Set your water softener to the hardness of your incoming water in grains per gallon, plus 5 for every ppm of iron if a test found any. If your water report gives mg/L or ppm, divide by 17.118. If it gives a range, use the high figure. A setting that’s too high wastes salt; one that’s too low lets hard water through before the next regeneration.
On this page
Below: how to get the number right, a table showing what a wrong setting costs, and the special cases (iron, blended supplies, a range in the report, and appliances that want a different unit).
Why the hardness setting matters
A demand-initiated softener doesn’t measure hardness. It counts gallons. The control valve divides its capacity (in grains) by the hardness you entered (in grains per gallon) to work out how many gallons it can soften, and it regenerates when that many have gone through. Our guide to how often a softener should regenerate has the intervals for common sizes.
So the setting is a guess about your water that the valve trusts completely. Enter a bigger number and it regenerates early, burning salt and water on resin that wasn’t used up. Enter a smaller one and it waits too long: the resin runs out and hard water flows to your taps until the next cycle.
How to find the right number in four steps
1. Get your hardness
On mains water, it’s in your utility’s annual water quality report (the Consumer Confidence Report in the US). Our guide to what hard water is explains where to look. On a well, use a test kit or a lab test.
2. Convert it to grains per gallon
Divide mg/L or ppm by 17.118 (full gpg to ppm table). A report figure of 280 ppm becomes 16.4 gpg. Not sure which unit your report used? Compare it with the water hardness scale: a “hardness” of 16 is almost certainly gpg, while 280 can only be mg/L.
3. Pick the high end of any range
Many reports give a low, high and average. Austin Water’s 2025 report, for example, lists 70 to 126 ppm with a 93 ppm average. Use 126 ppm (7.4 gpg) so the softener never underestimates. The extra salt is small: setting 7.4 instead of 5.4 gpg uses about 37% more salt, against the risk of hard water getting through on days the supply is at its hardest.
4. Add the iron allowance
If a test found clear-water (ferrous) iron, Pentair’s owner’s manual tells you to add 5 to the hardness setting for every 1 ppm of iron. With 18 gpg hardness and 0.6 ppm iron, enter 18 + 0.6 × 5 = 21. Check your own manual: it may give its own iron limit and allowance, and the Pentair manual describes iron reduction as generally for levels below 3 ppm.
The calculator below applies the same rules and shows how often a given softener will regenerate at your setting:
This calculator needs JavaScript. Grains per day = people × 80 gallons × hardness setting.
What a wrong setting costs: worked table
Take a family of four on 15 gpg water using 80 gallons each a day (320 gallons a day), with a 32,000-grain softener at a medium salt dose of about 25,600 usable grains. At the right setting, it regenerates every 5.3 days. Here’s what happens at other settings:
| Setting | Gallons per cycle | Days between regenerations | Regenerations per year | Salt per year | Result |
|---|---|---|---|---|---|
| 10 gpg | 2,560 | 8.0 | 46 | 375 lb | Hard water for about 2.7 days each cycle |
| 12 gpg | 2,133 | 6.7 | 55 | 450 lb | Hard water for about 1.4 days each cycle |
| 15 gpg | 1,707 | 5.3 | 68 | 563 lb | Correct |
| 18 gpg | 1,422 | 4.4 | 82 | 675 lb | Soft, 20% more salt |
| 20 gpg | 1,280 | 4.0 | 91 | 750 lb | Soft, 33% more salt |
| 25 gpg | 1,024 | 3.2 | 114 | 938 lb | Soft, 67% more salt |
| 30 gpg | 853 | 2.7 | 137 | 1,126 lb | Soft, double the salt |
The pattern is lopsided. Overstating hardness costs money but the water stays soft; understating it saves salt only because the softener stops doing its job for part of each cycle. If you have to round, round up a grain or two, not ten.
Special cases
Your utility blends sources
Some cities mix surface water and groundwater, and the blend changes with the season. The Las Vegas Valley Water District, for example, adds groundwater from April to October in parts of the valley, and still advises setting softeners to 16 gpg. Where a utility gives its own setting advice, follow it; otherwise use the highest figure in the report or test your tap in summer and winter.
You’re on a private well
Test, then test again a few months later: well water can change. Include iron and manganese in the test, because they decide whether a softener alone is enough.
Your valve asks for a different unit
Almost all US softener valves use gpg. Some European and UK softeners use °dH (German degrees) or ppm. Use the hardness converter instead of entering the gpg number into a valve that expects something else.
Your dishwasher also has a hardness setting
Dishwashers with a built-in softener need their own setting, usually in German degrees or on a level scale from the manual. If the house water is already softened, the dishwasher’s setting should reflect the softened water, not the raw supply; check the manual for its lowest setting.
You also have a capacity or salt setting
Some valves ask for the resin capacity and salt dose too. Those change the grains per cycle. The salt dose has as much effect on running cost as the hardness setting: see how much salt a water softener uses for the trade-off.
How to check the setting is right
Test your softened water with a hardness strip a day before the next regeneration, when the resin is closest to exhausted. If it reads 0 to 1 gpg, the setting is doing its job. If it reads hard, raise the setting a step at a time, or check whether the softener is actually regenerating (see water softener not using salt). If the water tests soft right after every regeneration and the salt goes down fast, you can try lowering the setting toward your measured hardness.
Frequently asked questions
What happens if the water softener hardness setting is too high?
The softener regenerates more often than it needs to. Your water stays soft, but you use more salt and water. In the table above, setting 20 instead of 15 adds a third to the salt bill.
What happens if it’s too low?
The softener runs out of capacity before it regenerates, so hard water comes through for part of each cycle. You’ll notice scale and spotting coming back.
Should I set my softener higher than my water hardness?
Only by the iron allowance, or to the top of a published range. Adding several grains “to be safe” mostly buys extra salt use.
What should I set my softener to with 0 iron?
Just your hardness in gpg. The iron allowance only applies when a test finds iron.
Does the hardness setting affect water quality or only salt?
Both, in the cases above: too low gives hard water at the end of each cycle, too high only costs salt and water.
