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Water softeners are sold in different sizes indicating how many hardness grains they can remove before they need to regenerate.
Choosing the right size is important because a softener that is too small will exhaust its softening capacity ahead of schedule, reduce water pressure and flow rate, require more time and money to maintain, and wear out prematurely.
On the other hand, a system that is too big is not only expensive to buy, infrequent regeneration cycles will also cause damage to its resin bed.
Simply put, properly sizing a water softener is key. How do you do that? Check out the following article to learn all you need to know.
5 steps are involved in properly sizing a water softener:
We will discuss each step in detail and provide you with plenty of examples along the way so everything is easy to understand.
If you want to learn about all the things to consider before you buy, take a look at our water softener buying guide.
Please note: Knowing your water’s exact hardness level is crucial. Mere guessing will likely result in improper sizing.
Let’s start by defining what water hardness actually is:
Water classifies as “soft”, “moderately hard”, “hard”, etc. depending on its calcium carbonate concentration. Levels are measured in ppm (parts per million), mg/L or gpg (grains per gallon). The latter is the industry standard.
If you want to convert from ppm or mg/L to gpg, divide by 17.12.
1 ppm = 1 mg/L and 1 gpg ≈ 17.12 ppm = 17.12 mg/L
|Soft||0 – 60||0 – 60||0 – 3.5|
|Moderately hard||61 – 120||61 – 120||3.56 – 7.01|
|Hard||121 – 180||121 – 180||7.06 – 10.51|
How can you find out what hardness level you are dealing with? That’s simple! You have a couple different options:
Furthermore, you have to pay attention to high iron levels, particularly if you receive water from a well. For each 1 ppm of iron, add 4 gpg to the total hardness value. Some experts also recommend adding 4 gpg per 1 ppm of manganese.
Calculation Example: Hardness Measured at 140 ppm, Iron at 1 ppm
140 ppm / 17.12 = 8.18 gpg
8.18 gpg + 4 gpg (equals iron at 1 ppm) = 12.18 gpg (total hardness)
Step 2 is to estimate your family’s daily water consumption. If you want to take out all the guesswork you should consult your latest water bill. It’ll list your consumption within a certain time period – think month, quarter or year. Use this number to calculate the daily average.
What you can do if you don’t have access to this information is to multiply the number of people living in your home by 80 to 100. This is how many gallons of water the USGS estimates a person uses per day. If you like to take really long showers, multiply by 100. If you try to conserve water as best you can, multiply by 80.
Calculation Example 1: 36,000 Gallons Usage in Q4 2018
36,000 gallons / 90 days = 400 gallons (per day)
Calculation Example 2: 4-Person Family, 90 Daily Gallons Per Person
4 persons x 90 gallons/person = 360 gallons (per day)
Lastly, multiply your hardness level by your daily water usage. The result is your daily softening requirement, so how many hardness grains a softener has to remove from your water on a daily basis.
Calculation Example: Water at 10 gpg, 360 Gallons Per Day
10 hardness grains/gallon x 360 gallons/day = 3,600 hardness grains (per day)
As you can tell from the calculation example, if your water measures at 10 gpg and you use 360 gallons a day then your potential new softener would need to remove 3,600 hardness grains each day – so far, so good.
Now, a water softener can only remove so many hardness grains before it has to regenerate. This has to do with how the softening process works. At some point, all resin beads inside the softening tank will be saturated, meaning that they can’t bind any more calcium or magnesium ions.
Regeneration involves washing out and draining away the accumulated minerals, and recharging the resin bed with sodium or potassium.
Most experts recommend to regenerate a softener about every six to seven days. This provides for a good balance between high softening efficiency, low wastewater production and protecting the head valve against wear. It’ll also conserve the resin protecting it against iron and sediment.
Multiplying our daily softening requirement of 3,600 grains by 7 days we get a total softening capacity of 25,200 grains:
3,600 hardness grains per day x 7 days = 25,200 grains (per week)
Theoretically speaking, this would allow us to regenerate a water softener with a 25,200-grain rating once a week and never run out of soft water. Why theoretically speaking? Because up to this point we haven’t taken into account salt efficiency a.k.a. brine efficiency.
