What Size Water Softener Do I Need?

Tape measure rolled out in the form of a chart, representing different sizes of water softenerIf you’re looking for a new salt based water softener system, the question what size water softener do i need? has probably occurred to you. If it hasn’t, it should. Water softener sizing is an important factor in determining the initial cost of your system, its maintenance costs going forward, and even its longevity. You don’t want to buy a system that’s too large or too small for your needs.

The job of conventional ion exchange water softeners is to take out dissolved grains of hard water minerals and replace them with something else (usually sodium). Therefore, water softener size is measured by grain capacity. Grain capacity is the number of grains of hard water minerals a softener can remove before the system needs to recharge. When we’re talking about water softener size, that’s the key number we’re looking for.

Note that salt free water softeners are different. Most of these systems are one-size-fits all. However, check individual model details.

The Water Softener Sizing Formula

Once you know the hardness of your water (how many grains of hard water minerals it has) and the amount of water you use, then you can easily calculate the grain capacity you need. Here’s the simple formula:

water hardness (in grains per gallon) x amount of water used daily (in gallons) x number of days between regenerations = grain capacity needed

Water hardness. Most water service companies will do a free water hardness test for you (in exchange for the opportunity to sell you a water softener). However, you can easily do the test yourself using water hardness test strips.

Amount of water used daily. Ideally you should look at a water utility bill to determine exactly how much water you use. However, a rough estimate is 75 gallons of water per day per person. So, most families of four use around 300 gallons of water a day.

Number of days between regenerations. For the sake of convenience, and to reduce wear and tear, you generally want your softener to regenerate once per week. So, in most cases, this number should be 7. However, it can be as little as 3.


Let’s look at a household with hard water (10 grains per gallon), four people using the average amount of water, seeking the typical regeneration frequency.

10 grains per gallon x 300 gallons (4 people x 75 gallons per day) x 7 days = 21,000 grain capacity

What Size Water Softener Do I Need?

Now that you know what grain capacity you need, you can look for the right water softener to meet your needs. For quick reference:

Less than 20,000 grain capacity = Small water softener

20,000-40,000 = Medium water softener

More than 40,000 = Large water softener

Beware of people trying to sell you a softener that’s larger than what you need, or that uses too much salt to achieve the desired grain capacity. As demonstrated by the example, most families only need a small or medium sized water softener to keep their home’s water soft.