Say this if it’s possible to change the fit of something.

For example, imagine that you’re trying on a ring at a jewelry store, but it’s a little too small for your finger. The saleswoman takes the ring and shows you that it can be pulled apart to become larger:

Here you go. It’s adjustable.

Other things that are "adjustable" include settings on a machine, like the volume, brightness, or color balance of a TV.

In general, words that end in "-able" mean "able to be ___ed". Some other examples:

• washable
• bendable
• winnable