Understanding Water Quality Scores
Learn how we calculate water quality ratings and what your city's score means for your health and safety.
Environmental Data Analysis
Expert Reviewed
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: February 2026
How Water Quality Scores Are Calculated
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.
Scoring Factors
Contaminant Levels (50%)
How detected contaminants compare to EPA Maximum Contaminant Levels (MCLs) and health guidelines. Contaminants exceeding legal limits significantly reduce the score.
Total Contaminants (20%)
The number of different contaminants detected in the water supply. Fewer contaminants result in higher scores, even if levels are within legal limits.
EPA Violations (20%)
Historical compliance with EPA regulations. Recent violations lower the score, while resolved violations have less impact. No violations boost the score.
Health Guidelines (10%)
Comparison to health organization guidelines, which are often more protective than EPA legal limits. Meeting stricter guidelines improves the score.
Water Quality Grade Scale
A+ to A- (90-100)
Excellent water quality. All contaminants well below legal limits. No recent violations. Safe to drink without filtration, though some may prefer it for taste.
B+ to B- (80-89)
Good water quality. Meets EPA safety standards. Some contaminants detected at low levels. Generally safe to drink. Filtration optional for extra protection.
C+ to C- (70-79)
Fair water quality. Meets minimum EPA standards but has contaminants worth monitoring. Consider water filtration, especially for vulnerable populations.
D+ to D- (60-69)
Below average water quality. Multiple contaminants at elevated levels or recent violations. Water filtration recommended for all household members.
F (Below 60)
Poor water quality. Contaminants exceed EPA limits or significant compliance issues. Certified water filtration strongly recommended. Consider bottled water for drinking and cooking.
Limitations of Water Quality Scores
While water quality scores provide a helpful overview, they don't tell the complete story:
- EPA limits aren't perfect: Legal limits may not reflect the latest health research. Some contaminants have health guidelines lower than EPA legal limits.
- Individual sensitivities vary: Vulnerable populations (infants, pregnant women, immunocompromised individuals) may need stricter standards.
- Home plumbing matters: Even with high-quality municipal water, old pipes or fixtures can add lead or copper to your tap water.
- Testing isn't universal: Not all contaminants are tested regularly. Emerging contaminants like PFAS may be under-reported in some areas.
- Taste and odor: A high score doesn't mean the water tastes good. Chlorine, minerals, and other factors affect taste even when water is safe.
What To Do About Your Water Quality Score
Score 80-100 (A or B Grade)
Your water quality is good. Consider these optional steps:
- Simple carbon filter for taste improvement
- Annual water testing if you have older plumbing
- Monitor your water utility's annual Consumer Confidence Report
Score 70-79 (C Grade)
Your water has some concerns. We recommend:
- NSF-certified water filter targeting specific contaminants
- Home water testing to verify municipal data
- Extra precaution for vulnerable family members
- Review your city's specific contaminant data
Score Below 70 (D or F Grade)
Your water quality needs attention. Take action:
- Install a comprehensive filtration system (reverse osmosis recommended)
- Get professional water testing for your home
- Use filtered or bottled water for drinking and cooking
- Contact your water utility about specific concerns
- Consider contacting local health department
Frequently Asked Questions
Why do scores differ from EPA compliance status?
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.
How often are scores updated?
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.
Can water quality vary within a city?
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.
What should I do if my city has a low score?
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.