Three units for one measurement
Hardness is the concentration of calcium and magnesium in the water, and it is reported in whichever unit the laboratory prefers. In the US it is usually grains per gallon; on a utility water quality report and on most lab printouts it is milligrams per litre, written as ppm and expressed as calcium carbonate. The conversion is a single number:
Grains per gallon = ppm ÷ 17.1
So 250 ppm is 14.6 gpg, and 10 gpg is 171 ppm. Getting this backwards is the most common error in softener sizing and it is not a subtle one — it puts the answer out by a factor of seventeen in whichever direction. If a number for household water looks like it belongs between 1 and 30, it is probably grains; if it looks like it belongs between 50 and 500, it is probably ppm.
| Grains per gallon | ppm as CaCO3 | Common description |
|---|---|---|
| under 1 | under 17 | Soft |
| 1 - 3.5 | 17 - 60 | Slightly hard |
| 3.5 - 7 | 60 - 120 | Moderately hard |
| 7 - 10.5 | 120 - 180 | Hard |
| over 10.5 | over 180 | Very hard |
The load arithmetic
A softener is rated in grains of hardness it can remove before the resin is exhausted and has to be flushed with brine. So the sizing question is how many grains the household presents per day.
Grains per day = people × gallons per person per day × grains per gallon
Four people at 75 gallons a day each, on 15 gpg water, is 4,500 grains a day. Over a seven day interval that is 31,500 grains, and with a reserve on top you are shopping for something in the region of 38,000. That is a common answer, which is why units cluster around that capacity.
The gallons per person figure is the one most often wrong. It should be total water passing through the softener, not indoor consumption in the abstract. If the outside hose bibs and the irrigation are plumbed ahead of the softener — which they normally should be, since plants have no use for softened water and it is a waste of salt — they do not count. If they are plumbed after it, they count and they will dominate everything else in summer. The irrigation zone calculator gives an idea of the volumes involved, and they are large.
Iron, and where a softener stops being the right machine
Dissolved iron is removed by a softener's resin along with the hardness, and it consumes capacity while doing it. The usual convention is to add a few grains per gallon of equivalent hardness for each ppm of iron, with three to five being the range people use. That compensation is a working allowance rather than a physical constant.
It also stops being a useful model past a few ppm. Iron fouls the resin bed progressively, coating the beads in a way that ordinary brine regeneration does not fully clear, and a softener asked to remove several ppm of iron degrades over a year or two rather than lasting. Manganese, sulphur and low pH each behave differently again and none of them are hardness. Past about 3 ppm of iron, the water needs testing properly and treating ahead of the softener, and that is a conversation with someone who works on wells rather than a calculation.
Salt dose and what efficiency actually means
The capacity of a cubic foot of resin depends on how much salt you regenerate it with, and the relationship has strongly diminishing returns. At 6 lb of salt per cubic foot a bed gives around 20,000 grains; at 15 lb it gives around 30,000. That is 50 percent more capacity for 150 percent more salt, so the efficiency falls from about 3,300 grains per pound to about 2,000.
Which end of that curve to sit on is a real choice rather than an obvious one. The efficient end means less salt bought, less chloride discharged and a lower running cost, at the price of a physically larger unit. The high dose end means a smaller unit keeping up with a heavy load, at the price of salt. Where the discharge goes matters too: brine to a septic system, or to a municipality with a chloride limit, is a constraint that some jurisdictions regulate directly, and it is worth knowing your local position before choosing a high dose. A softener discharge also has a drain connection that has to be made properly with an air gap, which is one of the points where the installation is not a casual job.
Running cost for the household water in general is a separate matter and is covered by the water bill calculator; if the softener is upstream of the water heater, the water heater sizing calculator is the neighbouring question. Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
What size water softener do I need for a family of four?
Work it from the hardness rather than the head count, because the same four people on 5 gpg water and on 25 gpg water need units several sizes apart. Four people at 75 gallons a day on 15 gpg present 4,500 grains a day. If you want a week between regenerations that is 31,500 grains, plus a reserve, so a unit rated somewhere around 32,000 to 40,000 grains fits. On 25 gpg water the same household presents 7,500 grains a day and needs roughly 60,000 grains for the same interval. Get the hardness tested first; everything else follows from it.
How do I convert ppm hardness to grains per gallon?
Divide by 17.1. One grain per gallon is 17.1 milligrams per litre of hardness expressed as calcium carbonate, which is the convention every lab uses. So a report showing 214 ppm is 12.5 gpg, and 300 ppm is 17.5 gpg. The reverse direction, grains to ppm, is multiplication by the same figure. If you are reading a utility water quality report, check the units carefully — some report hardness as calcium rather than as calcium carbonate, and that is a different number again.
Does a bigger softener use less salt?
Per gallon treated, generally yes, because a larger unit can be run at a lower salt dose per cubic foot and still make it to the next regeneration. Salt efficiency is roughly 3,300 grains per pound at 6 lb per cubic foot and only about 2,000 at 15, so the same water treated by a bigger bed at a lower dose costs meaningfully less salt over a year. The offsets are purchase price, physical space, and water sitting longer in a larger bed between cycles. Demand-initiated controls that regenerate on measured volume rather than on a timer save more salt than the sizing choice does, since a timer regenerates whether the capacity was used or not.
Should the outside taps run through the softener?
Normally not. Softened water gives no benefit to a lawn, a garden or a car wash, and irrigation volumes are large enough to dominate the grain load and multiply your salt consumption. The usual arrangement takes the hose bibs and irrigation off the line before the softener. A cold drinking tap is sometimes taken off ahead of it as well, for people who would rather not drink the sodium the exchange puts in, though the amount is small at ordinary hardness levels. Where the split is made is part of the plumbing design, and moving it later means opening walls.
Can this size a whole-house filter or an iron filter as well?
No. Those are different processes sized on different variables. A sediment or carbon filter is sized on flow rate and on the pressure drop it imposes at that flow, not on a grain capacity, and it is replaced or backwashed on a schedule rather than regenerated with brine. An iron filter uses an oxidising media and its sizing depends on the form the iron is in, the pH and the flow. What this page can tell you is what the softener behind them has to handle once they have done their part, which is the compensated hardness figure at the top of the results.