Short answer: a four-person home on hard city water (10–15 gpg) usually needs a 32,000–48,000-grain softener; a couple or a three-person home on moderate water fits a 24,000; well water with iron pushes most families to 48,000–64,000. The exact number is the grains of hardness the softener has to reduce each week, from four inputs — people, gallons per person per day, hardness and iron — and the calculator below runs that math for your house.
Tell us about your home & water
Roughly 300 gallons of water used per day.
Not sure? Here's how to find your water hardness — or start with “Hard”, the U.S. average.
A city water report or a home test kit gives you this number. 1 gpg = 17.1 ppm.
Recommended size
A 32,000-grain softener regenerates about every 9 days for your home — an efficient cycle that helps reduce hardness without wasting salt.
Your Aquonyx match
Diamond Soft 30,000 Grain (DM-S-30K)
The smallest Aquonyx softener that covers your requirement.
See the Diamond Soft line- Water / day
- 300 gal
- Hardness load
- 2,700 gr/day
- Regen every
- ~9 days
This is a sizing estimate. A certified Aquonyx dealer confirms the exact system with a water test (hardness, iron, pH, flow rate).
Want your exact numbers? A local Aquonyx dealer can test your hardness, iron, and pH — free.
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Below the calculator: the method a dealer uses at the kitchen table, a grain-capacity chart for 2–6 people, and the two mistakes — undersizing on the rated number and oversizing "to be safe" — that account for most of the softeners that disappoint. Related reading: rated vs. usable capacity, sizing for city water, and the well water testing guide for the hardness and iron numbers the math needs.
The four inputs
1. People in the household. Count everyone who lives there full time. A home office or frequent guests can justify rounding up by one.
2. Water use per person per day. The standard U.S. sizing estimate is 75 gallons per person per day, which covers showers, laundry, dishes, and the taps. Large soaking tubs, a big family that does laundry daily, or a home with teenagers can run 90–100; a retired couple often runs under 60. When in doubt, use 75 — it is the figure the industry and the calculator assume.
3. Hardness, in grains per gallon (gpg). This is the number everything hangs on, and it has to come from a test, not a guess. Municipal water quality reports list hardness, usually in mg/L (also written ppm); divide by 17.1 to get gpg — the city-water sizing guide shows where to find that report and what else to check on municipal supplies. A test strip gets you in the band; a titration kit or a lab report gets you the number. Well water must be tested — it varies from one property to the next.
| Band | Hardness | What the home usually sees |
|---|---|---|
| Soft | 0–3 gpg | Easy lather, no spotting |
| Moderate | 3–7 gpg | Light spotting on glassware |
| Hard | 7–10 gpg | Spots, scale on fixtures, dry skin |
| Very hard | 10–15 gpg | Heavy scale, shorter appliance life |
| Extreme | 15+ gpg | Rapid scale — often well water |
4. Iron, in ppm. Dissolved (clear-water) iron is exchanged by softener resin just as hardness is, and it consumes capacity fast. The working rule is to add 4 gpg of hardness for every 1 ppm of iron. Water at 12 gpg with 2 ppm of iron is sized as 20 gpg. Above roughly 3 ppm, or with any red-water (oxidized) iron, the softener is the wrong first step — dedicated iron treatment goes ahead of it, and the well water testing guide covers how to make that call.
The math, step by step
Once you have the inputs, the arithmetic is short.
- Daily water = people × gallons per person per day
- Compensated hardness = hardness (gpg) + iron (ppm) × 4
- Daily grains = daily water × compensated hardness
- Weekly grains = daily grains × 7
- Required rated capacity = weekly grains ÷ 0.75
Step 4 exists because softeners are sized to regenerate about once a week — often enough to keep the resin healthy, seldom enough to keep salt and water use down. Step 5 is the part most guides skip. The grain number on a softener is its capacity at the maximum salt dose; at an efficient dose it delivers roughly 75% of that. Sizing on the rated number alone is how a "48,000-grain" softener ends up regenerating every four days. The rated vs. usable capacity guide has the full salt-dose curve if you want to see why.
Then choose the smallest standard size whose rated capacity is at or above the required number. Standard residential sizes run 24,000, 32,000, 40,000, 48,000, 64,000 and 80,000 grains; above that you are into large single tanks, twin-tank systems, or light commercial equipment.
Worked example
A family of four, city water tested at 12 gpg, no iron.
- Daily water: 4 × 75 = 300 gallons
- Compensated hardness: 12 + 0 = 12 gpg
- Daily grains: 300 × 12 = 3,600 grains
- Weekly grains: 3,600 × 7 = 25,200 grains
- Required rated capacity: 25,200 ÷ 0.75 = 33,600 grains
The smallest standard size at or above 33,600 is 40,000 grains. A 32,000-grain unit would be undersized by this method — it would regenerate roughly every 6–7 days at an efficient salt dose with no reserve, and any guest weekend pushes it into hard water before the next cycle.
