Water Softener Size Calculator: Grain Capacity, Salt Use and Regeneration
Size a water softener from your water hardness (gpg, mg/L or °dH), iron, household size and water use: daily grain load, grains needed per regeneration, the resin size (24k–96k), how often it regenerates, salt per regeneration and per year, salt cost, efficiency and a salt-setting comparison.
How it works
Compensated hardness (gpg) = hardness + iron (ppm) × 4–5 · Daily load (grains) = compensated hardness × people × gallons per person per day · Grains per regeneration = daily load × days between regenerations × (1 + reserve) · Resin (ft³) = grains per regeneration ÷ capacity per ft³ at the salt setting, rounded up to a standard size · Salt per year = ft³ × salt per ft³ × 365 ÷ days between regenerations · 1 gpg = 17.1 mg/L as CaCO₃
A water softener swaps calcium and magnesium for sodium on a bed of ion-exchange resin. Its capacity is counted in grains of hardness, so sizing starts with how many grains your household removes each day: hardness in grains per gallon times the gallons you use. Dissolved iron is counted as extra hardness (sizing guides add 4 to 5 gpg for every 1 ppm) because it coats the resin. Multiply the daily load by the days you want between regenerations, add a reserve so the softener never runs dry, and pick the smallest standard tank that holds that many grains. How many grains a cubic foot of resin delivers depends on the salt setting. Standard resin gives about 20,000 grains per ft³ when regenerated with 6 lb of salt, 24,000 with 8 lb, 28,000 with 10 lb and 32,000 with 15 lb. The marketed size (a “32k” or “64k” softener) is the 15 lb figure, but that last 4,000 grains costs 5 extra pounds of salt, so a slightly larger tank on a lower setting can use far less salt: about 3,300 grains per pound at 6 lb against 2,100 at 15 lb. The second table shows that trade-off for your water. The resin capacities are typical published figures for standard 8% cross-linked resin with co-current regeneration; your softener’s data plate or manual gives its own table, which you can enter as a custom setting. Regeneration water is a rough estimate and sodium is the theoretical amount exchanged for the hardness removed.
Worked example
Water at 15 gpg with 0.5 ppm iron is 15 + 0.5 × 5 = 17.5 gpg compensated. Four people using 75 gallons each (300 gal/day) give a daily load of 17.5 × 300 = 5,250 grains. A 7-day cycle with a 10% reserve needs 5,250 × 7 × 1.1 = 40,425 grains. At the efficient 6 lb/ft³ setting (20,000 grains per ft³) that is 2.02 ft³, so the next standard size is 2.5 ft³, sold as an 80k-grain unit. It holds 50,000 grains, regenerates every 8.7 days (about 42 times a year) with 15 lb of salt, and uses about 632 lb a year: 15.8 bags of 40 lb, or $126 at $8 a bag.
Sources: Nelsen Corporation: Softener sizing (compensated hardness, worked example) · SoftPro: The water softener sizing formula (7-day cycle, safety margin) · SoftPro: Grain capacity vs salt efficiency (capacity by salt dose) · Water Conditioning & Purification: resin capacity at 8, 10 and 15 lb/ft³ · Alberta Agriculture: Water softening (Agdex 716 D32)
Frequently asked questions
What size water softener do I need for a family of 4?
Multiply your hardness in gpg by about 300 gallons a day (4 × 75), then by the days between regenerations. At 10 gpg that is 3,000 grains a day or 21,000 a week, so a 1–1.5 ft³ (32k–48k) unit works; at 20 gpg, 42,000 grains a week calls for a 2 ft³ (64k) unit or larger, depending on the salt setting.
How do I convert ppm or mg/L to grains per gallon?
Divide by 17.1: 256 mg/L (ppm) as calcium carbonate is 15 gpg. German degrees (°dH) are 17.8 mg/L each, so multiply °dH by 1.04 to get gpg. Under 1 gpg is soft, 1–3.5 slightly hard, 3.5–7 moderately hard, 7–10.5 hard and above 10.5 very hard.
How much salt does a water softener use?
Each regeneration uses the salt setting times the resin volume: a 2 ft³ softener at 8 lb/ft³ uses 16 lb. A typical family with hard water uses roughly 400 to 800 lb a year (10 to 20 bags of 40 lb). Lower salt settings and a metered valve cut that substantially.
What does the grain rating of a softener mean?
It is the most hardness the resin can remove between regenerations, usually quoted at the maximum salt setting of 15 lb per ft³ (about 32,000 grains per ft³). At an efficient 6 lb setting the same unit delivers only about 20,000 grains per ft³, so size from the setting you will actually use.
Is a bigger softener always better?
Not much beyond a cycle of 7 to 10 days. A bed that regenerates rarely can let bacteria and iron build up, and the larger tank costs more. Size for the load, then choose efficiency through the salt setting.
How much sodium does softened water contain?
About 7.9 mg per litre for every grain per gallon removed, so 15 gpg water gains about 118 mg/L. A 250 mL glass then holds about 30 mg. People on a sodium-restricted diet can keep an unsoftened tap for drinking or use potassium chloride instead of salt.
Related calculators
Capacities are typical for standard 8% cross-linked cation resin; check your unit’s own capacity and salt-setting table. Softened water adds sodium (about 7.9 mg per litre per gpg removed); people on sodium-restricted diets may want an unsoftened drinking tap or potassium chloride.