Calculator · formula shown
Water softener size calculator
Four numbers from your water report and your household, and you get the capacity you need between regenerations, the standard size to buy and the flow your plumbing will ask for.
- Compensated hardness12 gpg
- Grains removed per day3,600
- Grains between regenerations25,200
- With a 25% reserve33,600
- Peak flow to plan for9 to 11 GPM
Covered by a 40,000-grain softener with the reserve. Nameplate grains are measured at the highest salt dose; on an efficient setting most resin delivers less, which is why the reserve matters.
How the calculator works
The short answer: multiply the people in your home by the gallons each one uses per day and by your hardness in grains per gallon. That is the hardness your softener removes every day. Multiply by the number of days you want between regenerations, add a 25% reserve, and round up to the next standard size (24,000, 32,000, 40,000, 48,000, 64,000, 80,000 or 96,000 grains).
compensated hardness (gpg) = hardness (gpg, or mg/L ÷ 17.1) + 5 × iron (ppm)grains per day = people × gallons per person per day × compensated hardnessgrains needed = grains per day × days between regenerationssuggested size = smallest standard size ≥ grains needed ÷ 0.75
Each step has a reason. Hardness reports come in two units, and 1 gpg equals 17.1 mg/L, so a city report of 200 mg/L as CaCO3 means 11.7 gpg. Clear-water iron uses resin capacity too: published sizing guides add 3 to 5 gpg per ppm, and we take 5 because it errs on the side of a tank that keeps up. The 25% reserve covers houseguests, a week of laundry catch-up, and the fact that nameplate grain ratings assume the maximum salt dose. A 32,000-grain tank set to save salt delivers closer to 20,000 to 24,000 grains per regeneration, depending on the valve and the setting.
The flow line comes from the brands in our data. SpringWell sizes its CF filters at 9 GPM for 1-3 bathrooms, 12 GPM for 4-6 and 20 GPM for 7 or more, and its SS softener at 11 GPM for 1-3 bathrooms and 13 GPM for 4-6. If your softener cannot pass that flow, you feel it when two showers and the washer run at once, which is exactly when you notice a water system at all.
Where to find your numbers
City water: your utility publishes a Consumer Confidence Report every year, and most list hardness in mg/L. If it is missing, the customer service line usually knows it. Well water: you need a test. A lab panel for hardness, iron, manganese, pH and bacteria is worth more than any spec sheet on this site, because iron above a few ppm or any hydrogen sulfide changes what you should buy first. Our guide to removing iron from well water explains the order of treatment.
Not sure you need a softener at all? Read whole house water filter vs water softener first, then come back. If you already know, the softener ranking compares 7 softeners and conditioners at their 1-3 bathroom size, with prices checked on November 9, 2026.
Questions
What size water softener do I need for a family of 4?
It depends on your hardness. With 4 people at 75 gallons each and 10 gpg, the household removes 3,000 grains a day, or 21,000 a week: a 32,000-grain softener covers it with the 25% reserve. At 20 gpg the week needs 42,000 grains, so look at a 64,000-grain unit (or a 48,000 that regenerates every 5 days). Run your own numbers above.
How do I convert ppm or mg/L of hardness to grains per gallon?
Divide by 17.1. A city report that lists 250 mg/L (or ppm) as calcium carbonate means about 14.6 gpg. Some reports give hardness in "mg/L as CaCO3": that is the figure to use.
Why does iron change the size?
The resin in a salt-based softener takes up dissolved (clear-water) iron the same way it takes up calcium and magnesium, so iron uses capacity. Published sizing guides add 3 to 5 grains per gallon for each ppm of iron; this calculator uses 5, the cautious end. Above a few ppm, most brands, SpringWell and SoftPro included, point you to a separate iron filter first.
Is a bigger softener better?
Up to a point. A slightly larger tank regenerates less often and can run on a more efficient salt setting. Too large, and it may sit for weeks between regenerations; several brands add an automatic refresh for that case. Pick the smallest standard size that covers about a week with the 25% reserve.
Does this calculator work for salt-free conditioners?
Not for grains: salt-free conditioners do not have a grain capacity. They are sized by flow rate and by a maximum hardness (75 gpg for the SpringWell FutureSoft and the SoftPro Fortress). Use the flow check below and the hardness limit on each review.