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Water softener size calculator
A water softener is sized from the hardness it removes between two regenerations. This page multiplies the people in the house by the water each uses and by the hardness, adds 5 grains per gallon for each ppm of iron, and multiplies by the days between regenerations. 4 people at 75 gallons and 10 gpg make 3,000 grains per day and 21,000 grains over 7 days, which takes the 24,000 grain unit. The page then checks the peak flow of the house against the service flow rate of the unit. Resin type, salt dose and water chemistry beyond hardness and iron are not modeled. The same water also runs through the water heater size calculator and, on a private well, the well pressure tank size calculator.
- Give the people in the house, the water use per person, which the guides read at 75 gallons per day, the hardness in grains per gallon and the iron in ppm. The page opens on a family of 4 at 10 gpg.
- Choose the days between regenerations, about 7 in most guides, and whether the unit runs at its rated capacity or on a salt-saving setting that holds about 75 percent of the label.
- The page takes the smallest standard size whose capacity covers the load per regeneration. 21,000 grains takes the 24,000 grain unit. Then it compares the peak demand with the service flow rate you type from the spec sheet.
Example result for the default inputs
4 people, 10 gpg, regenerating every 7 days: 24,000 grains.
Softener size
24,000 grains
Load per day
3,000 grains
Load per regeneration
21,000 grains
Days at rated capacity
8.0 days
| Hardness used | 10 gpg, 10 gpg of hardness plus 0 ppm of iron at 5 gpg per ppm |
|---|---|
| Daily load | 3,000 grains per day, 4 people x 75 gallons x 10 gpg |
| Load per regeneration | 21,000 grains over 7 days |
| Capacity needed | 21,000 grains on the label, on a rated basis |
| Standard size | 24,000 grains, 24,000 usable, the smallest listed size at or above the load |
| Regeneration interval | 8.0 days at that size |
| Peak flow | 10 gpm against a service flow of 12 gpm: within the rating |
- Iron is 0 ppm here, so the load is hardness alone. Well water that carries iron needs the iron entered, at 5 gpg per ppm.
- The load is compared with the rated capacity on the label. A unit set to a salt-saving dose holds about 75 percent of that, and on that setting this load needs the 32,000 grain unit. Switch the capacity basis if it will run that way.
- The peak of 10 gpm is within the 12 gpm service flow rate entered. Service flow depends on the model and on the pressure drop it is quoted at, so read it from the spec sheet of the unit you pick.
- Resin type, salt efficiency, total dissolved solids, manganese, tannins and twin-tank softening are not modeled here. The result is a planning size, and a water test and the manufacturer's sizing chart decide the unit.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the softener size is worked out
This page sizes a softener from the hardness it has to remove between two regenerations. The daily load is the people in the house times the water each uses times the hardness, with iron added to the hardness at 5 grains per gallon for each ppm. Multiplied by the days between regenerations, that is the load per cycle, and the page names the smallest of the standard sizes of 24,000, 32,000, 40,000, 48,000, 64,000, 80,000 grains whose capacity covers it. A separate check compares the peak demand of the house with the service flow rate of the unit. These are public sizing guides and not a standard, and the page says where they differ.
