Follow these steps to wisely use and dispose of your leftover paint.
Buy only the paint you need. Reduce the chance of having paint left over in the first place. Ask your paint retailer to help you estimate the amount of paint you'll need based on the size (square footage) of the area that you are painting. Many retailers even have online paint calculators.
Use up all your paint. If you have a third of a gallon or so leftover, why not keep painting until you use it all? Freshen up a hallway or apply an extra coat to the ceiling. Or you can mix smaller quantities of similar latex paint colors to use as an undercoat on larger jobs.
Store paint so it lasts. Want to keep leftover paint on hand for touch-ups, small projects or crafts? When properly stored, paint can last for years. Cover the opening with plastic wrap, and make sure the lid fits securely. Here's the key step: store the paint can upside down. With the plastic wrap in place, the paint will create a tight seal around the lid, keeping the paint fresh until you need it again.
Donate or Exchange your paint. If you can't find another use your leftover paint, donate it to neighborhood associates, theater groups, schools, churches, or neighbors who need or want it.
As a last resort: dispose of paint properly! If your leftover paint is usable or too small an amount to save, make sure you dispose of it properly.
Dry leftover paint and discard it with your regular trash. Remove the lid and let your leftover latex paint air dry away from children and pets. You may stir in a cup of cat litter to speed dry the material. You may dispose of the dried paint along with your normal trash.
This material is hazardous. Don't discard with your regular trash.