What Size Water Softener Do I Need? (How to Size a System)

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The right-sized water softener for you depends on your water hardness and usage. 

  • If you have a smaller home or don’t use much water, and your water isn’t very hard, you might be fine with a 24,000-grain model. 
  • If you have a larger home or very hard water, you might need to upgrade to a bigger 40,000-grain water softener. 

The right-sized water softener for you depends on your household size and your water hardness

It’s important to correctly size a water softener so that it doesn’t regenerate more frequently than once every three days (to minimize water waste and salt usage) but regenerates at least once every 14 days (to avoid damage to the resin). 

How to size a water softener

đź“Ź How to Size A Water Softener: The Calculation

To work out the size of a water softener for your home, use the following three-step calculation: 

Step 1: Number of people in your home x 701 = N
Step 2: N x your water hardness (measured in GPG2) = grains per day (GPD) number
Step 3: Multiply the GPD number by 7 (the number of days in the week). 

You can compare this number to the different water softener capacities available and choose the most suitable size for you. 

If your water contains dissolved iron, make sure you add 5 GPG to your water hardness measurement for every PPM of iron present in your water. 

1 70 refers to the national average volume of water used per person, per day, measured in gallons per day. 
2 You can convert to PPM or mg/L by dividing the GPG measurement by 17.1 

Example Calculation For Measuring Water Hardness

Using the two-step calculation above, here’s an example of how you might measure water hardness if you have three people in your home and a water hardness of 18 GPG. 

What you need to know in advance: 

Step 1: Multiply the number of people in your home by 70.

3 x 70 = 210

Step 2: Multiply 210 by your water hardness (measured in GPG2) to get the GPD number.

210 x 18 = 3,780 GPD

Step 3: Multiply 3,780 by 7 (the number of days in the week).

3,780 x 7 = 26,460

It’s always best to go up in size rather than slightly undersizing, so in this case, you would go with a 32,000-grain water softener. 

We’ve designed this calculator to simplify the process for you.


📲 How to Calculate Water Softener Service Run Length

A water softener’s service run length is the estimated amount of water it can treat before needing to regenerate. It measures the softener’s efficiency and capacity to deal with your specific water hardness.

There’s a simple formula to calculate a water softener’s service run length. 

What you need to know in advance:

  • Your required water softener’s resin capacity (in grains)
  • Your water hardness (in grains per gallon) 
HardnessGrains per Gallon (GPG)Parts per Million (PPM) & mg/L
Soft<1 0 – 17
Slightly Hard1.5 – 517 – 60
Moderately Hard3.5 – 7 60 – 120
Hard7 – 10120 – 180
Very Hard>10>180

Use this data in the following calculation: 

Resin capacity (grains) / total hardness (GPG)

Example Calculation for Water Softener Service Run Length

Let’s say that your water softener’s resin capacity is 20,000 grains, and your water’s total hardness is 24 GPG. Here’s the calculation you would use: 

20,000 / 24 = 833 gallons

If you don’t enjoy crunching numbers, you can use this handy calculator for estimating your service run length.


  • Jennifer Byrd
    Water Treatment Specialist

    For 20+ years, Jennifer has championed clean water. From navigating operations to leading sales, she's tackled diverse industry challenges. Now, at Redbird Water, she crafts personalized solutions for homes, businesses, and factories. A past Chamber President and industry advocate, Jennifer leverages her expertise in cutting-edge filtration and custom design to transform water concerns into crystal-clear solutions.

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