A decimeter, abbreviated "dm", is a metric unit of length. It is defined as one tenth of a meter. One decimeter is equivalent to 3.93700787 inches.
To convert a measurement in decimeters to inches, you multiply the length in dm by 3.93700787. Generally it's okay to just use 3.94. So, if you measure an object with a ruler, and it's 3 dm long, the length in inches is 3.94 x 3 = 11.82 inches. You can perform the conversion with some calculators or do an online search for "length unit calculator" to find a web-based conversion tool. Apps are available that allow you to perform the conversion with a smartphone.