A summary of "grains", a water filtration industry measurement. //

A “grain” is a water filtration industry measurement equal to about 17.1 parts per million, or 17.1 milligrams per liter.

A deionizing cartridge rated for 256 grains, therefore, will remove approximately 4530 “parts per million” of dissolved solids (minerals). In practical terms, if you plan to use this cartridge to treat tap water that has 350 parts per million total dissolved solids, the cartridge will treat only about 13 gallons of water. //

Similarly, water softener sizes are usually expressed in grains. A 26,000 grain softener is one that will treat 26,000 grains of hardness before it needs to regenerate itself. This means that if your water has 12 grains of hardness, the softener will need to regenerate after about 1260 gallons.