If your home has a water softener, chances are good that it’s performing reliably and dependably. After all, water softeners don’t see a whole lot of rough and tumble action. But as they age, they can develop a number of problems with clogged filter screens, leaky valves, or worn out resin. All these are pretty easy to replace.

My water softener is one of those tall, 6-inch diameter torpedo-shaped tanks that looks like it fell off the back of a welding supply truck. The tank is filled with brown resin beads that soften the water. Screwed into the top is a control head containing several valves and an electromechanical timer. Water enters the control head, flows into the tank, gets softened and exits. There are also two pieces of tubing extending from the unit, one of which leads to the brine tank while the other takes water from the regeneration process to a floor drain. Like most sensible water softeners, it’s been quiet and uncomplaining over the years.

That is, until a few days ago. That’s when, in the dead of night, it began spraying water from its control head all over the place. Fortunately, only a few boxes got a little damp and most of the water trickled away to a nearby floor drain. A screwdriver and flashlight revealed that one of the screw lugs that keeps all the valves tightly sealed to the side of the control head had broken off. Vexed, I turned to Google and found that replacing the control head would cost about two thirds the price of a new water softener. Given that our softener looked to be well over 15 years, replacing it with a newer model that used less electricity, salt, and water sounded like a no-brainer. Swapping out the old one for the new one would be about as easy as replacing a new light bulb.

Just one nagging problem: there was no information anywhere on the old unit about its size!

Fine, I said to myself. All I have to do is figure out how to choose a water softener system of the correct size.

It turns out that determining how to choose a water softener size involves a little testing, a little estimating and a little calculating.

How Water Softeners Work

Water softeners work by removing calcium, magnesium and certain other metal ions from hard water. Softeners contain ion exchange resin beads, which are micro-porous to take up as many ions as possible. These beads weaken the chemical bonds of the minerals in the water and replace them with other metals, such as sodium. During the regeneration cycle, the calcium and other metal ions are washed out from the beads and then brine or salt water fills the tank to charge the beads with sodium ions. The result is water with less calcium and other minerals in it.

How to Choose a Water Softener Size

Water softeners are rated by their total daily grain capacity or size in grains per gallon (gpg). For example, there are 18,000 grain, 23,700 grain, and 34,900 grain capacity water softeners. To figure out how much mineral content you need removed, you need to know how hard your water is. This information may be available from your water department, so it’s worth checking with them first. If you have a well, you can collect samples yourself and send it off to a lab or purchase a home water test kit.

Water chemistry (including hardness) is measured in parts per million (ppm) and water hardness is rated:

 Soft 0-17 ppm
Slightly hard 17-60 ppm
Moderately hard 60-120 ppm
Hard 120-180 ppm
Very hard Above 180 ppm

To convert ppm to grains per gallon (gpg), 17.137 ppm = approximately 1 gpg.

Now, my water is so hard that I’m surprised it’s not carved out from a marble quarry. On my meager test kit, water hardness came in a 800+ ppm. To get gpg, I had to divide 800 by 17.137. That gave me about 46.68 gpg. Alternatively, you can use a handy online calculator.

Because iron is also usually present in hard water, I tested for it and came up with 5 ppm. Iron bonds so tightly to the resin in a water softener that it’s really difficult for the sodium to dislodge it. Consequently, there’s a rule of thumb that for every ppm of iron, you need to add 3-5 grains for hardness. Given that some of the older plumbing fixtures had lots of iron staining, I went with a 1 to 5 ratio. That added 25 ppm to the grain total, giving me a final count of 71.68 gpg.

Find the Total

Now that I had a ballpark figure for the amount of hardness per gallon, I needed to find our total daily water use in order to calculate the total daily grain capacity of the new softener:
  1. Estimate the daily gallons used per person in your household and multiply by the number of members of your household.
  2. Multiply total gallons used per day by your gpg.

I went with 80 gallons per person and multiplied that by the five people in my home. That gave me 400 gallons per day for total use. To calculate the total daily grain capacity, I multiplied 400 gallons by the 71.68 gpg I got for water hardness. That gave me 28,672 gpg.

Now, I’m ready to look for a new water softener that has a capacity greater than 28,672 gpg. I’m also going to factor in a little extra capacity for occasional high demand and wiggle room. Needless to say, there’s still a lot of comparison work to do to find the right water softener model, but having found the right size, I’m now on my way.