reflect what’s been reported.
A monitoring station a short distance away is a good proxy for a beach but not a perfect one.
Bacteria are an indicator, not a complete measure of safety — rip currents, algae, and chemical hazards are separate concerns.
BeachGrade is informational and is not a substitute for official public-health guidance.