Water Softener Size Calculator: Grains, Resin Volume and Salt
A household of 4 using a typical 75 gallons per person per day uses 300 gallons a day. At 15 grains per gallon (gpg) of hardness that is 4,500 grains to remove each day, or 31,500 grains over 7 days between regenerations, which calls for a 32,000-grain softener (1 cu ft of resin) using about 15 lb of salt per regeneration and 64 lb a month at the maximum salt setting. Enter your own hardness from a water test; add 4 gpg per ppm of iron if you have it. This is sizing guidance from a standard formula; the dealer or the maker’s capacity table has the last word.
Figures based on the sources listed belowUpdated How we calculate
32,000-grain softener.
32,000-grain softener
- Method: ion-exchange softener, sized on the grains to remove
- Your hardness in other units: 15 gpg = 257 mg/L as CaCO3 = 14.4 °dH = 25.7 °f: very hard (USGS)
- Compensated hardness: 15 gpg (15 gpg hardness)
- Grains to remove per day: 4,500 grains per day (4 people × 75 gal × 15 gpg)
- Capacity needed between regenerations: 31,500 grains for 7 days
- Recommended size: 32,000 grains, regenerating about every 7.1 days at this use
- Resin: 1 cu ft in the recommended size (0.98 cu ft needed at 32,000 grains per cu ft)
- Salt per regeneration: about 15 lb (15 lb per cu ft × 1 cu ft)
- Salt per month: about 64 lb (4,500 grains a day × 30.4 days ÷ 2,133 grains per lb)
Shopping list
- Water softener salt pellets
- Water hardness test strips
- Water softener resin cleaner
- Water softener bypass valve and fittings
- Brine tank cleaning brush
Your project
- People in the household: 4
- Water use per person per day: 75 gal
- Water hardness: 15
- Hardness unit: Grains per gallon (gpg)
- Iron in the water: No iron, or not known
- Days between regenerations: 7
- Salt setting: Maximum, about 15 lb per cu ft (rated capacity)
Also pick up
- Water softener salt pellets
- Water hardness test strips
- Water softener resin cleaner
- Water softener bypass valve and fittings
- Brine tank cleaning brush
Reopen this calculation: /calculators/water-softener-size/?s=eyJyb29tIjp7Imxlbmd0aEZ0IjowLCJ3aWR0aEZ0IjowfSwiZGVkdWN0aW9ucyI6W10sIndhc3RlUGN0IjowLjEsInBhcmFtcyI6eyJwZW9wbGUiOjQsImdhbGxvbnNQZXJQZXJzb24iOjc1LCJoYXJkbmVzcyI6MTUsImhhcmRuZXNzVW5pdCI6MCwiaXJvblByZXNlbnQiOjAsImlyb25QcG0iOjEsImlyb25GYWN0b3IiOjQsImRheXMiOjcsInNhbHREb3NlIjowfX0
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Typical standard softener sizes, resin and salt
| Rated size (grains) | Resin (cu ft) | Capacity at 15 lb/cu ft | Salt at 15 lb/cu ft | Capacity at 6 lb/cu ft | Salt at 6 lb/cu ft |
|---|---|---|---|---|---|
| 24,000 | 0.75 | 24,000 | 11 lb | 15,000 | 5 lb |
| 32,000 | 1 | 32,000 | 15 lb | 20,000 | 6 lb |
| 40,000 | 1.25 | 40,000 | 19 lb | 25,000 | 8 lb |
| 48,000 | 1.5 | 48,000 | 23 lb | 30,000 | 9 lb |
| 64,000 | 2 | 64,000 | 30 lb | 40,000 | 12 lb |
| 80,000 | 2.5 | 80,000 | 38 lb | 50,000 | 15 lb |
Grains per day and size by household and hardness
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg | 30 gpg |
|---|---|---|---|---|---|
| 1 | 375 / 24,000 | 750 / 24,000 | 1,125 / 24,000 | 1,500 / 24,000 | 2,250 / 24,000 |
| 2 | 750 / 24,000 | 1,500 / 24,000 | 2,250 / 24,000 | 3,000 / 24,000 | 4,500 / 32,000 |
| 3 | 1,125 / 24,000 | 2,250 / 24,000 | 3,375 / 24,000 | 4,500 / 32,000 | 6,750 / 48,000 |
| 4 | 1,500 / 24,000 | 3,000 / 24,000 | 4,500 / 32,000 | 6,000 / 48,000 | 9,000 / 64,000 |
| 5 | 1,875 / 24,000 | 3,750 / 32,000 | 5,625 / 40,000 | 7,500 / 64,000 | 11,250 / 80,000 |
| 6 | 2,250 / 24,000 | 4,500 / 32,000 | 6,750 / 48,000 | 9,000 / 64,000 | 13,500 / over 80,000 |
