Sizing a softener is a matter of arithmetic applied to your own household. The unit can only remove so much hardness before it needs to regenerate, so the question is how many days of heavy use it can cover between cycles. Consider a home with two adults, three children, a daily laundry load, and guests on weekends. Their average day looks very different from their busiest day, and the unit has to handle the busiest day.
Start with the hardness figure
Use the hardness number from your water test, and confirm the unit it is measured in. A figure in grains per gallon and one in milligrams per liter cannot be used interchangeably without conversion. If your test is incomplete or old in the sense of not reflecting your current supply, resolve that before you size anything. For how to get a usable report, see the guidance on the buyer's guide: The Missouri Water Softener Buyer's Guide.
Count the heavy days, not the average
List the water uses that run on your busiest day: showers, laundry loads, the dishwasher, any outdoor hose use, and any guests who stay over. Estimate the gallons for each, then add them to get a rough daily total for a heavy day. A household that runs two laundry loads on the same day as four showers will need a different unit than a couple who travel often and rarely run the washer twice in a day.
Work out how many days the unit lasts
Multiply your hardness figure by the gallons you use in a day to get the daily hardness load. Then compare that against the capacity the manufacturer lists for the unit. Divide the capacity by the daily load to see how many heavy days the unit will cover before it regenerates. Ask the installer to show you this calculation on paper, using your numbers, instead of accepting a single sizing answer.
Look at regeneration timing
A unit that regenerates on a small load may run at night, which is convenient for most homes. A unit that is too small for your busiest week may run during the hours you are using the most water, or may run out of capacity before it has a chance to regenerate. Ask when the unit is set to regenerate, and what happens if you have a heavy day in the middle of a cycle.
Resize when the household changes
Sizing is not a one-time answer. A new child, adult children moving back home, a regular houseguest, or a change in how the laundry is done can all shift the daily load. A home that is sold to a larger family may also need a different unit. Note the figures you used when you sized the system, and revisit them when the household changes so you are not relying on a number that no longer describes the house.