A water softener is sized by two numbers working together: how much water your household uses in a day and how hard that water is. Capacity is measured in grains, and hardness is measured in grains per gallon. A unit that is too small for your daily use runs out between regeneration cycles, and a unit that is far too large can sit for long periods between cycles, which is not always efficient and costs more up front. Before you compare quotes, get both numbers for your own house.
Start with your hardness number
Ask for a hardness test on the water at your home, not a figure for the city. Ask which tap was tested, whether the result came from a lab or a test strip, and whether the sample was taken before any existing equipment. A lab result is more reliable than a strip, and a written result is easier to compare across companies. If the test also shows iron, ask the installer how they plan to handle it, because a standard softener may not be the whole answer.
Measure your daily water use
Installers often estimate daily use from the number of people in the home and a per-person figure. Ask which figure they used and why. You can check it yourself by reading your water meter at the same time each morning for a week, recording the difference from one day to the next, and averaging the results. Use a week of normal habits, not a week when you are away or have guests. If the installer's estimate and your meter readings are far apart, ask them to explain before you accept the size.
Check peak flow, not just the daily total
A softener has to deliver softened water when several fixtures run at once, such as a shower, a dishwasher, and a washing machine. Ask the installer for the flow rate the unit delivers and how that compares with your pipe size and the number of fixtures you run together. A softener with enough capacity can still give weak showers if its flow rate is too low for the way you use water.
Ask how often it will regenerate
Ask for the expected number of days between regeneration cycles, based on your numbers. A unit that regenerates every day or two may be undersized, or it may be working hard for a large household. A unit that regenerates very rarely may be oversized. Also ask what the installer will do if your real use turns out higher than their estimate.
Make the quotes comparable
Two quotes can only be compared if they state the same capacity, the same hardness figure, the same daily use assumption, and the same expected regeneration rate. Ask each company to write these four numbers on the quote. If one company proposes a larger unit with no stated assumptions, ask for the reasoning before you compare prices. Also ask whether the quote includes the resin tank, the brine tank, the bypass valve, and the first salt fill, so the prices cover the same equipment.
Confirm the space and the salt plan
The unit needs room for the resin tank, the brine tank, and clearance for servicing. Salt has to be stored and loaded. Ask who will refill the salt, how often that will be needed, and how the unit signals a low salt level. If you plan to handle refills yourself, confirm that the access route is clear and the tank lid can be reached. Fit in your space is part of the sizing decision, not an afterthought.
What to bring to the call
Bring your meter readings, your hardness test result if you have one, a list of fixtures and appliances that use water, and the number of people who live in the home. Ask the installer to put the sizing reasoning in writing. A company that can explain your numbers plainly is giving you what you need to decide.