Sizing a softener is usually framed as a one-time question about the tank. In practice, the right size depends on who lives in the house and how they use water, and that changes over the years. A couple with no children, a family with a toddler, and a home with two teenagers and a grown child visiting every other month do not need the same answer, even if the square footage is identical. The goal is a unit that still fits the household you will have in a few years, not just the one standing in the kitchen during the estimate.
Count the people and the habits
Start with the people who use water every day, then add the habits that change the total. A teenager with long showers, a home where someone works from a bath-equipped room, a guest room that gets used most weekends, and a daytime home presence all change the water load. Write each one down. Do not rely on the number of bathrooms alone, because two bathrooms with light use can need less than one bathroom with heavy use.
Plan for the changes you can see coming
Ask yourself which changes are likely in the next five to ten years. A new baby, an adult child moving back, a parent moving in, a dog that needs washing in the utility sink, or a home office with a shower can all shift the load. None of these need a precise forecast. The point is to ask the installer whether the sizing they recommend still works if the household grows by one or two people or adds a regular heavy user.
Ask how the sizing was figured
Ask the company to show the sizing math in writing. Which household size did they use? Which daily water use did they assume? What hardness reading was the sizing based on, and when was it taken? If the reading is old, the sizing may be off. Ask what happens to the unit's regeneration schedule if the household grows, and whether you would need to call the company to change settings or whether the unit adjusts on its own.
Ask whether the tank size is fixed once installed. If the answer is no, find out what an upgrade involves, who does it, and whether it is a bigger tank or a second unit. A plan to add capacity later is only useful if you know how hard it is to do.
Questions to ask before the tank is chosen
How many people and how much daily use did you assume in the sizing? What changes in the household would make this unit undersized, and what would I do then? Does the regeneration schedule change as the household changes, and who sets it? Can the tank be upgraded later, and what does that involve? Will you put the sizing note and the assumptions in writing?
Compare the answers across the companies you are considering. A sizing answer that is confident but thin, with no assumptions shown, deserves a second question. For a starting list of companies to compare, see our ranked list of St. Louis softener companies, and read how we rate them to see what we checked.