Water Softener Sizing Calculator
Turn your water hardness and daily water use into a softener load (grains per day) and a regeneration interval — so you can pick a grain capacity that fits.
Calculator
At 20.0 gpg and 300 gallons/day your water carries about 6,000 grains/day; a 32,000-grain softener would regenerate roughly every 5.3 days. If hardness is in ppm, divide by 17.1 first. Sizing a softener is not a water test — test with a certified lab.
A water softener works by loading resin beads with grains of hardness until the resin is full, then flushing it clean with brine (a regeneration). To size one you need two numbers: how hard your water is and how much of it you use. Multiply them and you get the grains of hardness your softener has to remove every day; divide the unit’s grain capacity by that daily load and you get how often it will regenerate. You can enter hardness in grains per gallon (gpg) directly, or convert from a lab report in ppm / mg‑L by dividing by 17.1.
A softener that regenerates roughly every 3–7 days is a good target for a typical home — much more often and the unit is undersized and wears its resin quickly; much less often and softened water can sit in the resin bed for too long between cycles.
Formula
Two identities from stable water-chemistry conventions:
Grains per day = hardness (gpg) × daily gallons
Regeneration interval (days) = grain capacity ÷ grains per day
And, if your hardness came back from the lab in ppm (mg‑L as CaCO3):
gpg = ppm ÷ 17.1
Grain capacity is a property of the softener you buy, so this tool asks for it rather than assuming a brand or model.
Worked example
A home with 20 gpg hard water using 300 gallons a day loads 20 × 300 = 6,000 grains per day. A 32,000‑grain softener divided by 6,000 grains/day regenerates about every 5.33 days — a comfortable interval. If that same house had 40 gpg water it would load 12,000 grains/day and the same unit would regenerate every 2.7 days, a sign you would want a larger capacity.
How softener sizing works
Grain capacity on the box is usually the maximum capacity at a high salt dose; the efficient capacity (less salt per regeneration) is lower, so many installers size a unit so it regenerates every few days at a moderate salt setting rather than running it flat out. If you have iron in the water, add roughly 4–5 “grains” of hardness for each ppm of iron, because iron also loads the resin.
Hardness is only one property of your water. Sizing a softener does not tell you whether your water is safe: bacteria, nitrate, arsenic and other contaminants are invisible to a hardness number. Test your well water with a certified laboratory before you decide what treatment you actually need — a softener, a filter, a UV lamp, or a combination. Use the hardness scale below to read your result.
Reference table
Water-hardness classification (grains per gallon and the equivalent ppm / mg‑L). Divide a ppm reading by 17.1 to get gpg.
| Classification | Grains per gallon | ppm (mg‑L) |
|---|---|---|
| Soft | 0.0–1.0 | 0–17 |
| Slightly hard | 1.0–3.5 | 17–60 |
| Moderately hard | 3.5–7.0 | 60–120 |
| Hard | 7.0–10.5 | 120–180 |
| Very hard | 10.5+ | 180+ |