Iron compensation
| Iron | Compensated hardness | Grains per day | Recommended size (grains) |
|---|---|---|---|
| 0 ppm | 15 gpg | 4,500 | 32,000 |
| 0.5 ppm | 17 gpg | 5,100 | 40,000 |
| 1 ppm | 19 gpg | 5,700 | 40,000 |
| 2 ppm | 23 gpg | 6,900 | 64,000 |
| 3 ppm | 27 gpg | 8,100 | 64,000 |
Hardness converter: gpg, mg/L, °dH and °f
| Grains per gallon | mg/L as CaCO3 | German degrees | French degrees | USGS class |
|---|---|---|---|---|
| 1 gpg | 17.1 mg/L | 1 °dH | 1.7 °f | soft |
| 3 gpg | 51.3 mg/L | 2.9 °dH | 5.1 °f | soft |
| 5 gpg | 85.5 mg/L | 4.8 °dH | 8.6 °f | moderately hard |
| 7 gpg | 119.7 mg/L | 6.7 °dH | 12 °f | moderately hard |
| 10 gpg | 171 mg/L | 9.6 °dH | 17.1 °f | hard |
| 15 gpg | 256.5 mg/L | 14.4 °dH | 25.7 °f | very hard |
| 20 gpg | 342 mg/L | 19.2 °dH | 34.2 °f | very hard |
| 30 gpg | 513 mg/L | 28.7 °dH | 51.3 °f | very hard |
Hardness unit factors
| Unit | Equals | How to convert |
|---|---|---|
| Grain per US gallon (gpg) | 17.1 mg/L | mg/L ÷ 17.1 = gpg (the exact grain and US gallon give 17.118) |
| German degree (°dH) | 17.848 mg/L | 10 mg of CaO per litre, expressed as CaCO3: mg/L ÷ 17.848 = °dH |
| French degree (°f) | 10 mg/L | 10 mg of CaCO3 per litre: mg/L ÷ 10 = °f |
| ppm | 1 mg/L | in water, ppm and mg/L are the same number |
USGS hardness classification
| Class | mg/L as CaCO3 | Grains per gallon |
|---|---|---|
| Soft | 0 to 60 mg/L | 0 to 3.5 gpg |
| Moderately hard | 61 to 120 mg/L | 3.6 to 7 gpg |
| Hard | 121 to 180 mg/L | 7.1 to 10.5 gpg |
| Very hard | more than 180 mg/L | more than 10.5 gpg |
Methodology & assumptions
A water softener is an ion-exchange tank: resin beads hold calcium and magnesium (the hardness) until they are full, and a regeneration with salt brine renews them. It is sized by the grains of hardness it has to remove between regenerations. 1) Hardness. Softeners are sized in grains per gallon (gpg). A water test or utility report may give mg/L (the same as ppm) as CaCO3: gpg = mg/L ÷ 17.1. The hardness converter on this page also handles German degrees (°dH, 17.848 mg/L each) and French degrees (°f, 10 mg/L each). 2) Iron. Dissolved (ferrous) iron also takes up resin capacity, so it is added to the hardness as compensated hardness = hardness (gpg) + ppm of iron × 4. Four gpg per ppm is a typical factor; some makers use 3 to 5, so the factor is editable. Most softeners are rated for only a small amount of iron (about 2 ppm or less), and above that the iron is better treated by a separate filter. 3) Water use. Gallons per day = people × gallons per person per day. This tool starts at 75, a typical figure used for softener sizing and inside the USGS average of 80 to 100 gallons per person per day for indoor home use; use your own if your bills show it. Only the water that runs through the softener matters, so a household that keeps outdoor taps on unsoftened water uses less. 4) Grains per day = gallons per day × compensated hardness. 5) Capacity needed = grains per day × days between regenerations (default 7; dealers often use 3 to 7). 6) Resin and size. Softeners are rated at the capacity they reach with the maximum salt dose, about 15 lb of salt per cubic foot of resin, which is a typical 32,000 grains per cubic foot. Resin needed = capacity needed ÷ grains per cubic foot, and the tool picks the smallest of the typical standard sizes (24,000, 32,000, 40,000, 48,000, 64,000, 80,000 grains, which hold 0.75, 1, 1.25, 1.5, 2, 2.5 cu ft of resin) that covers it. Real products vary in the resin they hold, so a 48,000-grain model of one maker may hold slightly more or less resin than another. 