Most sizing advice starts with how many people live in the house. That is a reasonable first guess, but it misses the appliance that uses the most softened water in the home: the washing machine. A family of four that runs one load a week and a family of four that runs five loads a day will ask a softener to do very different amounts of work, even though the head count is the same.
Start with your water test, not the brochure
Before you talk to any provider, get the hardness figure for your own water. Your provider should either test it on site or accept a lab result for your supply. Write the number down. If a quote does not state the hardness it was sized for, ask for it. A sizing built on a different hardness level will be wrong no matter how careful the arithmetic is.
Count the water that actually goes through the softener
Ask the household to estimate, for an ordinary week, how many washer loads, dishwasher cycles, showers and baths the home runs. Note whether the washer is a high-efficiency model or an older top-loader, since older machines often use more water per load. Note any other routine that adds water use, such as frequent pet bathing or filling a tub often. These figures do not need to be exact. They need to be honest and written down, so you can compare them with what the provider used.
Ask how the provider turned that into a capacity
A useful quote shows how the capacity was reached: the daily water estimate, the hardness used, and the regeneration interval that results. Ask how often the system will regenerate. If it regenerates every few days in a home you described as heavy on laundry, ask why the number is that low. If it regenerates daily in a home that runs only a few loads, ask whether the unit is larger than it needs to be, since an oversized unit can waste salt and water.
Check that the whole house is in the count
Laundry is not the only heavy user. A large water heater, a second bathroom with a tub, or a dishwasher that runs daily all belong in the same count. Ask whether the sizing covers the whole house or only the fixtures you mentioned. Then set that answer beside your list and look for gaps.
What to compare before you choose
Put two or three quotes side by side. For each one, check the stated hardness, the daily water figure, the expected regeneration interval, and whether the tank size matches those numbers. A larger quote is not automatically better, and a smaller one that cannot keep up will leave you with hard water on the busiest days. If you want to see how we check these details, read Methodology: How We Match Systems and Compare Providers.