I have posted some of these before either as independent posts or as responses to some of the members on NextDoor. Consolidating everything at one place so that it is easy to find information. If you need any help, feel free to reach out to us directly.
You must have noticed water softeners are generally rated / labelled 30k grains, 40k grains capacity etc. Let's try and understand what is a grain? It is a measure! One grain of hardness equals 1/7000 pound of rock (generally calcium). One grain is equivalent to 17.1 milligrams per liter of water or sometimes referred to as PPM ( parts per million). In other words, if your city water hardness is measured 19 GPG (grains per gallon), it means there are 324.9 (19 x 17.1) milligrams of calcium &/ magnesium molecules present per gallon of water.
Now let's try to understand what does it mean when water softener capacity is labeled 40k grains! In simplistic terms it means, the resin tank in that softener can hold maximum of 40k grains of hardness before the regeneration / recharge of softener. Regeneration / recharge is a process via which softener disposes all the unwanted molecules it collected ( hardness) into drain and makes resin tank ready again to collect unwanted molecules.
To size water softener, by age old formula, you need to use the following: No. of people x avg. consumption of water / person / day ( generally around 50 gallons) x hardness of the water. For a house hold of 4, it means, 4 x 50 x 19 ( hardness in Pleasanton) = 3800. It means the softener needs to remove 3800 grains (hardness) / day in that household. I took 50 gals as average (excluding water that goes to yards and based on the water monitoring device that I installed prior to softener). This helps get true average use of softened water required. If you don't have a device like that go with readings from your city water consumption report for your house less average water/day for yards. Traditional softener's typically does not regenerate every day. It is assumed they regenerate once every week. Again this is a topic that goes very deep and wide depending on type of regeneration (auto, smart etc.). Given the fact that regeneration in many modern softeners may not follow good old rule of 7 days wait before regeneration and given water softener controllers have become very efficient in some models, you need to be bit cautious in using old rule as standard when sizing.
Now back to good old days of sizing, if you multiply 3800 grains / day x 7 , it comes to 26,600 grains removal per week. It is generally recommend to not max out capacity of resin tank for holding the grains before regeneration. So, planning for about 60-70% of capacity is considered a good idea. Assuming every manufacturer's model follows this mechanism (which is not), you would go with 35k or 40k grains unit for household of 4 people. That said, each model and make generally have their own way to process ( regeneration, salt consumption, water monitoring etc.) and is rated for how many people it is good for.
If you want to avoid getting too technical but still be able to make a right choice, go with what each manufacturer recommends specific to model under consideration. There are many models that are labeled 40k grains and good for a household up to 6 people. There have been several technical improvements to controllers, type of resin used and so sizing the same way for every manufacturer & model may not be the best idea. Hope this helps someone here looking for an understanding on this topic. Cheers. Feel free to reach out to us if you need any help!