- Daily load
grains per day = people x gallons per person per day x effective hardness (gpg)Penn State Extension works a household of 4 at 75 gallons and 10 gpg as 3,000 grains per day. The page opens on that case.- Iron
effective hardness (gpg) = hardness (gpg) + 5 x iron (ppm)Two guides add 5 gpg for each ppm of iron, one adds 4, and one reports 3 as typical. 12 gpg with 3 ppm of iron becomes 27 gpg, and the same family of 4 then carries 8,100 grains per day. Iron of 1 ppm or more calls for a filter ahead of the softener in one guide.- Load per regeneration
grains per regeneration = grains per day x days between regenerationsGuides aim at about 7 days, with 7 to 10 days as the target in one and 3 to 5 days as typical in another. 3,000 grains per day over 7 days is 21,000 grains. With 1 ppm of iron the same family carries 4,500 grains per day and 31,500 per cycle.- Standard size
size = smallest of 24,000, 32,000, 40,000, 48,000, 64,000, 80,000 grains with usable capacity at or above the load21,000 grains takes the 24,000 grain unit, which regenerates about every 8.0 days. 31,500 grains takes the 32,000 grain unit, at 98.4 percent of its capacity. A salt-saving setting holds about 75 percent of the label, so the same 31,500 grains takes the 48,000 grain unit, which holds 36,000.- Peak flow
peak demand (gpm) at or below the service flow rate of the unit (gpm)Add the flow of the fixtures that can run together, such as a shower, the kitchen faucet, a toilet refill and the clothes washer, and compare it with the service flow rate from the manufacturer. One guide lists about 8, 10, 12 and 15 gpm for 1 to 2, 3 to 4, 5 to 6 and 7 or more people. The page takes both numbers as typed.
Worked examples
The default case is a family of 4 at 75 gallons per person and 10 gpg with no iron. It makes 3,000 grains per day and 21,000 grains over 7 days, so the 24,000 grain unit covers it at 87.5 percent of its capacity and regenerates about every 8.0 days. With 1 ppm of iron the load becomes 31,500 grains and the 32,000 grain unit, and on a salt-saving setting the 48,000 grain unit, which holds 36,000.
Standard sizes and the grains each holds at the rated capacity and on a salt-saving setting of 75 percent.
| Label size, grains | Rated | Salt-saving, 75% |
|---|---|---|
| 24,000 | 24,000 | 18,000 |
| 32,000 | 32,000 | 24,000 |
| 40,000 | 40,000 | 30,000 |
| 48,000 | 48,000 | 36,000 |
| 64,000 | 64,000 | 48,000 |
| 80,000 | 80,000 | 60,000 |
Eight cases below run through the same functions the form uses. Iron changes the size, so 10 gpg with 1 ppm of iron needs 15 gpg of effective hardness and 12 gpg with 3 ppm needs 27. A longer interval needs a bigger unit, and 14 days between regenerations takes the 48,000 grain unit for the default family.
| Case | Setup | Result |
|---|---|---|
| Family of 4 at 10 gpg | 4 people, 10 gpg, 0 ppm iron, 7 days, rated capacity, peak 10 gpm on 12 gpm | 24,000 grains, 3,000 per day, every 8.0 days, 87.5% loaded, peak flow within the service flow |
| Same family with 1 ppm of iron | 4 people, 10 gpg, 1 ppm iron, 7 days, rated capacity, peak 10 gpm on 12 gpm | 32,000 grains, 4,500 per day, every 7.1 days, 98.4% loaded, peak flow within the service flow |
| Same family with iron, salt-saving setting | 4 people, 10 gpg, 1 ppm iron, 7 days, salt-saving setting, peak 10 gpm on 12 gpm | 48,000 grains, 4,500 per day, every 8.0 days, 87.5% loaded, peak flow within the service flow |
| Two people at 8 gpg | 2 people, 8 gpg, 0 ppm iron, 7 days, rated capacity, peak 10 gpm on 12 gpm | 24,000 grains, 1,200 per day, every 20.0 days, 35.0% loaded, peak flow within the service flow |
| Six people at 20 gpg, peak above the service flow | 6 people, 20 gpg, 0 ppm iron, 7 days, rated capacity, peak 14 gpm on 12 gpm | 64,000 grains, 9,000 per day, every 7.1 days, 98.4% loaded, peak flow above the service flow |
| Six people at 20 gpg, 4 days, salt-saving setting | 6 people, 20 gpg, 0 ppm iron, 4 days, salt-saving setting, peak 10 gpm on 12 gpm | 48,000 grains, 9,000 per day, every 4.0 days, 100.0% loaded, peak flow within the service flow |
| Family of 4, 12 gpg and 3 ppm of iron | 4 people, 12 gpg, 3 ppm iron, 7 days, rated capacity, peak 10 gpm on 12 gpm | 64,000 grains, 8,100 per day, every 7.9 days, 88.6% loaded, peak flow within the service flow |
| Family of 4 at 10 gpg, 14 days between regenerations | 4 people, 10 gpg, 0 ppm iron, 14 days, rated capacity, peak 10 gpm on 12 gpm | 48,000 grains, 3,000 per day, every 16.0 days, 87.5% loaded, peak flow within the service flow |
Sizing guides and data sources
- Penn State Extension, Water Softening - A sample calculation of 75 gallons per person per day, 10 gpg and 4 people as 3,000 grains per day, and a 20,000 grain unit regenerating about every 6 to 7 days. The article gives no iron factor and no flow rate.
