Grain & GallonWhat size water softener do you need? A calculator with iron and salt settings
Find what size water softener you need: enter your household, hardness and iron to get the resin volume and grain rating at your salt setting. All sourced.
For a household of 4 on water with 15 grains per gallon (gpg) of hardness and no iron, the right water softener size is about 1.5 ft³ of resin at the efficient salt setting, sold as a roughly 48,000-grain softener. Enter your own household and water test below to get the resin volume for your home, the salt it uses and a warning if your iron is more than a softener should handle.
Water softener size calculator
Enter your household and your water test. You get the resin volume to buy at the salt setting you choose, the salt it uses per regeneration, and a warning when your iron is more than a softener should handle.
Resin to buy
ft³
- Compensated hardness
- gpg
- Hardness removed per day
- grains
- Salt setting
- lb per ft³
- Resin capacity at this setting
- grains per ft³
- Resin needed (exact)
- ft³
- Sold as about
- grains
- Salt per regeneration
- lb
- Days between regenerations at this size
- days
- Salt per month
- lb
- Conservative continuous flow
- gpm
- Above 5 ppm dissolved iron. Penn State Extension recommends softening for iron only below 5 mg/L. The usual fix is an oxidizing iron filter ahead of the softener.
- Iron plus manganese is 3 to 10 ppm, the range where Penn State lists an oxidizing filter (greensand, Katalox-type media or air injection). Consider one before the softener.
- Iron plus manganese is over 10 ppm. Penn State lists oxidation (for example chlorination) followed by filtration, and NDSU says over 10 ppm may need a dedicated iron removal system. Size the softener for the water after that treatment.
- Check your model's iron limit. Nelsen says most softeners are rated to about 2 ppm; the manuals we read list 10 ppm (WaterBoss 900, Whirlpool WHES40E) and 12 ppm (Whirlpool WHES44). The WaterBoss manual notes that Wisconsin caps the claim at 5 ppm.
- Manganese isn't added to the size: no manufacturer manual we read gives a factor for it. Follow your softener manual's manganese guidance, or remove it first with an iron and manganese filter.
- Enter clear-water (ferrous) iron only. Red, already oxidized iron clogs resin, and Penn State says to filter it out before the softener.
- Penn State lists softening for iron when hardness is between 3 and 20 gpg. Yours is higher, so confirm with the manufacturer before relying on the softener for iron.
- Penn State lists softening for iron when hardness is between 3 and 20 gpg. Yours is lower, so an iron filter may suit you better than a softener.
- You need more than 2 ft³ at this setting, the largest common size in the products we list. Regenerate more often, choose a higher salt setting, or ask the seller about a larger or twin-tank system.
- Higher salt settings get more grains out of each cubic foot but fewer out of each pound of salt: about 3,500 grains per lb at 6 lb per ft³ against about 2,070 at 15 lb (WC&P design table).
How this works: Compensated hardness is your hardness plus your iron times the factor in your manual (Whirlpool and WaterBoss use 5; Nelsen 4; Hellenbrand 3). Manganese is not counted. The daily load is compensated hardness times people times gallons per person (75 gallons per the WaterBoss manual and Penn State). Resin needed is the daily load times the days between regenerations, divided by what 1 ft³ of resin removes at your salt setting: 21,000 grains at 6 lb, 26,000 at 9 lb, 29,000 at 12 lb and 31,000 at 15 lb (WC&P design guidelines). It is rounded up to 1, 1.5 or 2 ft³. The "sold as" rating uses the common convention of 32,000 grains per ft³ at maximum salt, so treat it as approximate. The flow figure uses 5 gpm per ft³, a conservative continuous-flow design guide (WC&P); spec sheets often list more. Hardness in mg/L divided by 17.1 gives gpg.
How to size a water softener
Sizing is three multiplications and a division. First, add your iron to your hardness to get the “compensated hardness”. Then multiply by the water your household uses in a day. That gives the grains of hardness the softener has to remove each day. Multiply by the days you want between regenerations, and divide by what one cubic foot of resin removes at your salt setting.
