Sizing a water softener can be a little tricky and is often complicated by the numerous opinions offered from lots of different people.  It is critical to size a water softener properly in order to maximize the lifespan of the valve and also to minimize the amount of water and salt used to regenerate the system.  A properly sized water softener should regenerate about once a week and should provide an uninterrupted source of softened water.  Before we dive into the calculations used for sizing a water softener we need to define a couple of terms:

Water Hardness:  This is simply the amount of dissolved magnesium and calcium that is in your water

Grains per Gallon:  These are the units used to measure hardness in water.  More grains = harder water

Cubic Feet of Resin:  This is the amount of resin in a water softener measured in cubic feet

The two most important elements of sizing a water softener are understanding the amount of water that is going to be used in the home, and knowing the water hardness value - usually measured in Grains per Gallon (GPG).  It should be noted that some laboratory tests may measure hardness as Parts per Million (PPM) or Milligrams per Liter (mg/L).  In this article, we will use GPG for all calculations.  Multiply PPM or mg/L by 17.1 to convert to GPG.

Estimating the water use for a home is quite straightforward.  Many independent studies have shown that in a residential setting most people use about 75 gallons of water per day, per person.  This number may sound very high but keep in mind it takes all water uses into account.  This includes flushing toilets, showers, dishwashers and washing machines, not to mention what is consumed in drinking and eating.  To estimate the amount of water used per household multiply the number of people in the home by 75.  We're often asked if a softener should be sized based on the maximum occupancy of the house.  For instance, many people are concerned that even though they have 4 people in their home for 360 days of the year, should they be sizing the softener based on the 6 extra people living there over the holidays?  The answer is no - you want to size a water softener for the typical water use scenario.  The softener will simply regenerate more often to accomodate for those days of the year when more water than normal is used.

The next step in sizing a water softener is taking the total water used per day and multiplying it by the Grains per Gallon (GPG) of hardness.  The resulting number is the amount of hardness that the the water softener will need to remove per day.  We will call this value the DAILY SOFTENING REQUIREMENT.  Calculate this number and write it down, we'll need it again later.

Before moving on to the next step it's important to take a moment to describe the way in which water softeners are sized and how their sizing is described to consumers.  Water softeners are sold by the amount of "grains capacity" they have.  For instance, a very common water softener is a 32,000 grain water softener.  This title simply means that there is enough water softening resin inside the softener to remove a total of 32,000 grains of hardness before the resin is saturated and needs to be regenerated.  Based on this description, a very common conclusion is to to divide the DAILY SOFTENING REQUIREMENT into the overall size of the softener and find a system that will need to regenerate once per week.  Unfortunately, there is a flaw in this logic - and here's why:

When you purchase a water softener it is because you want to have soft water all the time.  If we assume that a 32,000 grain water softener will soften 32,000 grains before regenerating, we are also assuming that the softener will regenerate immediately after it has reached capacity - otherwise you'd get hard water breaking through.  We also don't know exactly when the softener will reach capacity.  It could occur right in the middle of the day.  It is not practical to have the water softener regenerate in the middle of the day, nor is it desirable to have anything but soft water all the time.  So for this reason, all metered water softeners maintain a "reserve capacity".  The reserve capacity exists so that if your softener comes close to reaching capacity in the middle of the day, there is still enough reserve to soften the water for the rest of the day and then regerate in the middle of the night when nobody needs to use the water.  Without the reserve capacity, softeners would run out of soft water and/or would have to regenerate in the middle of the day - and both of these situations are not ideal.  The typical reserve capacity used by a water softener is 20%.  This means that the water softener will not regenerate in the middle of the night as long as it has greater than 20% of it's capacity remaining.  If the value is lower than 20%, the softener will regenerate.  Please note that some softeners are smarter than others, and use a complicated mathematical model to predict water use for the next day based on historical household data.  But, for sizing calculations it is important that we take this reserve into account.  What most water professionals typically do is multiply the actual capacity of the softener by 0.8 giving the actual "working capacity".  So, a 32,000 grain softener has 25,600 grains of actual working capacity.  You can then divide your DAILY SOFTENING REQUIREMENT into that modified capacity in order to determine how often a given water softener will have to regenerate based on your water hardness and water usage.

All of the calculations above can also be rolled into one simple formula that will help you determine the size of softener that's best for you:

Softener Size = Number of People in Home X Hardness (GPG) X 75 Gallons X 7 Days between regenerations

This calculation will give you a rough number that you can then round up to the size of water softeners that are typically offered.  Standard sized softeners are the following:

• 24,000 Grain
• 30,000 Grain
• 32,000 Grain
• 40,000 Grain
• 45,000 Grain
• 48,000 Grain
• 60,000 Grain
• 64,000 Grain