Test the water before you buy anything. Every number on this site is worked from a measured figure, and sizing equipment for a problem nobody has
measured is the most expensive mistake in this category. On city water, start with the Consumer Confidence
Report your utility publishes every year, which is free, then test at your own tap for anything that changes
inside your own plumbing. On a private well, nobody tests it but you, and only a certified laboratory test
tells you what you actually have. Hardness is the one figure you can genuinely read at home with a strip.
Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics. They are invisible and tasteless, so nothing tells you whether equipment is working. The limits are legal
figures rather than preferences, and lead does not even have a maximum contaminant level: it has an action
level of 0.015 mg/L, which is a trigger for utility action rather than a safe concentration, and the health
goal is zero. No filter pitcher addresses any of these. Talk to your local health department or a licensed
water treatment professional before buying equipment for them.
A certification covers a specific claim, not the whole unit. NSF/ANSI 42 is aesthetic effects such as chlorine taste and odor. 53 is health effects, claim by claim. 44
is softeners. 55 is ultraviolet, where Class A delivers 40 mJ/cm2 for water not known to be safe and Class B
delivers only 16 as a supplement to water already considered safe. 58 is reverse osmosis. 401 is emerging
compounds. 372 is the lead CONTENT of a fitting, not lead removal. The phrase "NSF certified" with no standard
number and no claim attached tells you almost nothing.
Researched, not professional advice. HomeWaterCalc publishes calculators and buying guidance compiled from published standards, regulatory
limits, manufacturer specifications and the consensus of verified owner reviews. We do not test equipment in
person and we never claim to. Nothing here diagnoses a water problem or says any product makes water safe to
drink. Published figures and rules of thumb are labeled separately on every page that carries both, and where
a published figure does not exist we say so rather than printing an estimate.
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