Reverse osmosis
Whole-home filtration
Water softener
Main job
Makes the cleanest drinking water
Improves the water throughout the house
Prevents hard-water scale
Installed
Under the kitchen sink, with its own faucet
Where water enters the home
Where water enters the home
Treats
Drinking and cooking water
Every tap, shower, and appliance
Every tap, shower, and appliance
Reduces
A wide range of dissolved contaminants, such as lead, arsenic, and nitrates, depending on certification
Sediment, chlorine, and chemicals that affect taste and smell
Calcium and magnesium (hardness)
Doesn't do
Treat the whole house or disinfect on its own
Remove hardness or filter as finely as RO
Remove chlorine, lead, PFAS, or most other contaminants
Ongoing care
Filter and membrane changes
Filter changes
Salt or potassium refills