How Do You Size a Water Softener?

Traditionally, water softeners have been sized using the following formula:

  • Step 1 – Enter the hardness in gpg (grains per gallon);
  • Step 2 – Enter the iron in ppm (parts per million) multiplied by 4;
  • Step 3 – Enter the manganese in ppm multiplied by 6;
  • Step 4 – Add Steps 1-3 to get what is called Compensated Hardness;
  • Step 5 – Multiply the number in the family by 75 to 100 gallons (that’s how much water the average person uses a day); and
  • Step 6 – Multiply Step 4 times Step 5 and that gives you the total grain capacity you need each day.

What happened above, is that an arbitrary number was assigned to iron and manganese as a factor which was added to the actual hardness to determine how much hardness was in each gallon of water. Then, you just figured how many gallons of water you used each day and multiplied that by the hardness in each gallon and PRESTO – you knew how many grains of hardness were needed to be removed from the water so that it would be soft.

Next, you decided how often you wanted to regenerate (usually 2-4 days between regenerations) and you picked a softener in that size range.

I am going to suggest that sizing a water softener that way is outdated, and in fact, can get you in a lot of hot water if you are a dealer or cause you to have intermittent hard water if you are the customer.

The problems is that you can have enough “daily capacity” when you size a softener this way, but you might not have enough “hourly capacity.”

What am I talking about?  Well, let’s say you and your spouse have 3 kids, from ages 8 to 16.  You have 4 bathrooms and when everyone gets up for school or work, there is about an hour period of time where 70-80% of your daily water usage can be utilized for showers, toilets flushing, dishwasher running, lavs and kitchen sinks running and even a washing machine doing a quick load before work.  You have enough capacity to treat hundreds of gallons of water a day, but you don’t have enough hourly capacity because the water flow through the softener exceeds it’s ability to soften (softening is a matter of “contact time” – if you don’t have enough contact, you will have “hardness leakage”).

Averages aren’t always good.  As an example, put one foot in boiling water and another in ice water and see if “on average” you are comfortable. 

The solution is simple – slightly oversize the softener so that it has more contact time with the resin.  For example, your compensated hardness indicates that you might need a 35,000 grain water softener, but you know you use a ton of water between 7 AM and 8 AM, so you up-size the softener to the next size up which is a 53,000 grain unit.  It costs about $100 more but you got a 50% larger water softener.  It has to regenerate less.  It doesn’t work as hard.  It has more contact time with the water so the water quality is consistently better. There is no hardness leakage and you will love your water.

At US Water, we have used a formula to size water softeners dramatically different from the above formula.  It has worked for us for over 20 years and involves just sizing the system to the number of people in the family and the number of bathrooms.  To that end, we are releasing our new Water Softening Calculator this month, which will tell you what size system is right for you.  Just enter the number in the family and the number of baths and PRESTO – it tells you what size is best for you.

Watch for it on our website soon.

This article has 2 Comments

  1. I am in the market for my first softener and having read a lot of information on sizing, I would appreciate your feedback on the following information I recently read.

    Basically what the article states is that a water softener rated at 32,000 based on lab results may only perform around 30,000 in real world usage. Also stated is that a system operating on high efficiency will use much less salt but will only regenerate at approximately 2/3 of the systems capacity, which can create several issues.

    I just touched on some of the details of the article but can you let me know if this is true since it would have an impact on the size unit I would need to purchase to get the results I am looking for.

    The Full article is located at

  2. I want you to understand what I am about to say is not meant to be disrespectful in any way. Ion-exchange theory and application is something that we could discuss for several days… and just scratch the surface. I take issue with the attached article because it is true, but not accurate. There’s a lot more to that article than what you read. Sizing a water softener is a lot more than grains capacity. If you are commenting on this blog, you will know how we size a water softener.

    What I read from Aquatell sounds like an indictment, but that’s how softening works. It’s just silly to rely on outdated sizing techniques.

    It would take me too long to debunk all that is in that article and explain how ion-exchange really works that you don’t have enough time and neither do I.

    Sometimes, you just have to trust the experts.

    Here’s the simple way to size a softener:

    How many in your family and how many bathrooms?

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