Softeners carry several numbers and they are not interchangeable. One describes how much hardness the unit removes between regenerations, one how fast it can pass water, one how efficiently it uses salt, and one whether an independent body has verified any of it. Comparing units means comparing the same number, which product pages do not always make easy.
Grain capacity
The headline figure: how much hardness the resin holds before it must be regenerated. It is the number sizing calculations produce and the one to match against your hardness and household water use. The complication is that capacity depends on how much salt is used to regenerate, so a single unit is often quoted at several capacities depending on the salt dose assumed. When comparing two products, check whether both figures assume the same salt setting, because a high salt figure flatters a smaller unit.
Service flow rate
How much water the unit can pass without hardness breaking through and without an unacceptable pressure drop. This matters in a house with several bathrooms or a high simultaneous draw, and it is a different constraint from capacity: a unit can have ample capacity and still deliver hard water during a peak because water is passing through the resin too quickly to exchange properly. Check it against the household's plausible simultaneous draw rather than average use.
Salt efficiency
How much hardness is removed per unit of salt used, and it is the figure that decides running cost. The relationship is not linear: a high salt dose extracts more capacity from the same resin but uses disproportionately more salt for the last part of it, which is why a larger unit run at a modest salt setting is usually cheaper to operate than a smaller one pushed hard. Certification programmes exist for efficiency, and a unit that publishes the figure is easier to compare than one that does not.
Certification and what it covers
Independent certification against the recognised standards for softeners verifies capacity claims and material safety, and certifiers publish listings naming the model. That is checkable in a minute and worth more than a mark on the box. It does not verify build quality or valve reliability, which is what long term reviews are for. Sizing still has to be done from your measured hardness and use. Our softener installation guide covers what the chosen size means for the install.
Questions people ask about water softener ratings
What does grain capacity mean?
How much hardness the resin holds before regenerating. It depends on the salt dose assumed, so compare like with like.
Why does service flow rate matter?
A unit can have ample capacity and still let hardness through during a peak, because water passes the resin too quickly to exchange.
What is salt efficiency?
Hardness removed per unit of salt, which decides running cost. A larger unit at a modest salt setting is usually cheaper to run.
What does certification verify?
Capacity claims and material safety against recognised standards. It does not verify build quality or valve reliability.