7) Salt efficiency. Capacity depends on how much salt is used. At the maximum dose of about 15 lb per cubic foot a resin gives about 32,000 grains per cubic foot, or 2,133 grains per pound of salt; at about 6 lb per cubic foot it gives about 20,000 grains per cubic foot, or 3,333 grains per pound. The efficient setting uses 60% less salt per cubic foot (6 lb against 15 lb) to give about 62% of the capacity (20,000 against 32,000 grains), so a softener that will run on it needs more resin for the same job: the Salt setting field switches the sizing to it. These are typical figures; the dose is set in the control valve, and the data sheet of the valve and resin gives the exact curve. 8) Salt. Per regeneration = resin of the chosen size × salt per cubic foot. Per month = grains per day × 30.4 days ÷ grains per pound of salt, which does not depend on the size of the unit. Water is also used and discharged at each regeneration; this tool does not count it. What this is not. Water hardness changes from season to season and the incoming flow rate matters too: the softener must also pass the peak flow of the house without a pressure drop, which this tool does not check. It does not cover sodium in the softened water, other contaminants, or salt-free conditioners (which do not remove hardness). A reserve is built in only by the choice of days between regenerations: some dealers add 20 to 30% on top, which this tool does not.
- US Geological Survey, Water Science School, FAQ: Do you have information about water hardness in the United States?: the generally accepted classification of water as soft (0 to 60 mg/L as calcium carbonate), moderately hard (61 to 120), hard (121 to 180) and very hard (more than 180), and the note that the EPA has no hardness standard
- US Geological Survey, Water Science School, Water Q&A: How much water do I use at home each day?: on average each person uses about 80 to 100 gallons of water per day for indoor home uses, the basis for the typical 75 gallons per person per day used here
- Nelsen Corporation, Softener Sizing (technical page): the sizing method of compensated hardness × gallons per day × days between regenerations, rounded up to the next available softener size, with iron added at 4 gpg per ppm; the page also notes that most softeners are rated only for about 2 ppm of iron or less. Some dealer pages use 5 gpg per ppm, so the factor is editable here
- Water Quality Association hardness classification, as reproduced by university extension publications such as the University of Maryland Extension fact sheet Hardness in Household Water: soft below 1 gpg, slightly hard 1 to 3.5, moderately hard 3.5 to 7, hard 7 to 10.5 and very hard above 10.5 gpg, and 1 gpg = 17.1 mg/L. The WQA scale is finer than the USGS one used in the converter on this page
- Chem-Aqua, How to Calculate Softener Capacity: a salt dose of 15 lb per cubic foot of resin is typically used for an exchange capacity of about 30,000 grains per cubic foot, and lower salt doses reduce capacity but use salt more economically
- aQuatell, Understanding true water softener capacity: about 6 lb of salt per cubic foot of resin yields roughly 20,000 grains of capacity, about two thirds of the maximum for about a third of the salt (a dealer article; the 15 lb figure on the maximum dose varies from one article to another, from 15 to 18 lb)
- SoftPro Water Systems, Grain Capacity vs. Salt Efficiency: the trade-off between capacity and salt use, about 3,333 grains per pound of salt at 6 lb per cubic foot against about 2,133 grains per pound at 15 lb per cubic foot, which are the ratios of the 20,000 and 32,000 grains per cubic foot used here (a dealer article)
