Water Softener Size Calculator: How Many Grains?

Correctly sizing a water softener can save you more than $500 per year. That’s because the biggest mistake most first-time water softener buyers make is ‘bigger capacity is always better’.

That’s an easy way out for choosing the size of the water softener (and unnecessarily using up to 1000 lbs of salt too much). The reason why is simple:

Calculating the correct size of a water softener is not easy. Factors include the water expenditure, hardness of water and potential iron content.

If you’ve Googled ‘How big a water softener do I need’, you were probably disappointed by the explanation. Water softener size calculators don’t really give you the correct grain capacity (instead you get a recommendation of a water softener), and even experts try to get away with ‘most households need a 32,000-grain capacity water softener’.

That’s why we at Waternity Lab decided to explain in the most simple terms how to size a water softener. We used publically available statistical approximations to estimate the size of water softener down to 1,000-grains.

Here’s the deal:

The correct grain-capacity of a water softener is simply matching your water quality and water use in order to:

  • Minimize salt use (some people use 2,000+ lbs of salt per year while 1,000 lbs would do the same job).
  • Maximize water softener capacity (using it to it’s fullest).
  • Prolong the lifespan of your water softener.
how big should a water softener be
Correctly sizing water softener can reduce your salt costs for more than $500/year.

To truly simplify everything, we have developed a water softener size calculator (you’ll find it at the end of this article).

First, however, let’s look at how the size of a water softener can be calculated:

What Does The Grain Capacity Even Mean?

The capacity of a water softener is measured in grains. Essentially, water hardness is measured in ‘grains per gallon’ or GPG. This unit can be converted to parts (of hardness) per million or ppm.

The whole point of a water softener is to reduce the hardness of the water. Look at how many ppm tap water has and what is considered soft water in the sketch below:

hardness of water in parts per million

Basically, a water softener reduces hard tap water at 300 ppm (or 17.5 GPG) to soft water (below 60 ppm or 3.5 GPG).

For example, to turn 1 gallon of hard tap water into soft water, we would need 14-grain capacity:

17.5 GPG – 3.5 GPG = 14-grain capacity

Of course, water softeners have 32,000-grain, 48,000-grain, and even 60,000-grain capacity. Most people are wondering what does a grain-capacity of a water softener even mean. Here’s how that goes:

Example: A water softener will have a 48,000-grain capacity.

What that means: A water softener has the capacity to remove 48,000 grains of water hardness after 1 regeneration cycle.

In short, a 48,000-grain capacity is capable to turn 3,428 gallons of hard water (300 ppm) into soft water (below 60 ppm) before we need to regenerate it again.

A family of 5, for example, can use up to 500 gallons of water per day. To have continuous access to soft water, you would need to regenerate such a softener every 9 or so days.

How To Calculate What Capacity Water Softener Do You Need?

To calculate the grain water softener capacity, we need to figure out two things:

  1. How much water we use (per day).
  2. What is the actual hardness of our water?

An additional consideration is how often do we run a regen cycle. Some people run it every 3 days, others every 2 weeks. Here is how often should a water softener regen:

  • Every 3 days – TOO OFTEN.
  • Every week – JUST RIGHT.
  • Every 2 weeks – NOT OFTEN ENOUGH.

In short, running a regen every 7 days (1 week) is ideal. That keeps the resin bed properly regenerated and it doesn’t damage the valve in the long run. Usually, you do it on Sunday evening when before going to sleep.

To properly size a water softener, we thus need to calculate how many hard water grains a softener needs to remove in one week. Here is how this equation looks like:

Water Use Per Week (In Gallons) * Water Hardness = Minimum Grain Capacity

Let’s figure out how much water you use and how to measure the water hardness:

How Much Water Does Your Household Use Per Week?

Water in a household is used for washing, dishes, drinking, and so on. There are two ways how to estimate your water use:

  1. Check your monthly water bill.
  2. Or estimate the water use based on a statistical average.

The easiest way is to check your monthly water bill. You will find how many gallons of water you use per month and can calculate how much water you use per week.

Example: Checking the water bill, you see that you used 10,000 gallons of water the previous month.

Calculation: If you used 10,000 gallons in a month, you can fairly estimate that your water use per week is about 2,500 gallons (1 month = 4 weeks).

Another way is to estimate water usage. According to the USGS:

On average, each person uses about 80-100 gallons of water per day.

When we try to figure out what size water softener do we need for a family of 4, we can estimate that such a household uses 320-400 gallons of water per day.

Weekly usage: Between 2,240 and 2,800 gallons.

Now that we know how much water we need to soften, let’s look at how hard this water is:

How Hard Is Your Water?

Around 85% of US households have hard water. In order to properly size a water softener, it’s useful to know exactly how hard your water is.

Here is a table with water hardness classification by the U.S. Geological Survey Office:

Classification Hardness in ppm Hardness in GPG
Soft Water 0-60 ppm 0-3.5 GPG
Moderately Hard Water 61-120 ppm 3.56-7.01 GPG
Hard Water 121-180 ppm 7.06-10.51 GPG
Very Hard Water Above 181 ppm Above 10.57 GPG

To precisely measure the hardness of your water, you can buy a water hardness tester. It costs $10 to $20 and will tell you exactly how hard your water is.

However, you can also estimate that you’re among that 85% of US households that have hard water. That means that water that flows from your faucets and is used for washing clothes is between 7.06 GPG and 10.51 GPG. For simple calculation purposes, we can take the middle-point of hard water hardness: 8.79 GPG.

Water Softener Size Calculator

Let’s first solve an example using the equation the water softener size calculator is based on:

Water Use Per Week (In Gallons) * Water Hardness = Minimum Grain Capacity

Example: Family of 4 uses 350 gallons of water per day; that’s 2,450 gallons per week. They live in Ohio and have an average water hardness of 8.79 GPG.


2,450 gallons * 8.79 GPG = 21,535.5 Minimum Grain Capacity

We see that this family of 4 would need a water softener with about 22,000-grain capacity.

Here you can use the water softener size calculator to estimate what size of a water softener is optimum for your home:

Using the calculator, you can estimate how big a water softener you need.

Keep in mind that the sizing of a water softener is relevant for salt-based water softeners. You will find the best water softeners here (some of them do use salt and utilize either upflow or downflow regen cycle).

It gets even better:

If you choose a modern water softener that doesn’t require salt for functioning, you don’t really have to properly size a water softener. For more information, you can check the best salt-free water softeners here.

Leave a Comment