- Mid-Atlantic Water, What Size Water Softener Do I Need - The sizing formula with iron at 5 grains per ppm, the 75 gallons per person per day that it attributes to the Water Quality Association, and a family of 4 at 10 gpg with 1 ppm of iron as 4,500 grains per day and 31,500 for the week. Its table of label sizes and the grains each holds on a salt-saving setting gives the 75 percent used here.
- WaterHardness.org, Water Softener Size Calculator - The daily grain formula at 75 gallons per person, iron at 5 gpg per ppm (3 ppm with 12 gpg is 27 gpg), a 7 to 10 day regeneration target, and sizes from 24,000 to 80,000 grains.
- Nelsen Corporation, Softener Sizing - The same formula by days between regenerations, with iron at 4 gpg per ppm, hardness in ppm divided by 17.1, regeneration typically every 3 to 5 days, capacities at full salt and at an efficient dose, and a note that most softeners are rated for up to 2 ppm of ferrous iron.
- Quality Water Lab, How to Size a Water Softener - The peak flow check, where the flow of the fixtures running together is compared with the continuous service flow rate of the unit, and iron at 3 to 5 grains per ppm with 4 as the midpoint.
Limitations of this calculator
- The size comes from public sizing guides and not from a standard. NSF/ANSI 44 certifies softener capacity claims, and this page does not apply it.
- Iron is added at 5 gpg per ppm, the largest of the allowances in the guides, which use 3, 4 and 5. A smaller factor gives a smaller unit. Iron above 2 ppm belongs to an iron filter ahead of the softener, and the page only warns.
- Water use per person is a planning value of 75 gallons per day. Metered use, a large household with high-flow fixtures or irrigation on the softened line changes it, and the field accepts 20 to 150 gallons.
- Resin type and volume, the salt dose, the brine and rinse water and the real capacity at a given dose differ by model. The capacity here is the label figure or 75 percent of it, so read the manufacturer's capacity table for the unit you pick.
- Hardness is typed in grains per gallon. A result in mg/L or ppm as calcium carbonate is divided by 17.1 first, and the page does not convert it.
- Total dissolved solids, manganese, tannins, hydrogen sulfide and pH are not modeled, although they affect resin life and softener performance. Water with those needs a treatment train that a water test should specify.
- Twin and alternating tank systems, reserve capacity percentages and split softening of the hot and cold lines are not sized. A load above the 80,000 grain unit is refused with the interval that would fit.
- The flow check compares two numbers you type in. Service flow rate is quoted at a pressure drop that differs by maker, and the page does not read the pressure drop, the line size or the supply pressure.
- This is a planning estimate and not a design drawing. A water test, the manufacturer's sizing chart and a licensed water treatment or plumbing professional decide the unit.
Common softener sizing mistakes
- Leaving the iron out. Iron adds to the hardness. It is easy to miss. A family of 4 at 10 gpg with 1 ppm of iron carries 4,500 grains per day and not 3,000, and the weekly load of 31,500 grains takes the 32,000 grain unit and not the 24,000 grain unit. That is a full size up.
