Water Softener Size Calculator
Water softener sizing starts with the amount of hardness the system must remove, not the number printed on a product label. Enter your hardness, household water use, and any measured iron. The calculator will show the grain load for several regeneration intervals and identify every assumption it uses.
Calculate your grain load
Hardness and household size are the only required fields. Everything else refines the result, and every value the calculator supplies for you is named in the output.
Example result
Showing a worked example for 4 people at 15 gpg. Enter your own numbers to replace it.
25,200 grains of hardness between regenerations, at 7 days
- Hardness entered
- 15 gpg (257 ppm)
- Daily water use
- 240 gal (estimated)
- Iron adjustment
- 0 gpg
- Manganese adjustment
- 0 gpg
- Compensated hardness
- 15 gpg
- Daily grain load
- 3,600 grains
| Days between regenerations | Grains to remove |
|---|---|
| 3 days | 10,800 |
| 5 days | 18,000 |
| 7 days | 25,200 |
Daily water use is estimated from household size using Pentair’s 60 gallons per person per day sizing assumption. A metered figure from your water bill will give a more accurate load.
How the calculation works
Five steps, in order. Every constant comes from a published source, listed at the bottom of this page. If JavaScript is off, the arithmetic below is all you need to do it by hand.
| Step | Formula | Source of the constant |
|---|---|---|
| 1. Convert hardness | gpg = ppm ÷ 17.1 | Whirlpool states 1 gpg equals 17.1 mg/L or 17.1 ppm, and instructs dividing a ppm result by 17.1. |
| 2. Estimate daily use | gallons/day = 60 × people | Pentair sizes its residential chart on 60 gallons per person per day. |
| 3. Compensate hardness | compensated gpg = gpg + (iron ppm × 4) + (manganese ppm × 8) | Pentair's published hardness factors: 1 ppm iron counts as 4 gpg, 1 ppm manganese as 8 gpg. |
| 4. Daily grain load | grains/day = gallons/day × compensated gpg | The method EcoWater also describes: multiply daily water usage by hardness in grains per gallon. |
| 5. Load between cycles | grains = grains/day × days | Pentair sizes on a minimum of three days between regenerations, so 3, 5 and 7 days are shown side by side. |
A worked example you can check
Four people, 10 gpg, no iron, no metered figure. Daily use is estimated as 4 × 60 = 240 gallons. Daily load is 240 × 10 = 2,400 grains. Over seven days that is 16,800 grains. Add 2 ppm of iron and the compensated hardness becomes 10 + (2 × 4) = 18 gpg, so the seven-day load rises to 30,240 grains. Both figures are recalculated by the automated tests on every build.
Where the assumptions could be wrong for you
The 60 gallons per person per day figure is a planning assumption for drawing a chart, not a measurement of your household. It is worth knowing that published estimates disagree substantially: EcoWater puts average use at 80 to 100 gallons per person per day and expects a family of four to use 320 or more. That is well above the Pentair figure this calculator defaults to.
We have not averaged the two, because averaging two planning assumptions produces a third number that nobody published. Instead the calculator uses the Pentair figure, tells you it is doing so, and takes a metered number from your water bill in preference to any estimate. If your household is heavier on water than average, enter the real figure — it changes the result more than any other input.
What to compare when you look at a specific model
Capacity alone does not tell you whether a unit suits a house. Check all of these:
- Certified capacity at the salt dose you would actually run. Ask for the capacity at the efficient minimum dose, not only the headline maximum.
- Rated service flow rate, in gallons per minute, against the peak demand of your house. Whirlpool notes that continuous operation above the rated service flow may affect capacity and efficiency.
- Pressure drop at that flow, and your incoming pressure.
- Supply line size and the fittings the unit expects.
- Maximum hardness and maximum clear water iron the model is rated for. The WHES30 lists 95 gpg and 8 ppm; the WHES33 lists 105 gpg and 9 ppm. Above a model's iron rating the answer is usually separate iron treatment ahead of the softener, not a bigger softener.
- What the softener will not remove. A softener addresses hardness and, in moderate amounts, clear water iron. Whirlpool states its units will not remove ferric (red water), bacterial or colloidal iron, and will not remove sediment.
Sources checked
Each link goes to the manufacturer or organisation that published it. We do not host copies.
- Pentair American Plumber Softeners & Filters Catalog PDF
- Whirlpool Water Solutions Hard Water & Water Softening Systems FAQ
- EcoWater Systems How Much Does a Water Softener Cost?
- Whirlpool Water Solutions Whirlpool Water Softener Owner’s Manual — Models WHES30 and WHES33 PDF