How To Size A Water Softener? 4-Factor Sizing Guide + 3 Examples

“Our household of 4 has hard water. We are looking at different sizes and brands of water softeners. How do we determine the size of the water softener we need to buy?”

This is the classic water softener sizing problem every family has to solve. If you buy an undersized one, you will still have hard water. If you buy an oversized one, you will needlessly spend $100s extra, have problems with regen cycles, and so on (general overkill). You will very often see water softeners that are too big installed since water softener dealers really like to push for bigger (reason = $$$) units.

To help everybody out, Waternity Lab has prepared this all-in-one “How To Size A Water Softener” guide (with formula), along with the 3 solved sizing examples at the end. We are going to take a structured approach here, going step by step, to finally answer the following question:

Do you need a 24000 grain (smallest), 30000 grain, 36000 grain, 48000 grain, 56000 grain, or 64000 (biggest) grain water softener?

Note: This article explains how to calculate the water softener size needed (math and considerations). If you need to size a water softener, you should use this water softener size calculator that does all this math and considerations automatically right away.

These are the water softener sizes or capacities in terms of grains for residential use.

36000 grain water softener installed for a 4 family household
Example of a 36,000-grain water softener installed for a family of 4.

(Very) Quick Example: Let’s say your family uses 1,000 gallons of water per week. You measured that water hardness is 15 GPG (equal to 205.42 PPM; this is considered “Very Hard” water), and iron content is insignificant. What size water softener do you need?

You want to decrease water hardness below 3.5 GPG (categorized as “Soft” water), ideally to 0 GPG. That means we are aiming to remove 15 GPG from every gallon of water you use; at 1,000 gallons/week, that’s 15,000 grains. Now, we compensate for 75% saturation regeneration cycle activation (we’ll explain this further on), and we figure we need a water softener to remove 20,000 grains of water hardness. Naturally, we have to opt for the smallest 24,000 grain unit.

Note: We are also going to explain the common errors made in most water softener sizing guides you find on the internet. The two key factors that are left out are 1) water softener regen cycle activation at 75% resin saturation, and 2) size increase for hard water containing iron ions (Fe2+).

Let’s first outline how to size a water softener in general using all 4 factors. Even more importantly, after the explanation, we will show you exactly how to calculate the size of water softener system by solving 3 practical examples:

How To Determine Size Of Water Softener (Complete Overview)

To figure out what size water softener we need, we have to start with 2 pillars (first 2 factors) of water softener sizing:

  1. How hard is your water? We can express this is PPMs, GPG, mg/L, mmol/L of various calcium molekules. The harder water you have, the bigger the water softener you should have. Note: For softener sizing, using GPG or Grains Per Gallon water hardness is perfect. You can check the water hardness scale for PPM and GPG here.
  2. How much water do you use? A household using 2,000 gallons of water per week will likely (not necessarily, also depends on water hardness) need a bigger water softener than a household using 1,000 gallons of water per week. Note: It’s perfect to use a weekly water usage in gallons. We will show you how to calculate this from your monthly water bill.

Now, the recommended regeneration cycle for most units (SpringWell, Culligan, Pelican, Fleck, etc.) is 1 week. Based on this, we also have to account for 2 additional water sizing factors, namely:

  1. Usable water softener capacity is 75% of its total capacity. When the unit hits the regeneration cycle, it still has to have 25% of the grain capacity left. That means that the true capacity of 24,000 grain unit is actually 18,000 grains, the true capacity of 36,000 grain unit is 27,000 grains, and so on.
  2. Iron content. If we simplify, we can say that water hardness is caused by the calcium and magnesium ions. However, iron ions, if present, also add quite a lot of water hardness, and we do have to account for that (it creates these rusty brownish-red stains that can congest your piping as well). The rule of thumb is that we add 5 GPG per every 1 PPM of iron present in your water. Example: If you have a lot of iron in your water (say 2 PPM), we have to add 10 GPG to the complete water hardness.

Accounting for all these 4 factors, we can write the water softener size equation like this:

Water Softener Size (Grains) = (Water Hardness (GPG) + Iron Content (PPM) × 5) × Weekly Water Usage (Gallons) / 0.75

This water softener sizing formula may seem a bit complex. Let’s break it down and see how you can DIY water softener sizing accurately:

1. Water Hardness (1st Factor)

Water softeners use grains of salt to extract calcium ions (Ca2+) and magnesium ions (Mg2+) out of water; making it soft water. To determine how much water has to be softened (corresponding to the size of the water softener), we first need to figure out how hard our water actually is.