Simply put, regenerating a softener’s resin bed to 100 percent requires a disproportionately high amount of salt, whereas partial regeneration is much more efficient. For instance:
In order to fully regenerate a 33,000-grain unit 14.1 lbs of salt are needed. The same model requires no more than 2.6 lbs of salt to regenerate 13,000 grains. And with 8.3 lbs of salt you could regenerate 27,600 grains.
In other words, while a 33,000-grain system can remove 33,000 hardness grains from water between two regeneration cycles, it would demand an excessive amount of salt doing so.
However, only recharging approx. 40% of the softening bed requires no more than one-fifth of the salt amount, while three-fifth of the salt would lead to roughly 84% regeneration. Obviously, this is a much cleverer way to operate a water softener.
The relation between regeneration level and required salt dose is illustrated by the following chart:
The higher the salt dose, the higher the softening capacity, but the lower the efficiency per pound of salt.
Bottom line: If you require a water softener that can remove 25,200 hardness grains per week as calculated in our example above, actually choosing a system with a higher grain capacity – e.g. 40,000 grains – will save you a lot of money on salt in the long run.
What’s more, servicing will be much more convenient and less salt will be discharged into the environment threatening aquatic life. It’ll also benefit your sewage system in case you have one.
Just to clarify, the reason why you need a higher grain capacity than calculated is because if you do not let your softener regenerate to 100%, it’ll exhaust faster. This effect can be compensated by a bigger system.
Grain Capacities & Tank Sizes
A softener’s grain capacity is the maximum number of hardness grains the system can remove before it has to regenerate. The lower the rating, the higher the required regeneration frequency.
Grain capacity can also be described as cubic feet of softening resin. Higher volume means larger tank size.
|Resin in Cubic Feet||Tank Dimensions (Ø x Height)||Maximum Grain Capacity|
|0.75||8″ x 44″||15,000|
|1.0||9″ x 48″||20,000|
|1.5||10″ x 54″||30,000|
|2.0||12″ x 52″||40,000|
|2.5||13″ x 52″||50,000|
|3.0||14″ x 65″||60,000|
You should always check the salt efficiency of a model which is expressed in grains per pound of salt. The number indicates how many hardness grains a softener can remove per pound of salt that it uses at the minimum salt dose and under real-life conditions.
One model might have 4,410 @ 2.9 lbs and another one 5,060 @ 2.7 lbs. The latter is more efficient. Softeners certified against NSF/ANSI Standard 44 must have a salt efficiency rating of at least 3350 grains per pound of salt so that’s something to look out for. The efficiency for systems installed in California must be at least 4000 grains per pound.
Daily and weekly softening capacity is all well and good, but if your softener cannot deliver enough soft water at times of peak consumption, hard water will leak through and/or you will see a drop in pressure and flow.
This can especially be the case in the morning when everyone gets up for work and school. There might be multiple showers and taps running, toilets flushing, and maybe you start the dishwasher before leaving the house.
Thus, make sure to pay attention to service flow rates measured in gallons per minute (gpm).
Generally speaking, larger homes require higher flow rates. Single households might do with 6 or 7 gpm. For families with 2 or more bathrooms we recommend a minimum flow rate of 12 gallons per minute. Big families should look for systems providing at least 15 gallons per minute. (Source)
Typical Water Use of Modern Outlets
There are many variables that go into sizing a commercial water softener. Water temperature, pressure and flow rate are only some of them.
However, this sizing guide is aimed at private home owners. For sophisticated commercial water softener sizing and configuration we recommend you contact a specialist in your area. Just know that this is not going to be cheap.
Do you now know what grain capacity you need? Great! Click here to find our reviews of suitable systems.
If you have any questions or thoughts about how to size a water softener please don’t hesitate to leave a comment below!