Before you compare any softener, you need one number: how hard your water is. Hardness is the amount of dissolved calcium and magnesium, and it is what a softener is built to reduce. If the number is low, a softener may not solve a problem you actually have, and the money may be better spent elsewhere.
If you are on city or town water
Public water suppliers must publish an annual water quality report for their customers. In the US it is often called a Consumer Confidence Report. Look for a line for hardness, sometimes listed as total hardness. Check the units. Hardness is usually given in grains per gallon or in milligrams per liter as calcium carbonate. One grain per gallon is about 17.1 milligrams per liter, so you can convert if the report and a seller's material use different units.
The report measures water at the treatment plant or a sampling point, not at your kitchen tap. Pipes in the street and your home plumbing can change what reaches you. If the report is hard to find, call the supplier's customer line and ask for the most recent hardness result for your area.
If you are on a private well
A well has no utility report, so the only reliable figure comes from a lab test of a sample from your own water. Ask the lab to report hardness along with iron, manganese, and any other metals or minerals it can check. Many county extension offices and state-certified labs run these tests. Ask for results in grains per gallon or milligrams per liter so you can compare them with the figures in this guide.
Check what else the report shows
A softener handles hardness. It does not reliably handle iron, manganese, or sulfur, and high iron levels can foul the resin over time. If the report or lab result lists any of these, note them before you talk to anyone. The equipment you need may be different, and a company that quotes a softener without asking about them has not finished the job.
What to bring to a company
Put the hardness number, its units, and the source on one page. Ask every company to explain how its recommendation uses that number. If they cannot connect the figure to their sizing, that is worth knowing before you sign anything.