How we calculate

Every number on this site comes from a formula you can see and a source you can check.

Formulas

Each calculator lists its formula in plain text under “The formula”. Unit conversions use exact definitions (for example, 1 grain per gallon = 17.118 mg/L as calcium carbonate). Engineering estimates such as pump horsepower or reverse osmosis output are labeled as rough.

Defaults and assumptions

Default values come from cited sources: EPA WaterSense for household water use, NSF/ANSI standards for filter certification, CDC and state health departments for well disinfection, and manufacturer manuals for equipment ratings. Where we couldn't find an authoritative figure, the input is tagged Assumption so you know to replace it with your own number.

Input checks

Out-of-range inputs are flagged next to the field and clamped to the allowed range. Inputs that make a result physically impossible, such as a pressure switch cut-out below its cut-in, show an error instead of a misleading number.

EPA water data

The water quality lookup uses EPA SDWIS records and EPA's UCMR 5 PFAS sampling, processed into static files when the site is built. We describe these records carefully: a single PFAS detection isn't a violation, the legal limit (MCL) is different from a health guideline, and lead action levels apply to the 90th percentile of tap samples. Lead results below the lab's reporting limit are shown as “<DL”, and older results are flagged. Hardness values carry a confidence label, from a utility's own report (highest) down to a county estimate of untreated water (lowest).

Review

County water pages

We publish a county page only when its main water system serves at least 10,000 people, was sampled for PFAS under EPA's UCMR 5 and has a lead result. Counties are ranked by Census population (2024 estimates; Connecticut's former counties use the 2020 estimates because Census now reports planning regions). The main system is the one serving the most people in that county, counting a system that spans several counties only for its share. If that system isn't a city or town utility but a city utility serves at least 75% as many people, we feature the city utility instead, since that's what most residents look for. Every page still lists the other large systems.

Hardness uses the utility's own report when we have it. Where a system draws water from a different source than local streams and wells (for example Chicago's Lake Michigan supply) we don't show the county estimate at all.

Each calculator shows the date its sources were last checked. Pages are written and reviewed by the Tapped Out team. We are not a lab, utility or health authority, and nothing here is medical or professional advice. When in doubt, test your water.

Corrections

If you spot an error, please tell us through the contact page. We fix confirmed errors and update the review date.