Same family on a well at 18 gpg with 1.5 ppm iron: compensated hardness is 18 + 6 = 24 gpg, daily grains 7,200, weekly 50,400, required 67,200 — an 80,000-grain softener, and an iron filter ahead of it deserves a serious look.
Sizing lookup table
Rated capacity to select, assuming 75 gallons per person per day, weekly regeneration and an efficient salt dose. Use the compensated hardness (hardness + 4 × iron ppm) to pick the column.
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg | 25 gpg |
|---|---|---|---|---|---|
| 2 | 24,000 | 24,000 | 24,000 | 32,000 | 40,000 |
| 3 | 24,000 | 24,000 | 32,000 | 48,000 | 64,000 |
| 4 | 24,000 | 32,000 | 48,000 | 64,000 | 80,000 |
| 5 | 24,000 | 40,000 | 64,000 | 80,000 | 96,000 |
| 6 | 24,000 | 48,000 | 64,000 | 96,000 | 110,000 |
Two things to notice. First, the 24,000-grain size covers more homes than dealers expect — a couple on moderately hard water does not need a 48,000-grain tank. Second, the table climbs quickly once compensated hardness passes 15 gpg, which is where iron usually pushes it: a well at 12 gpg with 2 ppm of iron sizes in the 20 gpg column, not the 10.
When to go up a size — and when not to
Go up one size when:
- Household water use is well above 75 gallons per person (large tubs, daily laundry, a home with teenagers).
- The home has a high-flow fixture set — a multi-head shower or a 3-bath home with several running at once — and the smaller unit's flow rating would be exceeded. Flow, not grains, is the constraint here; check the valve's service flow rate at an acceptable pressure drop.
- The family is likely to grow, or the home regularly hosts guests.
- The calculation lands within a few percent of a size boundary. Reserve is cheap; a softener running at its limit is not.
Do not go up a size just to be safe. A softener that is far too large regenerates too rarely. Resin that sits loaded for weeks — especially with iron present — fouls, channels and packs, and the chloride per regeneration is higher than a right-sized unit's. Right-sized with a weekly cycle beats oversized every time. If the math says 40,000 and the customer wants 80,000 "to be safe," the honest answer is that it will cost more, use more salt per regeneration and not last longer.
Beyond 80,000 grains, or any home where water use is continuous (large households, home businesses, horse properties), the right answer is usually a twin-tank system or light commercial sizing rather than a bigger single tank — one tank regenerates while the other stays in service, so there is no hard-water bypass window and no compromise on regeneration frequency. That is a dealer conversation, and the calculator routes larger loads there for that reason.
Common sizing mistakes
Sizing by bathrooms. Bathroom count says nothing about hardness. A 4-bath house on 5 gpg water needs a smaller softener than a 2-bath house on 20 gpg.
Skipping the hardness test. "The neighbor has 15 gpg" is not a test. Wells differ across a street; municipal hardness changes seasonally and by source. Test the water that will go through the softener.
Ignoring iron. The single most common reason a properly sized softener is "not keeping up" in a well-water home is iron that was never added to the hardness load.
Sizing on the rated number. Covered above, but it bears repeating: the box number is the maximum-salt figure. Divide by 0.75 before you choose.
Confusing grains with flow. Grain capacity tells you how long the softener runs between regenerations. The valve's service flow rate tells you whether the home will see a pressure drop with three showers running. Both must be right, and a big-grain tank on a small valve solves neither problem well.
For dealers: what changes at the professional level
The method above is the one Aquonyx dealers use on every residential quote, and it is what the sizing calculator runs. Dealers sizing from a full water test — hardness, iron, manganese, pH, TDS and flow — refine it in two ways: manganese is compensated like iron, and the salt-dose setting is chosen deliberately so that the usable capacity and regeneration interval match the household rather than defaulting to the valve's factory program. The Diamond Soft residential line is offered in 30,000, 45,000 and 60,000-grain sizes for exactly this reason: the steps line up with the required-capacity bands most homes land in once the math is done properly.
If a system is already installed and the customer reports hard water between cycles, the diagnostic order is the same as the sizing order: confirm the hardness and iron with a fresh test, check the programmed capacity and salt dose against the actual load, then look at the valve. A Clack WS1 that will not draw brine produces the same symptom as an undersized tank, and it is far more common.
The short version
Count the people, multiply by 75, multiply by the tested hardness (plus 4 per ppm of iron), multiply by 7, divide by 0.75, and pick the smallest standard size at or above that number. Test the water, never guess it. Run the numbers in the water softener sizing calculator, and for anything above 80,000 grains or on iron-bearing well water, get a dealer to size it from a full test.