A sizing chart is a shortcut through arithmetic, and it works when your household matches the assumptions built into it. Those assumptions are rarely printed, which is why two houses with the same occupancy and hardness can end up with the same recommended unit and very different experiences of it.
What the chart is calculating
It multiplies three things: the number of people, an assumed daily water use per person, and your hardness, to get how much hardness has to be removed per day. It then picks a resin capacity so the unit regenerates at a sensible interval, neither daily nor once a fortnight. Both extremes are inefficient: frequent regeneration wastes salt and water, and very long intervals leave resin sitting fully loaded. The chart's output is a grain capacity, which is what you are actually buying.
The assumption that breaks first
Daily use per person is a default, and households vary widely around it. A power shower, a large garden watered from the softened line, a hot tub, or teenagers will exceed it comfortably, while a couple out all day will not approach it. If you have metered bills, use the real figure instead of the occupancy count, because sizing on people alone is the most common reason a correctly chosen unit regenerates far more often than its owner expected.
Hardness units, and iron
Charts are published in both common units and the numbers look nothing alike: the USGS reports hardness as milligrams per litre as calcium carbonate, soft at 0 to 60 through to very hard above 180, while much of the trade uses grains per gallon. Reading a chart in the wrong unit produces a wildly wrong answer. Iron also consumes capacity and must be added into the effective hardness, and Penn State Extension notes softeners handle dissolved iron only when pH is above 6.7, hardness is 3 to 20 grains per gallon and dissolved iron is under 5 mg/L.
Choosing between two adjacent sizes
When the chart lands between sizes, the larger unit generally regenerates less often and uses salt more efficiently, because a bigger bed can be run at a lower salt dose per regeneration and still deliver the household's needs. The costs are purchase price and space. A much oversized unit is not better, since resin sitting loaded for long periods is not ideal either. Our softener installation guide covers what the chosen size means for the install.
Questions people ask about water softener size chart
What does a sizing chart actually produce?
A resin grain capacity chosen so the unit regenerates at a sensible interval given your hardness and assumed water use.
Which assumption is most often wrong?
Daily water use per person. Use metered bills if you have them rather than an occupancy count.
Why do charts give such different numbers?
Two hardness units are in common use and their figures look nothing alike. Check which one the chart is using.
Should I round up a size?
Usually yes. A larger unit regenerates less often and uses salt more efficiently, at the cost of price and space.