We'll assume that you'll be purchasing from ion exchange water softeners because all the controversy surrounding magnetic and electronic water softeners might have turned you off buying one. So the question is: which softener is right for your home?
Know water hardness
Get the water tested to find out how hard it is and determine the grain capacity you'll need in a water softener. Most companies in the water softening business will do this for free, but you can also complete your own test and send it in to certain locations for examination. It's safe to assume your water is hard if soap scum is present in sinks, body and dish soap doesn't lather and cleaning supplies aren't working effectively.
Figure out the softener's capacity
Multiply the water's hardness level (in grains per gallon) by the amount of water that is used each day to find out how many grains will need to be removed. One person uses approximately 80 gallons of water each day. This number covers all usage, from drinking to doing laundry, showering and other needs. In a four-person household, about 320 gallons of water would be used daily. So, if the water has a hardness of 10, a four-person household would need 3,200 grains removed daily.
A good thing to consider is purchasing a system that goes longer between regenerations - a few weeks of coverage is a good start. Most water softeners have systems ranging in coverage from 20,000 to 100,000 grains per gallon. In the above case, a 45,000-grain softener would last the family about two weeks between refills.
The math: Hardness (10) x Daily Use (320) = (3,200) x Usage Length (14 days) = 44,800
Where to use soft water
Not all water in the home needs to be softened. The fewer pipes soft water is running to, the less you'll spend on operating costs. Decide in which areas soft water is essential and allow hard water to be delivered to the rest. Some water should never be softened, especially water flowing outdoors to be used on the lawn or in the garden.
Other impurities it can remove
Different water softeners can be certified to remove different contaminants. Depending on the quality of your water, you may want to choose one that can handle small amounts of iron, manganese or other hazards.
Look into the specific details of what the water softener does. Water softener options can save you time and money but often add a bit extra to the purchase price. Weigh the costs and benefits: the savings you reap could very well outweigh the money paid out initially.