What Size Softener Do I Need?

by Curtis Grossi | Last Updated: August 6, 2020

Water softeners are devices used to prevent ‘hard’ water from forming lime-scale build-up in water pipes, fittings and appliances, and to enhance the lathering property of soap and detergents which can be reduced by the presence of hard water. Water hardness is caused by elevated levels of calcium and magnesium ions in the water, which usually mean that the water has been sourced from an area containing limestone or dolomite.

water flow

When water flows through or over limestone or dolomite formations, before it is drawn into the water supply, it dissolves calcium and magnesium ions from these rocks. When the water is heated, these ions combine with carbon dioxide and oxygen molecules in the water to form carbonate crystals that cling to the heated surfaces in which the water is contained. This results in what is commonly known as ‘lime-scale’, a hard crust that coats the insides of electric water heaters, hot water pipes, electric kettles and irons, and hot water faucets.

Why Soften Water?

The buildup of so-called ‘lime-scale’ tends to reduce the efficiency and lifespan of pipes and appliances, increase maintenance costs and the amount of cleaning agents required. Hard water can also be unpleasant to use for bathing and showering because of its poor lathering ability and the formation of soap scum.

In order to avoid the problems caused by lime-scale build-up, it is necessary to ‘soften’ the household water supply by the removal or modification of the calcium and magnesium ions before they can form carbonate crystals.

Fortunately, water softener systems are available in a wide variety of sizes these days, from portable systems for your RV to a range of whole-house systems for your home, whatever size it is.

Water softeners technically refer to salt-based ion-exchange systems, whereas water conditioners, also termed ‘de-scalers’, use other technologies such as chelation, nucleation-assisted crystallization and electromagnetic pulses – the latter two generally classed as physical water treatment – to ‘modify’ the hardness ions to prevent them from forming lime-scale in water pipes and appliances.

Factors Determining Water Softener Size or Capacity

The size of water softener you will need is determined by two factors; the ‘hardness’ of your water supply, and the amount, or volume, of water per day that you wish to have softened.

It is always advisable to find out the hardness of your water supply, whether from your local water supply utility, or by having it measured yourself. Water hardness is measured in parts per million (ppm) or grains per gallon (gpg). One grain of hardness is equal to approximately 17 ppm. A measured hardness of 10.5gpg (180ppm) or more is considered very hard water which will need to be softened using a water softener, although above 7gpg (120ppm) you may also wish to soften your water.


Water hardness readings of 25gpg (425ppm) and above are not uncommon in some water supplies, especially those drawn directly from wells.

Water Softener Size Calculation

Households in the USA are considered to use an average of 75 gallons (284 liters) of water per person per day.

So, of your water hardness has been measured at 15gpg and there are 5 people in your household, your water softener will need to remove 15 x 5 x 75 = 5,625 grains of water hardness per day. Water softener manufacturers recommend regenerating the resin beads of a water softener system every seven days (but definitely not longer than 14-day intervals), resulting in a capacity requirement of 39,375 grains per week.

Incorporating Iron Removal

It should also be borne in mind that the presence of ferrous (dissolved) iron in water increases its hardness, so you should also enquire about, or test for, the presence of this mineral in your water supply. If iron levels are low, they can be removed via the normal operation of your water softener, but you should treat your resin tank with a proprietary iron remover once a year. Also note that each ppm of iron is equivalent to 3 to 4 grains of calcium or magnesium hardness, so this should be figured into the calculation of the capacity of water softener you require. If your water contains 1ppm of iron, then you will need to add 1,125 grains to your daily water hardness calculation, and 7,875 grains to your weekly requirement, using the earlier example above, resulting in a weekly requirement of 47,250 grains.

Efficiency and Salt Consumption


Water softener resin has a rated capacity of 32,000 grains per cubic foot (73,227 kilograms per cubic meter), but would require 16 lbs (7.26kg) of salt to regenerate to 30,000 grains. This would imply using a massive 832 lbs (59 stone) of salt per annum for a 30,000 grain system. Using only 6 lbs (2.72kg) of salt, or 37.5% of 16 lbs, it is possible to regenerate the cubic foot of resin to 20,000 grains, or 62% of its original capacity. The efficiency gain of 25% in terms of salt consumption with the accompanying reduction in costs, labor and environmental impact make it work considering up-sizing your water softener capacity by the factor indicated earlier, i.e. double the capacity of your basic calculation.

Given that water softener capacity ratings are based on test conditions in a laboratory, and not real world conditions, and that water softener manufacturers recommend that their systems ‘regenerate’ once per week, it is wise to purchase a water softener system with a capacity of twice the amount you calculate using this method.

Recommended Capacity

In summary, your water softener capacity requirement incorporates water hardness, weekly consumption, dissolved iron, and efficiency factors.

In the example of a five-person household used earlier, the basic capacity requirement was calculated at 39,750 grains per week, increasing to 47,250 grains per week in the presence of 1ppm of ferrous iron. A 48,000 grain system would therefore appear to be sufficient for this household. However, this system would require about 24 pounds of salt to regenerate and would recover only about 94% of its original capacity, that is, 45,000 grains, which is less than the required weekly capacity.

Using the efficiency factor, 37.5% of the salt can regenerate to 62.5% of the resin’s original capacity. Therefore one would look at a 96,000 grain system that would require 18 lbs of salt per week to regenerate to 60,000 grains, which will be more than adequate.

Using a water softener with a flow meter rather than a timer, the system can be set to only regenerate when required, which would be perhaps every 10 days rather than once per week. This will result in further savings to offset the cost of the larger system.