The easiest way to measure water hardness is by using home water hardness test kits. These may be testing strips (colors corresponding to water hardness), TDS meters, EC meters, and so on.

measure water hardness with a tds meter
Measuring water hardness with a TDS meter.

The key data point we want to get here is how many grains per gallon of water hardness we have. When we know that, we know how much water hardness our new water softener will have to extract per gallon.

Example: If we measure 10 GPG, we know that we need to extract 10 grains per every gallon we use (in a week). If we measure 25 GPG, we have to extract 25 grains per every gallon we use, etc.

Since water softener size is expressed in grains, we also need to measure water hardness in grains (GPG), or convert measured water hardness in other units to GPG. To help you out, we have prepared these converters of water hardness units to GPG:

  • If you measure water hardness in PPM, you should use this PPM to GPG converter to get water hardness expressed in GPG. Example: 200 PPM is equal to 11.68 GPG.
  • If you measure water hardness in mg/L of CaCO3, you should use this mg/L to GPG converter to get water hardness expressed in GPG. Example: 100 mg/L of CaCO3 is equal to 5.84 GPG.

Once we know how hard our water is (in terms of GPG), we can proceed to figure out how much water we use weekly:

2. Water Consumption On Weekly Basis (2nd Factor)

If you use a lot of water, you need a more sizable water softener. Namely, we need to determine how many gallons of water we use per week. Why per week? Because with water softener sizing, we know that it should start a regeneration cycle every 7 days. That means that the states water softener capacity – be it 24000 grains, 36000 grains, 48000 grains, and so on – should last for 1 week.

Now, there are these 2 ways of how to determine how many gallons of water you use per week:

  1. Theoretical approach (not very accurate). Here we use statistical water usage for homes (not your home specifically). EPA states that “the average American family uses more than 300 gallons of water per day” (that’s 2,100 gallons/week). You can take this number but it usually less accurate since this is the national average.
  2. Practical approach (very accurate). Here we take a look at our monthly water bill. On the bill, you will see how much water you have used in previous months. This is usually expressed as used gallons of water per month or per 30 days. To get our weekly water usage, we have to divide monthly usage by 4.3 since 1 month = 4.3 weeks.
monthly water bill to determine how many gallons of water you use per month
Find the water usage on the water monthly bill, divide these gallons per month by 4.3 to get your water usage in gallons per week.

Expert Tip: You should always get the weekly water consumption via monthly water bill (2nd option). This gives you the most real-life and, thus, accurate data on water usage in your home.

Quick Example: Let’s say you have a look at your monthly water bill. You see that you used 7,000 gallons of water in the previous month. We can use this data point to calculate how many gallons you use per week like this:

Weekly Water Usage = 7,000 Gallons/Month / 4.3 = 1,628 Gallons/Week

Now that we know both how hard our water is and how much we use it per week, we can make a rough (albeit a bit inaccurate) calculation for the water softener size we need.

For adequate sizing, however, we really need to account for the following 3rd and 4th factors. Here they are:

3. Account For 75% Usable Water Softener Capacity (3rd Factor)

From the get-go, most of us think that a 36,000-grain water softener can extract 36,000 grains of water hardness from our water, right? It seems quite a reasonable conclusion.

However, due to the way most water softeners operate, we cannot use all 36,000 grains of water in 1 cycle. Namely, about 25% of the grains inside the water softener must be left over when we start the regeneration cycle.

Here is how this real water softener capacity changes when we account for the 25% that must be left in the water softener (we did all the calculations here so you get the immediate feeling of the water softener’s effectiveness):

  • 24,000 grain unit will use 18,000 grains to extract water hardness, but 6,000 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 24,000-grain water softener is actually 18,000 grains.
  • 30,000 grain unit will use 22,500 grains to extract water hardness, but 7,500 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 30,000-grain water softener is actually 22,500 grains.
  • 36,000 grain unit will use 27,000 grains to extract water hardness, but 9,000 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 36,000-grain water softener is actually 27,000 grains.
  • 48,000 grain unit will use 36,000 grains to extract water hardness, but 12,000 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 48,000-grain water softener is actually 36,000 grains.
  • 54,000 grain unit will use 40,500 grains to extract water hardness, but 13,500 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 54,000-grain water softener is actually 40,500 grains.
  • 60,000 grain unit will use 45,000 grains to extract water hardness, but 15,000 grains will be left inside the unit when we start the regen cycle. That means that the true capacity of a 60,000-grain water softener is actually 45,000 grains.

