Water Softener Size Calculator
Four people at the 75 gallon example and 10 grains per gallon use 3,000.0000 grains/day. A 20,000 grain example capacity lasts 6.6667 days.
Figures checked . People open at 4, gallons at 75, hardness at 10 gpg, and capacity at 20,000. Both 75 and 20,000 are the Penn State sample, and both can be edited.
A water softener size calculator for the question what size water softener do I need. Daily grains are people times gallons times hardness. Days are the capacity you type divided by that daily total.
Hot-water storage is a different appliance: water heater size calculator. Water held inside a pipe: pipe volume calculator.
Grains per day and days of capacity
75 gallons and 20,000 grains are the sample in Penn State Extension’s water-softening article, not a required setting. Replace them with your use and the capacity printed for the unit. Hardness in mg/L or ppm is divided by 17 because that article says one grain per gallon is approximately 17 mg/L. No iron compensation is added. The article describes iron fouling and does not give a grain adder.
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Penn State’s sample, and the 17
Source: Paul D. Robillard, William E. Sharpe, and Bryan R. Swistock, Water Softening, Penn State Extension. The sample calculation is labeled “Sample Calculation for Determining a Regeneration Cycle”: 20,000 grains of capacity, 75 gallons as average person usage per day, 10 gpg, and 4 people. It multiplies 75 × 10 × 4 = 3,000 grains per day, then 20,000 ÷ 3,000 is “about 6-7 day regeneration.” The 75 is inside that sample. Table 2 on the same page is a different guide, with drinking and cooking at 1 gal/person/day, bathing at 25–60 gal/use, dishwashing at 6–19 gal/use, and clothes washing at 20–33 gal/use. Use those rows if you would rather build a gallon figure, then type it.
The same article says a grain per gallon equals approximately 17 mg/L or ppm. Table 1, “Water Hardness Classification,” is copied below. A value that lands on 3.5 or on 7.0 sits in two printed bands, and the panel names both.
| Classification | gpg | ppm or mg/L |
|---|---|---|
| Soft | Less than 1.0 | Less than 17 |
| Slightly hard | 1.0 to 3.5 | 17 to 60 |
| Moderately hard | 3.5 to 7.0 | 60 to 120 |
| Hard | 7.0 to 10.5 | 120 to 180 |
| Very hard | Greater than 10.5 | Greater than 180 |
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What the EPA guide adds
The EPA WaterSense publication Guide to Selecting and Maintaining a Water-Efficient Water Softener (file dated May 2026) says water above 120 mg/L, or 7 gpg, is generally considered hard, and softeners are often recommended above 180 mg/L or 10 gpg. It defines grain capacity with a 32,000-grain example: that unit removes 32,000 grains before it regenerates. It says not to oversize, because an oversized unit wastes water, and it says to prefer demand-initiated regeneration over a timer. A typical softener uses 20 to 70 gallons per regeneration, and efficient units can use 4 gallons or less per 1,000 grains removed. Those water-use ranges are not multiplied on this panel.
Worked grain totals
- Sample inputs: 3,000.0000 grains/day, 6.6667 days on a 20,000 grain capacity. 4 × 75 × 10 = 3,000, and 20,000 ÷ 3,000 = 6.666…
- Three people, 75 gallons, 120 mg/L, 32,000 grain capacity: hardness 7.0588 gpg, grains 1,588.2353 grains/day, days 20.1481 days.
- One person, 50 gallons, 180 ppm, 20,000 grain capacity: hardness 10.5882 gpg, grains 529.4118 grains/day, days 37.7778 days.
Water softener questions
What does the Penn State sample return?
Leave people at 4, gallons per person at 75, hardness at 10, the unit on grains per gallon, and capacity at 20000. Hardness reads 10.0000 gpg. Household gallons reads 300.00 gal/day. Grains per day reads 3,000.0000 grains/day. Days of capacity reads 6.6667 days. Penn State band reads Hard (7.0 to 10.5 gpg). The article calls 20,000 ÷ 3,000 about 6–7 days. 6.6667 is 20,000 ÷ 3,000 shown to four decimals.
How is 120 mg/L turned into grains?
Set people to 3, leave gallons at 75, set hardness to 120, set the unit to mg/L, and set capacity to 32000. Hardness reads 7.0588 gpg. Grains per day reads 1,588.2353 grains/day. Days of capacity reads 20.1481 days. 120 ÷ 17 = 7.0588. The 32,000 is an EPA sentence about what a grain capacity means, typed here as the example capacity. It is not a recommended unit.
What if one person uses 50 gallons and the report says 180 ppm?
Set people to 1, gallons to 50, hardness to 180, unit to ppm, and capacity back to 20000. Hardness reads 10.5882 gpg. Household gallons reads 50.00 gal/day. Grains per day reads 529.4118 grains/day. Days of capacity reads 37.7778 days. Penn State band reads Very hard (greater than 10.5 gpg). 180 ÷ 17 = 10.5882, which is past the “greater than 10.5” line.
Why divide by 17 and not 17.1?
Penn State writes that a grain per gallon equals approximately 17 mg/L or ppm, and Table 1 pairs “less than 1.0” grain with “less than 17” ppm. This page uses that 17. The EPA guide pairs 120 mg/L with 7 gpg and 180 mg/L with 10 gpg. Those two pairs are not one divisor, so they are quoted as the guide’s hardness sentences and are not the conversion in the box.
Does iron add extra grains?
Not on this page. Penn State discusses iron fouling: colorless reduced iron is removed by the resin, and red oxidized iron clogs it, so filtration belongs ahead of the softener. The article does not print a grains-per-gallon adder for iron. None is applied here.
Should the softener be larger than this grain total?
The EPA WaterSense guide says softeners are sized on grain capacity and that oversizing wastes water. It also says to choose demand-initiated regeneration rather than a clock that regenerates on a schedule. Days of capacity here is only capacity divided by the daily grains. It is not a brand recommendation.