- Reading the label as the usable capacity. A softener on a salt-saving setting holds about 75 percent of its label. 31,500 grains per week fits the 32,000 grain unit at the rated capacity, but on that setting it needs the 48,000 grain unit, which holds 36,000. Check the setting.
- Typing hardness in mg/L. Hardness in mg/L or ppm as calcium carbonate has to be divided by 17.1 to get grains per gallon. 171 mg/L is 10 gpg. The field refuses 171, but it accepts 85 mg/L typed as 85 gpg, a load 17.1 times too large. Divide first.
- Ignoring the days between regenerations. Days between regenerations scales the size directly. 3,000 grains per day over 7 days is 21,000 grains and takes the 24,000 grain unit, and over 14 days it is 42,000 grains and takes the 48,000 grain unit, which then regenerates about every 16.0 days.
- Treating grain capacity as the flow rating. Grain capacity and service flow are two checks. They answer different questions. The service flow rate comes from the spec sheet of the model, so a peak of 14 gpm needs a unit rated for 14 gpm or more, even where the grain size of 64,000 is right.
- Using a softener to remove a lot of iron. A softener is not an iron filter. At 3 ppm of iron the page adds 15 gpg and sizes 64,000 grains, but it also warns that most softeners are rated for up to 2 ppm of ferrous iron, so an iron filter comes first.
Common questions about softener size
How do I size a water softener?
Multiply the people in the house by the gallons each uses per day and by the hardness in grains per gallon, then by the days between regenerations. 4 people at 75 gallons and 10 gpg is 3,000 grains per day, or 21,000 grains over 7 days, which takes the 24,000 grain unit. Round up.
How many gallons per person per day should I use?
The guides read use 75 gallons per person per day, and Penn State Extension uses 75 in its sample calculation. Metered use is better when you have it, and the field takes 20 to 150. Use the meter reading if the house has one.
How does iron change the softener size?
The page adds 5 gpg of hardness for each 1 ppm of iron. 10 gpg with 1 ppm becomes 15 gpg, and a household of 4 goes from 3,000 to 4,500 grains per day. Other guides use 4 or 3, so a smaller factor gives a smaller unit. Iron matters.
What size water softener does a family of four need?
At 10 gpg with no iron the weekly load is 21,000 grains and the 24,000 grain unit covers it. Water with 15 gpg of effective hardness, which is 10 gpg plus 1 ppm of iron, gives 31,500 grains, and that takes the 32,000 grain unit at the rated capacity or the 48,000 grain unit on a salt-saving setting. It depends on the water.
How often should a water softener regenerate?
The guides aim at about 7 days between regenerations, with 7 to 10 days as the target in one and 3 to 5 days as typical in another. This page takes the interval you type, 3 to 14 days, and reports the interval of the unit it picks, 8.0 days for the opening case. Longer is not better.
Why does a 48,000 grain softener hold only 36,000 grains?
Guides list a label capacity and a lower figure for an efficient salt dose. Two of them put the efficient figure at 75 percent of the label, so the 48,000 grain unit holds 36,000. The page offers the label itself or that 75 percent as the basis. Salt dose sets it.
What is the service flow rate and why does it matter?
The service flow rate is the flow a unit passes at a stated pressure drop, and the manufacturer lists it. A peak demand above it restricts the water at the fixtures, and the page compares the two numbers you type. Take the service figure from the spec sheet. Do not guess it. Check both.
Do I need an iron filter before the softener?
One guide advises an iron filter ahead of the softener at 1 ppm or more, and another reads most softeners as rated for up to 2 ppm of ferrous iron. The page warns from 1 ppm and does not size a filter. Test the water first.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup. Run it as often as you like, for as many houses as you have on the list this week.
Used on the same job
Test the water, then size the unit
Hardness and iron come first, from a water test, and the household sets the daily use. The grains per regeneration and the days between regenerations then give the size, and the flow check is made against the spec sheet of the model. For the pipe that carries the water, see the water pipe size calculator. You can also ask the assistant to go over a water test result.
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