Here is the worked example behind the answer above:
- Compensated hardness: 15 gpg + (5 x 0 ppm iron) = 15 gpg
- Daily load: 4 people x 75 gallons x 15 gpg = 4,500 grains a day
- Load per regeneration: 4,500 x 7 days = 31,500 grains
- Resin needed: 31,500 / 21,000 grains per ft³ (at 6 lb of salt) = 1.5 ft³
The 75 gallons per person comes from the WaterBoss owner’s manual and matches the example in Penn State Extension’s softening guide, which works out a 6 to 7 day cycle for a family of 4. The calculator rounds your result up to the next common size: 1, 1.5 or 2 ft³.
Add iron and the answer moves fast. With 1 ppm of clear-water iron in the same example, compensated hardness becomes 20 gpg, the daily load 6,000 grains, and the resin needed 2 ft³. One part per million of iron raised the size by a third.
Why we size by resin volume, not the number on the box
The grain rating on a softener box is its capacity at the maximum salt setting. SoftPro’s grain-capacity page puts a 32,000-grain system at about 1 ft³ of resin, and the maximum-setting table on US Water Systems’ Matrixx page lists 32,000, 48,000 and 64,000 grains for 1, 1.5 and 2 ft³. That is roughly 32,000 grains per cubic foot, which is why 1.5 ft³ is sold as “48,000 grains”. Run the softener at an efficient salt setting and it removes far less per regeneration than the box says.
The manufacturers’ own tables show the gap. The Whirlpool WHES40E manual rates its 1.14 ft³ of resin at 40,000 grains with 15.5 lb of salt, 31,600 grains with 9.0 lb and 11,900 grains with 2.4 lb. US Water Systems lists its Matrixx 1.0 ft³ model at 35,000 grains maximum and 15,000 grains at its high-efficiency setting.
Sizing from the box number leads to one of two outcomes: you run the softener at a high salt setting to get the rated capacity, or it regenerates more often than you planned. Sizing by resin volume at your chosen setting avoids both. Most grain calculators stop at the box number; this one sizes by resin volume first and then shows the grain rating it is sold as. Use that “sold as about” figure to match the result to a product listing, but treat it as approximate. Our guide to water softener grain capacity sets three makers’ salt tables side by side, from the lowest dose to the highest.
Choosing a salt setting
The salt setting is the pounds of salt used per cubic foot of resin at each regeneration. More salt squeezes more capacity out of the resin, but each extra pound removes less hardness than the one before. The design table published in Water Conditioning & Purification magazine gives these figures for standard resin:
| Salt setting | Capacity per ft³ | Hardness removed per lb of salt |
|---|---|---|
| 6 lb per ft³ | about 21,000 grains | about 3,500 grains |
| 9 lb per ft³ | about 26,000 grains | about 2,890 grains |
| 12 lb per ft³ | about 29,000 grains | about 2,420 grains |
| 15 lb per ft³ | about 31,000 grains | about 2,070 grains |
Going from 6 lb to 15 lb gets you about 48% more capacity per cubic foot but uses 2.5 times the salt per regeneration. That is why the calculator defaults to 6 lb: you buy a little more resin once and use less salt every year after. The trade-off is water. On the same resin, a higher setting stretches the time between regenerations, so fewer regenerations send water to the drain over a year.
Our running cost calculator puts both sides in pounds of salt, gallons and dollars for your household.
Sizing a softener for well water with iron
Clear-water (ferrous) iron uses up resin capacity the same way hardness does, so manuals tell you to add it to the hardness number. They don’t agree on how much:
- 5 gpg per 1 ppm of iron: the Whirlpool and WaterBoss manuals
- 4 gpg per 1 ppm: Nelsen’s sizing guide
- 3 gpg per 1 ppm: the Hellenbrand H125 manual
Use the factor from the manual of the softener you’re considering. If you haven’t chosen one, 5 is the most common and the most cautious of the three. Our guide to the water softener hardness setting lists each maker’s factor and where to enter the result on the control.
Manganese is not added. None of the manufacturer manuals we read gives a manganese factor, and the figures on seller calculators come without a source. The calculator shows a warning instead when you enter manganese, because the right answer is usually to follow your softener manual’s manganese guidance or remove it with a filter first.
When a bigger softener is the wrong fix
Past a certain point, more iron calls for a different machine, not more resin. Penn State Extension recommends a softener for iron only when dissolved iron is below 5 mg/L, hardness is between 3 and 20 gpg and pH is above 6.7. It lists an oxidizing filter when iron and manganese together reach 3 to 10 mg/L, and oxidation followed by filtration above 10 mg/L.
