The Federal Reserve System usually adjusts the federal funds rate target by 0.25% or 0.50% at a time.

http://en.wikipedia.org/wiki/Federal_Reserve_System#cite_note-91

The Fed therefore tries to align the effective federal funds rate with the targeted rate by adding or subtracting from the money supply through open market operations. The Federal Reserve System usually adjusts the federal funds rate target by 0.25% or 0.50% at a time.