The useful question is not whether to test but what to include and when to repeat it. A panel covering everything is expensive and mostly reports absences, while the standard cheap panel can miss the contaminant that matters in your area. Building the list from your source, your area and your plumbing gets a useful answer for a sensible price.
On a municipal supply, start with the free report
The EPA notes that every community water system publishes an annual Consumer Confidence Report and maintains a finder for them, and it states the disinfectant in use, hardness, and the results of required monitoring for the system as a whole. That answers a great deal at no cost. What it cannot tell you is what happens between the works and your tap, so the tests worth paying for on a municipal supply are the ones about your own plumbing: lead in particular, drawn as a first draw sample from a tap that has stood.
On a private well, the baseline panel
The EPA notes private wells are not federally regulated under the Safe Drinking Water Act, so nobody samples on your behalf, and a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. A sensible baseline covers coliform bacteria, nitrate, pH, hardness, iron and manganese, plus total dissolved solids. To that, add what is known in your area: arsenic, radon, fluoride or agricultural chemicals, which local health departments and neighbouring well owners can usually tell you about.
When to repeat
Annually for bacteria and nitrate on a well, since both change and neither is visible. Every few years for the wider chemistry unless something changes. And immediately after any of these: flooding or heavy run off reaching the wellhead, work on the well or the pump, a change in taste, smell or colour, a new baby or pregnancy in the household, or a neighbour's well testing positive for something. Those events change the answer more than the calendar does.
Getting a result you can act on
Use a laboratory accredited for drinking water analysis, which supplies the correct bottles, preservation requirements and holding times, and follow the sampling instructions exactly since stagnation and technique change results more than people expect. Ask for results against the relevant benchmarks rather than raw numbers alone. Keep the reports, because the trend across years is more informative than a single figure. Our test kit guide covers packages and what each includes.
Questions people ask about drinking water test
What should a well water baseline include?
Coliform bacteria, nitrate, pH, hardness, iron, manganese and total dissolved solids, plus anything known to be a local issue such as arsenic or radon.
How often should I retest?
Bacteria and nitrate annually on a well, wider chemistry every few years, and immediately after flooding, well work or any change in taste or colour.
What is worth testing on a municipal supply?
Mainly what happens in your own plumbing, especially lead as a first draw sample. The utility's annual report covers the system itself.
Do I need an accredited laboratory?
For anything health related, yes. It supplies the correct bottles, preservation and holding times, and states the method and detection limit.