What Size Water Softener Do I Need? Grain Capacity Chart

A water softener's size is its grain capacity: how much hardness it can remove before it has to regenerate. A family of four with moderately hard water usually needs a 24,000 to 32,000 grain unit. To choose the right one, you need two things: how hard your water is, and how much of it your household uses.

Step 1: Find your water hardness

Hardness comes from dissolved calcium and magnesium. It is measured in grains per gallon (gpg) or in mg/L (ppm). To convert, divide mg/L by 17.1. You can find yours from:

• The water quality report from your utility, which is often online.
• A hardness test strip or drop-count kit, which costs a few dollars.
• A lab test (a good idea for well water, which can vary and contain iron).

Step 2: Add iron if you have it

Dissolved iron uses up the softener's capacity too. A common adjustment is to add about 5 grains of hardness for every 1 ppm of iron. Water with 3 gpg of hardness and 2 ppm of iron behaves like 13 gpg. Iron over a few ppm calls for other treatment as well.

Step 3: Work out daily use

The usual estimate is 75 gallons per person per day. The grains you need to remove each day are the people times gallons times effective hardness.

A household of four with 15 gpg water uses 4 × 75 × 15 = 4,500 grains per day, or 31,500 over a 7-day cycle, which points to a 32,000 grain softener.

Softener size by household and hardness

With 75 gallons per person per day, no iron, and a week between regenerations:

The water softener size calculator lets you adjust the people, hardness, iron and days between cycles.

How often should it regenerate?

Most manufacturers recommend sizing so that the softener regenerates about once a week. Much more often (every day or two) wastes salt and water and wears the system. Much less often (every two weeks) can let the resin bed foul, and stale water sit in the tank. If you are between two sizes, the larger unit is usually the better choice.

Other things to check

• Flow rate. Make sure the softener's service flow rate (in gallons per minute) covers your peak demand, such as two showers and a washing machine at once, typically 8 to 12 GPM in a family home.
• Salt efficiency. A softener's rated capacity depends on the salt dose per regeneration. At the efficient setting you get less than the headline number, so do not undersize.
• Water pressure and pipe size. A whole-house unit needs 25 to 120 psi and 3/4 or 1 inch plumbing.
• Salt-free "softeners." Conditioners that do not remove hardness minerals can reduce scale, but do not soften the water in the same way.
• Drain and power. The unit needs a floor drain or standpipe for the backwash, and an outlet.

Is it worth it?

Hard water leaves scale in kettles, water heaters and pipes, and makes soap and detergents work less well. Many people notice softer skin and clothes, and cleaner dishes. Soft water also makes water heaters last longer and run more efficiently, but the softener costs money, uses salt and water to regenerate, and puts a little sodium into the supply. Water that is only slightly hard may not justify one. Test first, and decide.