- Purolite, C100 Sodium Cycle Engineering Bulletin (ion-exchange resin maker): operating capacity of a softening resin depends on the amount and concentration of salt used for regeneration, the hardness and sodium of the water and the flow rate, shown in capacity curves
- Pentair, Fleck 5600 SXT user guide: the control valve sets the capacity and the salt per regeneration for the resin volume installed, by the brine fill time, so the salt dose is a setting of the valve and not a fixed property of the tank
- Typical figures on this page, not taken from a data sheet: the 75 gallons per person per day, the 4 gpg per ppm of iron, the 7-day interval, the standard sizes of 24,000 to 80,000 grains with their resin volumes (the rated size divided by 32,000 grains per cubic foot), and the 15 lb and 6 lb salt doses with 32,000 and 20,000 grains per cubic foot. The Clack and Fleck valve tables give the exact capacity for each salt setting; use them for the unit you are considering
How to size a water softener
- Get your hardness. Use a water test or your utility’s report. If it is in mg/L or ppm, divide by 17.1 to get grains per gallon, and add about 4 gpg for each ppm of iron.
- Find the water use. Multiply the people in the home by the gallons each uses per day; 75 is a typical figure.
- Grains per day. Multiply the gallons per day by the compensated hardness in gpg.
- Capacity between regenerations. Multiply by the days you want between regenerations (7 is a typical choice).
- Round up to a standard size. Choose the next size up from 24,000, 32,000, 40,000, 48,000, 64,000 or 80,000 grains, and check the salt setting and the flow rate on the unit’s data sheet.
Worked examples
| Project | Result |
|---|---|
| 4 people, 75 gallons each, 15 gpg, no iron, 7 days, maximum salt setting | 32,000-grain softener: 4,500 grains a day, 31,500 grains for 7 days, 1 cu ft, 64 lb of salt a month |
| The same household on the efficient salt setting (6 lb per cu ft) | 64,000-grain softener: 31,500 grains needs 1.57 cu ft at 20,000 grains per cu ft; 41 lb of salt a month |
| The same household with 2 ppm of iron | 64,000-grain softener: 23 gpg compensated, 6,900 grains a day |
| Very hard water: 25 gpg | 64,000-grain softener: 7,500 grains a day, 52,500 grains for 7 days |
| Hardness from a lab report: 250 mg/L | 32,000-grain softener: 250 ÷ 17.1 = 14.6 gpg, 4,386 grains a day |
| Regenerating every 3 days instead of 7 | 24,000-grain softener: 13,500 grains between regenerations |
| A couple at 10 gpg | 24,000-grain softener: 1,500 grains a day |
| A family of 6 at 20 gpg | 64,000-grain softener: 9,000 grains a day, 63,000 grains for 7 days |
| Lower use: 60 gallons per person per day | 32,000-grain softener: 3,600 grains a day |
Frequently asked questions
- What size water softener do I need?
- Multiply the people by the gallons each uses per day and by the compensated hardness to get the grains to remove per day, multiply by the days you want between regenerations, and round up to the next standard size. A household of 4 at 75 gallons each and 15 gpg removes 4,500 grains a day, 31,500 grains over 7 days: a 32,000-grain softener. Two people at 10 gpg need 24,000-grain softener, and six people at 20 gpg need 64,000-grain softener.
- How do I calculate grains per day?
- Grains per day = people × gallons per person per day × hardness in gpg. For 4 people at 75 gallons and 15 gpg that is 4 × 75 × 15 = 4,500 grains. Hardness in mg/L or ppm is divided by 17.1 first: 250 mg/L is 14.6 gpg.
- How many gallons per person per day should I use?
