Two softeners with the same price tag can be sized very differently, and the size decides how often the unit regenerates, how much salt and water it uses, and whether it runs out of soft water on a busy day. Before you accept any quote, ask the seller to show the sizing math. A seller who can explain it in a few minutes is giving you something to check. A seller who only quotes a model name is asking you to trust a number you cannot see.
Start with the hardness number you were given
Sizing begins with your water's hardness, measured in grains per gallon. Ask which number the seller used and where it came from. It should match your utility's water quality report or a recent well test, not a guess or a neighbor's figure. If you have not yet pulled that report, do it first, because every later calculation depends on it. Write the number down and keep it with your quotes so you can compare them directly.
Ask how much water the house really uses
Capacity is only half the picture. The seller should estimate daily water use from the number of people in the home and a per-person figure, then multiply the two. Ask which per-person figure was used and why. Ask whether the estimate includes guests, a dishwasher, a washing machine, or any outdoor use. An estimate that leaves out a big user is the most common reason a unit runs short.
Check the usable capacity, not the marketing capacity
Manufacturers and sellers often describe capacity in a headline figure, but the number that matters is usable grain capacity after the unit reserves a margin for regeneration. Ask the seller to state the usable capacity for the exact model and resin volume they are proposing. Then divide it by your daily grain load, which is your hardness multiplied by your daily gallons. The result tells you roughly how many days pass between regenerations. If the answer is only a day or two, the unit is likely too small for your home. If it is many days, the unit may be larger than you need.
Ask how the unit decides when to regenerate
Some controls regenerate on a fixed schedule, while others regenerate based on measured water use. A fixed schedule can waste salt and water when the house uses less than expected, especially if it is oversized. A meter-based control follows actual use. Ask which type the quote includes, how it is set at installation, and whether the seller can change the setting later if your household changes.
Compare the quotes on the same assumptions
Once you have two or more sizing explanations, line them up. Check that each seller used the same hardness number, the same household size, and the same per-person figure. If the assumptions differ, the sizes will differ too, and the cheaper quote may simply be the smaller unit. Ask each seller to explain any gap. A good answer names the assumption that caused the difference and says why it fits your home.
Questions to ask before you commit
Ask the seller which hardness number they used and where it came from. Ask for the per-person daily use figure and whether it includes guests and large appliances. Ask for the usable grain capacity of the exact model, and how many days pass between regenerations at your estimated use. Ask whether the control regenerates by schedule or by meter, and whether the setting can be changed later. If the answers are specific and match your water report, you have a sizing you can check. If they are vague, ask again in writing before you sign.