Where the numbers come from
Every number on this site comes from a published, citable source — never estimates we invented. Here is exactly where each metric comes from.
Water hardness (PPM)
State averages synthesized from USGS groundwater studies and EPA SDWIS records, as compiled by Water Hardness Lab (2026). Hardness is classified on the USGS scale: soft (0–60 PPM), moderately hard (61–120), hard (121–180), very hard (180+). Hardness varies by city and even by neighborhood — treat state averages as a starting point.
Tap water quality score
TapWaterData's 2026 state rankings: population-weighted averages of city-level scores built from EPA Safe Drinking Water Information System (SDWIS) violation data, annual utility reports and state agency databases covering 9,500+ cities. 100 = no contaminants exceeding EPA limits or health guidelines. The national benchmark shown is the unweighted average of the 51 state scores.
Health-based violations per water system
America's Health Rankings analysis of EPA SDWIS data via ECHO (2024): the average number of health-based drinking water violations per community water system in each state. The U.S. average is 2.5. Health-based violations mean a system exceeded a maximum contaminant level or broke a treatment rule — not paperwork violations.
City pages
City data comes directly from EPA's Safe Drinking Water Information System bulk records (via ECHO): active community water systems, the population each serves, and health-based violations over the past five years, deduplicated by violation ID. A water system is attributed to the first city it reports serving, so districts spanning several towns are counted once under their primary city. We show the top ten cities per state by population served, refreshed quarterly.
The A–F water grade
Each state's water score (0–100) is 75% its tap-water quality score (EPA contaminant compliance) and 25% a violations factor, where zero health-based violations per water system scores 100 and six or more scores 0. That score maps to a letter grade with plus/minus bands (A+ = 97+, A = 93–96, A- = 90–92, and so on down to F below 60). A city's grade starts from its state score and is adjusted for the city's own five-year health-based violation record per water system — so a clean city grades above its state and a troubled one below. City grades can reach F; state averages bottom out around D because a statewide average blends many systems. Every grade reflects the public regulatory record for an area's water systems, not a test of the water at any individual tap — for that, only a certified home test is definitive. The ZIP lookup resolves your ZIP to your city (from EPA served-area records) or, failing that, to your state.
Purification system costs
Calculator price ranges come from Angi's 2026 cost guides: under-sink reverse osmosis $150–$1,300 installed; water softeners typically $1,000–$2,500 installed; whole-house filtration $1,129–$3,540 (average $2,274); whole-house reverse osmosis $4,800–$8,000; professional water testing $50–$200.
Per-category sources
Water Hardness: State averages synthesized from USGS groundwater studies and EPA SDWIS records (Water Hardness Lab, 2026). PPM = parts per million of dissolved calcium carbonate; divide by 17.1 for grains per gallon.
Tap Water Quality: Quality scores: TapWaterData 2026 state rankings, population-weighted from EPA SDWIS violation data across 9,500+ cities (100 = no contaminants exceeding EPA limits or health guidelines). Violations: America's Health Rankings analysis of EPA SDWIS/ECHO 2024 data — average health-based violations per community water system (U.S. average: 2.5).
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