Methodology & Assumptions

Our calculators are estimating tools. This page lays out the formulas, the assumptions behind them, and the sources we checked. They give a reasonable starting estimate — not a professional specification. Always confirm your water by testing and consult a licensed installer before buying.

Water softener sizing — basis

We estimate daily softening demand as people × 75 gallons/day × water hardness (grains per gallon). Dissolved iron adds to that load, so we add about 5 grains per gallon of effective hardness per 1 ppm of iron. Multiplying daily demand by your regeneration interval (about 7 days by default) gives the capacity needed between recharges.

We then recommend the smallest standard unit whose working capacity covers that demand. A softener's advertised "grain" rating assumes a heavy, salt-inefficient recharge; at an efficient salt dose it delivers only about two-thirds of that rating, so we size against real working capacity (about 68% of the rated number) rather than the headline figure.

Assumptions: 75 gallons/person/day (a common industry planning figure); efficient working capacity ≈ 68% of rated; standard catalog sizes of 24,000–110,000 grains. Your actual usage and water chemistry will vary.

Reverse osmosis sizing — basis

A household uses only a few gallons of RO water per day, so membrane size is driven by how quickly the storage tank refills at peak draw, not by daily volume. We map household size and usage to standard membrane tiers (roughly 1–3 people → 50 GPD, 4–5 → 75, 6 → 100).

Assumption: a membrane's rated GPD is measured at 77°F and 60 psi; a typical home gets roughly half that, which we account for.

A note on local water hardness

Where we mention local water hardness, treat it as a typical, approximate figure that varies by utility, source, and season. For an accurate result, test your own water or enter your number into the calculator.

Sources we checked

• Water softener sizing formula & iron compensation
• Water Conditioning & Purification — softener salt efficiency / working capacity
• Culligan — what size water softener do I need
• USGS — hardness of water (classification thresholds)
• ESP Water — reverse osmosis buyer's guide

These are general references we consulted while building and checking the calculators; they do not endorse this site.