- Softener sizing commonly uses 75 gallons per person per day, which this tool starts with; it is a typical figure, not a measurement. USGS gives 80 to 100 gallons per person per day as the average for indoor home use. Your own use may differ; if your bill and your people count give a lower figure, enter it (at 60 gallons the default household needs 32,000-grain softener). Water that does not run through the softener, such as outdoor taps on a separate line, should not be counted.
- How do I allow for iron in the water?
- Iron takes up resin capacity, so it is added to the hardness: about 4 gpg for each ppm of iron is a typical factor, and some makers use 3 to 5 (the factor is editable). With 2 ppm of iron the default 15 gpg becomes 23 gpg, 6,900 grains a day, and the recommended size moves from 32,000-grain softener to 64,000-grain softener. Most softeners are rated for only about 2 ppm of iron or less, and iron makes the resin foul, so higher levels are better treated by a separate iron filter.
- How many days between regenerations should I choose?
- It is your choice, and it trades size against salt and water use. Dealers often size for 3 to 7 days; this tool uses 7 by default. A shorter interval allows a smaller softener that regenerates more often: the default household at 3 days needs 24,000-grain softener against 32,000-grain softener at 7. A larger softener regenerates less often but costs more.
- Does a 32,000-grain softener always remove 32,000 grains?
- The rated size is the capacity at the maximum salt dose, about 15 lb per cubic foot of resin. At about 6 lb per cubic foot the same resin gives about 20,000 grains per cubic foot, so a 1 cubic foot unit gives about 20,000 grains instead of 32,000. The efficient setting uses far less salt for each grain removed (about 3,333 grains per pound against 2,133), but needs more resin: the default household needs 64,000-grain softener on it, against 32,000-grain softener at the maximum setting. These are typical figures; the valve and resin data sheets give the exact ones.
- How much salt does a water softener use?
- Per regeneration it is the resin in cubic feet times the salt dose per cubic foot: a 1 cubic foot unit uses about 15 lb at the maximum setting and about 6 lb at the efficient one. Per month it depends only on the hardness removed: grains per day × 30.4 days ÷ grains per pound of salt. The default household uses about 64 lb a month at the maximum setting and about 41 lb at the efficient one, with the efficient setting sized up to 64,000-grain softener. A few pounds more or less are normal; the hardness, the water use and the setting decide it.
- How do I convert hardness in mg/L or ppm to grains per gallon?
- Divide by 17.1: gpg = mg/L ÷ 17.1, so 250 mg/L as CaCO3 is 14.6 gpg. To go back, multiply by 17.1. A German degree (°dH) is 17.848 mg/L and a French degree (°f) is 10 mg/L, so 1 gpg is 0.96 °dH or 1.71 °f. The unit selector in the calculator and the converter table on this page do the same.
- What counts as soft, hard and very hard water?
- USGS uses soft for 0 to 60 mg/L as calcium carbonate, moderately hard for 61 to 120, hard for 121 to 180 and very hard for more than 180 mg/L. In grains per gallon that is up to 3.5, up to 7.0, up to 10.5 and above 10.5 gpg. The Water Quality Association’s scale has finer steps. Water at or below about 7 gpg (120 mg/L) is often left untreated unless it causes scale or spotting problems.
- How much does hardness change the size?
- Directly, because grains per day scale with hardness. The default household at 15 gpg needs 32,000-grain softener; at 25 gpg it needs 64,000-grain softener. A softer supply (such as 5 gpg) may not need a softener at all. Have the water tested rather than guessing: hardness varies widely even between neighbouring areas.
- Is the size the same as the flow rate the house needs?
- No. The size in grains is the capacity between regenerations; the flow rate the softener can pass at an acceptable pressure drop is a separate rating (in gallons per minute) set by the valve and tank. Check that the unit passes the peak flow of your home; this tool does not.
- How is this different from a water heater or tankless size?
- A water heater is sized by hot water demand (see the water heater size and tankless water heater size calculators). A softener is sized by the grains of hardness in all the water that runs through it, so the hardness and the days between regenerations matter and the hot water demand does not. A softener protects a water heater from scale, which is one reason the two are often planned together.