Check the IDE "Files > Examples > Sensors" directory for Ping sonar code example,and also the Arduino Playground for other sonar examples. I'm not familiar with US-100, so it probably works differently.The way the distance is displayed is determined by how you scale it and print it AFTERyou take the reading. See the Ping example above. 1" = 2.54 cm = 25.4 mm, it's justa scalefactor.
distances from 20-800mm in 1mm increments