Quick Example: Let’s say we use 2,000 gallons of water per week, and the measured water hardness is 15 GPG. A quick, but inaccurate, calculation would tell us we need 2,000 gallons × 15 GPG = 30,000-grain unit. However, to be accurate, we have to account for the 25% grain leftover like this:

Water Softener Size = 2,000 Gallons × 15 GPG / 0.75 = 40,000 Grain Water Softener

Dividing by 0.75 accounts for 75% of usage grains during 1 regen cycle. Here we see that:

  • Inaccurate calculation (without 75% usable grains) would tell us we need a 30,000-grain unit. This is a wrongly sized unit.
  • Accurate calculation (accounting for 75% usable grains) tells us we need a 40,000-grain unit. This is a more adequately sized unit. Since there is no 40,000-grain unit on the market, we can opt for a 36,000-grain or 48,000-grain unit; a 30,000-grain unit would certainly be too small.

The final factor – iron content – is especially important for homes that have a lot of iron in their water (less significant for very low iron water):

4. Account For Iron Content (4th Factor)

With water hardness, we mostly use water softener grain capacity to neutralize calcium and magnesium ions. The great thing about water softeners, quite surprisingly, is that they can also extract unwanted iron ions (Fe2+).

We definitely want to take advantage of the iron-removing ability. Water with heavy iron content can rust our piping, leave brown-red stains on our faucets, and so on. Obviously, however, we do have to boost the water softener size to account for this extra iron-removing job.

why we include iron ion concentration in calculate how many grain water softener we need
Rusting outside and, more importantly, inside the pipes can be much more severe and quick if you have a lot of iron concentration in your water.

Here we use the following water softener sizing rule of thumb:

  • Add 5 GPG for every 1 PPM of iron ions present in our water.

Let’s go for a quick example to demonstrate how to account for iron ion concentration. Namely, let’s say that we have measured water hardness to be 10 GPG. Here are some examples:

  • If, in addition to 10 GPG water hardness, we also have 0.5 PPM iron content, we have to add 2.5 GPG to get the overall water softener size relevant water hardness of 12.5 GPG.
  • If, in addition to 10 GPG water hardness, we also have 1 PPM iron content, we have to add 5 GPG to get the overall water softener size relevant water hardness of 15 GPG.
  • If, in addition to 10 GPG water hardness, we also have 2 PPM iron content, we have to add 10 GPG to get the overall water softener size relevant water hardness of 120 GPG.
  • If, in addition to 10 GPG water hardness, we also have 3 PPM iron content, we have to add 15 GPG to get the overall water softener size relevant water hardness of 25 GPG.

As we can see, in homes with very high iron content, we will have to upsize a water softener quite a lot.

You can measure the concentration of iron ions in your water by using heavy metals water home test kits.

How To Calculate The Size Of Water Softeners (3 Examples)

Now that we understand what all parts of the water softener size formula mean, we solve 3 water softener sizing problems.

Example 1: A family of 5 uses 2,400 gallons of water per week. The measured water hardness is 12 GPG, and we also have some iron content (0.5 PPM, to be exact). How do we determine what size water softener this family of 5 needs?

Fairly simple, really. We use the water softener size formula from the 1st part like this:

Water Softener Size  = (12 GPG + 0.5 PPM Iron × 5) × 2,500 Gallons Per Week / 0.75 = 48,333 Grain Water Softener

In this case, we would choose a 48,000-grain water softener since it’s the closest to our adequately calculated grain number.

Example 2: In a 2,000 square feet house, the weekly water usage is 3,500 GPG. The water hardness is 8.5 GPG, and there is no iron content. Here is how to calculate the size of water softener we need for this 2,000 sq ft home:

Water Softener Size  = (8.5 GPG + 0 PPM Iron × 5) × 3,500 Gallons Per Week / 0.75 = 39,667 Grain Water Softener

We see that this 2,000 sq ft home will need to choose between a 36,000-grain and 48,000-grain unit. The 36,000-grain water softener would be the most optimum.

Example 3: A small 1,000 sq ft cottage uses only 1,200 gallons of water per week. However, this is very hard well water with 23 GPG water hardness. In addition, it also has 2.5 PPM of iron ion concentration. Here is how we calculate how big a water softener you need for this 1,000 sq ft cottage:

Water Softener Size  = (23 GPG + 2.5 PPM Iron × 5) × 1,200 Gallons Per Week / 0.75 = 56,800 Grain Water Softener

We see that despite the small size of the cottage, the water softener requires should have about 56,800 grain capacity (due to very hard water and high iron content). In this case, we would opt for a 56,000-grain unit.

Conclusion And Questions

We hope that this illustrates well how to go about sizing your water softener. Do account for all 4 factors, use the water softener sizing equation, and you will get a pretty accurate result.

If you need a bit of help or would like for us to do some water softener sizing math, you can use the comments below, give us some numbers, and we can help you out.

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