A filter takes five minutes to buy, but a good purchase starts with a diagnosis: what is actually in your water? The answer is almost always available for free — it just takes knowing where to look. The tools in this section exist for that: put numbers on your water before spending a dollar, so you buy the right level of treatment.

Your utility's report: the first move

Every community water system publishes an annual Consumer Confidence Report, and the results are public. You do not have to go hunting for it: I republish them city by city in the water quality register. The lines that matter for choosing a filter are the chlorine residual (the one responsible for the taste), lead and PFAS (which need certification or reverse osmosis, not a taste filter), and disinfection byproducts.

Testing station

What's in your tap water?

Enter your ZIP code and I'll pull up what your own water utility reported, straight from its annual Consumer Confidence Report.

Five minutes of reading saves you from the two symmetrical mistakes: over-equipping water that only needed activated carbon, or under-equipping water that deserved better.

Hardness: the one figure your report probably omits

Second reflex, and the one that surprises people: hardness is usually not in the report. It is not a regulated contaminant, so most utilities never publish it — roughly three reports in four skip it. Where it exists, this site shows it in grains per gallon. Where it does not, a test strip settles it in ten seconds.

The scale: under 3.5 gpg soft, 3.5 to 7 moderately hard, 7 to 10.5 hard, over 10.5 very hard. This number decides a whole side of your equipment: on very hard water, your white deposits are scale and no carbon filter will change that; on soft water, forget the softener. The method is in how to test your water hardness at home, and water hardness by city explains where the published figures come from and why they are patchy.

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Varify

Water hardness test strips (gpg)

4/5

Strong points

  • Reads in grains per gallon as well as ppm — gpg is the unit softeners are sized in
  • Costs less than a coffee and takes ten seconds
  • The only way to know your hardness when your utility does not report it

Weak points

  • Bands rather than an exact figure
  • Strips age: check the expiry date
  • Hardness only — nothing about lead or PFAS

Where to buy

Cost per gallon: the calculation that settles everything

To compare a pitcher, a faucet mount and an under-sink unit on the same basis, one calculation: cartridge price divided by its rated capacity in gallons. A pitcher cartridge ($7 for about 40 gallons) works out at roughly 17 cents a gallon; a faucet cartridge ($20 for 100 gallons) around 20 cents; an under-sink block ($49 for 1,000 gallons) under 5 cents. Bottled water sits between $1 and $2. Run this before every purchase, taking 20 to 30% off the rated capacities to match real use: the rankings in the comparisons are built on it.

Frequently asked questions

How do I find out the quality of my city's water?
Enter your city in the register: I republish what your utility reported — chlorine, lead, PFAS, nitrate, arsenic, disinfection byproducts and bacteria — taken from its own annual Consumer Confidence Report. See your city's report →
What does filtered water cost per gallon?
Roughly 20 cents a gallon for a faucet filter, 17 for a pitcher, and under 5 for a long-life under-sink block, against $1 to $2 for bottled water. Divide the cartridge price by its real capacity to get your own figure.
Does hardness change which filter I should buy?
Yes, in the negative: if your water is very hard, no carbon filter will reduce the scale. The filter stays useful for taste, but protecting the house goes through a softener. Understanding hardness by city →