I put decimeter (dm) in mathcad and it says that this variable is undefined. How can I define it then?
If you're using Prime, make sure that both dm and m are labelled as Unit.
Mathcad does not provide a multitude of multiples and submultiples of units of measurement. So much so that I, all those I need, I have defined and collected in a file, as you can see in the following picture. The file created in this way can be connected to the file in which you work by inserting the "reference".