Water Hardness GPG to PPM Calculator
Convert gpg, ppm and °dH.
Special
Water Softener Size Calculator
Work out how many grains of hardness your household removes each day and which softener rating — 24,000, 32,000, 48,000 grains or more — can handle it with a sensible regeneration interval.
How it works
- Choose whether to estimate water use from household size or enter gallons per day from a water bill.
- Enter the hardness from your water test and pick its unit; add iron if the test shows it.
- Set how often you would like the softener to regenerate (about weekly is typical) and your salt setting.
- Read the recommended rating and compare how every standard size would perform in the table.
Formula
Compensated hardness (gpg) = hardness + 4 × iron (ppm) Daily load (grains) = gallons per day × compensated hardness Capacity needed = daily load × days between regenerations Choose the smallest rating where rating × salt-setting factor ≥ capacity needed
Hardness in ppm (mg/L as CaCO3) is divided by 17.12 to give grains per gallon. Iron is counted at 4 gpg per ppm, a common dealer rule because iron also consumes exchange capacity. A softener’s rating is its capacity at the maximum salt dose; the medium setting delivers about 80% of it and the efficient setting about 67%. The days column keeps one day of water use in reserve.
Example
A family of five on a well with 18 gpg hardness and 1 ppm iron wants the softener to regenerate about once a week.
Inputs
- Water use from
- Household size
- People in the home
- 5
- Use per person
- 75 gal/day
- Water hardness
- 18
- Hardness unit
- gpg (grains/gal)
- Iron (optional)
- 1 ppm
- Target days between regenerations
- 7 days
- Salt setting
- Medium (≈9 lb salt / ft³)
Result
Recommended softener size80,000 grainsDelivers about 64,000 grains per cycle at the medium salt setting
Daily hardness load8,250 grains/day
Capacity needed per cycle57,750 grains
Est. days between regenerations≈ 6 days
Why a bigger softener is not always better
An oversized softener regenerates rarely, which sounds efficient, but resin that sits for weeks can channel and foul, and many installers set a calendar override anyway. An undersized unit regenerates every day or two, using more salt and water per gallon softened and risking hard-water bleed-through at peak demand.
Aiming for regeneration roughly every 5–10 days at a medium salt setting balances salt efficiency, resin health and reserve capacity. The table shows how the neighbouring sizes compare so you can judge the trade-off.
What this calculator does not cover
Flow rate matters as well as capacity: a household with several bathrooms may need a larger resin tank or valve for pressure reasons even if the grain capacity is small. Manganese, hydrogen sulfide, tannins and high iron may need dedicated filtration before the softener. A softener does not make water safe to drink; testing and treatment for contaminants are separate questions.
Frequently asked questions
What size water softener do I need for a family of 4?
With 75 gallons per person and 10–15 gpg hardness, a family of four removes about 3,000–4,500 grains a day, so a 32,000–40,000 grain softener regenerating about weekly at a medium salt setting is typical. Enter your own hardness above for a specific answer.
How do I convert ppm to grains per gallon?
Divide ppm (mg/L as calcium carbonate) by 17.12. For example, 257 ppm is about 15 gpg. The calculator accepts ppm directly, or use the GPG to PPM converter linked below.
Why does iron increase the softener size?
Softener resin removes small amounts of dissolved (clear-water) iron, but iron uses exchange capacity. A common rule adds 4 gpg of hardness for each ppm of iron. Higher iron levels usually call for an iron filter before the softener.
What does the grain rating on a softener mean?
It is the hardness the resin can remove between regenerations at the maximum salt dose. A 32,000-grain unit at a medium salt setting delivers roughly 25,600 grains per cycle.
Is a softened water supply safe to drink?
This calculator only sizes equipment for hardness. It does not assess drinking water safety. Have your water tested and follow local health guidance; people on sodium-restricted diets may want an unsoftened tap or reverse osmosis.