On a municipal supply a great deal of testing has already been done, published and paid for, so the useful question is not what is in the water but what a home kit adds. The answer is specific and worth knowing: it covers the stretch between the water main and your own fixtures, which no utility report describes.
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The EPA requires every community water system to publish an annual Consumer Confidence Report and maintains a finder for them. It covers the regulated contaminants, names the disinfectant used, usually gives hardness, and lists any violations during the year, all measured to laboratory standard on a defined monitoring schedule. Nothing a home kit does approaches that for the parameters it covers, so anything the report already answers is not worth buying a test for.
The gap the report leaves
It describes water leaving the treatment plant and sampled around the distribution system, not water leaving your kitchen tap. Lead is the clearest example, since it typically enters from a lead service line, from lead solder in older properties or from brass fittings, all on your side of the meter. Copper from corroding pipework is the same. So is anything contributed by an ageing water heater, a long dead leg, or a filter cartridge that has been in place for years.
Which tests are worth doing
A first draw lead test in any older property, sent to a laboratory rather than read from a strip, because this is the one result that genuinely changes decisions. Hardness, using a titration kit or strips, since the figure decides whether a softener pays and how it is set. Chlorine, to see what is arriving and to confirm a filter is still working. pH is quick and useful. Skip broad multi parameter strip packs promising dozens of results, which are imprecise and invite overreading.
Sampling to match the question
For lead, take a first draw after water has stood in the pipes for several hours, since that is what shows the plumbing's contribution, and a flushed sample afterwards to separate the service line from the fixture. For everything else, run until cold and steady with the aerator removed, so you are measuring the supply rather than what sat overnight. Use the same tap each time so results compare. Our water test kit guide covers when to escalate to a laboratory.
Questions people ask about city water test kit
Do I need to test if the utility publishes a report?
Not for the regulated contaminants. A home test covers what happens between the water main and your own tap.
What is the single most useful test on city water?
A first draw lead test in an older property, sent to a laboratory, since it is the result that most often changes a decision.
How do I sample for lead?
A first draw after water has stood several hours, plus a flushed sample, to separate the service line from the fixture.
Are broad multi parameter strips worth it?
Rarely. They are imprecise and encourage conclusions the chemistry does not support. Single purpose kits are more reliable.