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Learn how we calculate water quality ratings and what your city's score means for your health and safety.
Environmental Data Analysis
This content has been reviewed by Dr. Sarah Chen, PhD, Environmental Science Advisor, to ensure accuracy and alignment with current environmental science standards.
Last reviewed: April 2026
Water quality scores provide a simplified way to understand overall tap water safety. Scores range from 0-100, with higher scores indicating better water quality. The calculation considers multiple factors based on EPA data and health guidelines.
How detected contaminants compare to EPA Maximum Contaminant Levels (MCLs) and health guidelines. Contaminants exceeding legal limits significantly reduce the score.
The number of different contaminants detected in the water supply. Fewer contaminants result in higher scores, even if levels are within legal limits.
Historical compliance with EPA regulations. Recent violations lower the score, while resolved violations have less impact. No violations boost the score.
Comparison to health organization guidelines, which are often more protective than EPA legal limits. Meeting stricter guidelines improves the score.
Excellent water quality. All contaminants well below legal limits. No recent violations. Safe to drink without filtration, though some may prefer it for taste.
Good water quality. Meets EPA safety standards. Some contaminants detected at low levels. Generally safe to drink. Filtration optional for extra protection.
Fair water quality. Meets minimum EPA standards but has contaminants worth monitoring. Consider water filtration, especially for vulnerable populations.
Below average water quality. Multiple contaminants at elevated levels or recent violations. Water filtration recommended for all household members.
Poor water quality. Contaminants exceed EPA limits or significant compliance issues. Certified water filtration strongly recommended. Consider bottled water for drinking and cooking.
While water quality scores provide a helpful overview, they don't tell the complete story:
Your water quality is good. Consider these optional steps:
Your water has some concerns. We recommend:
Your water quality needs attention. Take action:
A city can be "EPA compliant" (meeting minimum legal requirements) but still have contaminants at levels that reduce the score. We consider health guidelines that are more protective than EPA legal limits, total contaminant count, and historical compliance patterns.
Water quality scores are updated quarterly based on the latest EPA data from the Safe Drinking Water Information System (SDWIS). However, testing frequency varies by contaminant type and water system size.
Yes. Large cities may have multiple water systems or treatment plants. Additionally, local infrastructure (pipes, fixtures) significantly affects water quality at the tap. Our scores reflect the primary water system serving each city.
First, review the specific contaminants detected in your water. Then, choose a certified water filter that removes those specific contaminants. Contact your water utility for their latest Consumer Confidence Report and ask about improvement plans. Consider home water testing to verify quality at your tap.