Red iron is a hard stop. If your water comes out of the tap orange or with rust-colored particles, the iron has already oxidized, and Penn State notes that it clogs softener resin. It needs filtering before the softener, whatever the size.
Softener manuals set their iron limits higher than Penn State: 10 ppm for the Whirlpool WHES40E and WaterBoss 900, and 12 ppm for the larger Whirlpool WHES44. Our guide to choosing between an iron filter and a water softener explains why the numbers differ and has a matcher that turns your test into a recommendation. If you’re not sure which numbers on your report matter, start with how to read well water test results.
Water softener size chart by household
These rows come straight from the calculator at 15 gpg hardness, no iron, 75 gallons per person and 7 days between regenerations:
| People | Resin at 6 lb setting | Salt per month at 6 lb | Resin at 15 lb setting | Salt per month at 15 lb |
|---|---|---|---|---|
| 1 | 1 ft³ (about 32,000) | 10 lb | 1 ft³ (about 32,000) | 16 lb |
| 2 | 1 ft³ (about 32,000) | 19 lb | 1 ft³ (about 32,000) | 33 lb |
| 3 | 1.5 ft³ (about 48,000) | 29 lb | 1 ft³ (about 32,000) | 49 lb |
| 4 | 1.5 ft³ (about 48,000) | 39 lb | 1.5 ft³ (about 48,000) | 65 lb |
| 5 | 2 ft³ (about 64,000) | 48 lb | 1.5 ft³ (about 48,000) | 82 lb |
| 6 | over 2 ft³ | 58 lb | 2 ft³ (about 64,000) | 98 lb |
At the efficient setting a household of 4 uses about 26 lb less salt a month for the same resin. For 6 people at 6 lb, the load needs 2.25 ft³. A 2 ft³ unit still works, but it regenerates about every 6 days instead of 7. Larger or twin-tank systems exist, but we haven’t verified them as common residential sizes, so the calculator stops at 2 ft³ and tells you when you’ve passed it.
If your water is harder than 15 gpg or you have iron, the table undersizes you. Use the calculator.
Check the flow rate too
A softener also has to keep up with your peak water use, such as two showers and a washing machine at once. The WC&P design guide uses 5 gallons per minute (gpm) per cubic foot of resin for continuous flow and 7.5 gpm per cubic foot for peak flow, so 1.5 ft³ is good for about 7.5 gpm continuous. That is the conservative figure the calculator shows.
Spec sheets usually list more. The Whirlpool WHES40E rates 8.5 gpm with 1.14 ft³ and the Matrixx 1.0 ft³ model 10 gpm of service flow. These are real ratings, often measured at a stated pressure drop, but they sit above the design guide. If your home has several bathrooms and your result is 1 ft³, look at the flow rating before the grain rating. On a dealer build the control valve is rarely the limit; our Fleck 5600SXT vs 5800 vs Clack WS1 comparison shows why the resin tank caps flow first.
Some brands skip grains and sell by flow or bathroom count instead. SoftPro sells the Elite by flow band and SpringWell sells its softeners by bathrooms; our SpringWell vs SoftPro comparison puts both brands’ published specs side by side.
Next step
With a resin volume in hand, you can shortlist models. Our best water softeners for well water lists each pick’s resin volume, iron limit and capacity at efficient and maximum salt, so you can match your result to a product. More sizing and cost guides are in sizing and running costs.
Questions people ask
What size water softener does a family of 4 need?
At 15 gpg hardness with no iron, a family of 4 using 75 gallons each needs about 1.5 ft³ of resin at the efficient 6 lb salt setting, sold as a roughly 48,000-grain unit. At the maximum 15 lb setting, 1.5 ft³ still fits, and 1 ft³ falls just short. Iron or harder water pushes it up, so run your own numbers in the calculator.
Can a water softener be too big?
A larger resin bed at the same salt setting mostly means more days between regenerations and a higher purchase price. If the calculator's "days between regenerations" comes out far above what you planned, the unit is bigger than your load needs, so check the flow you need and buy the smaller size that covers it.
How do I convert mg/L hardness to grains per gallon?
Divide mg/L (or ppm) as calcium carbonate by 17.1. A report showing 257 mg/L is about 15 gpg. Iron and manganese are reported in mg/L too, and for those, mg/L and ppm are the same number.