You have to use python in the field calculator. The basic operations are listed here under the Geometry Unit Conversion section. The basic syntax for filling in length from lines is:
You can convert units with expressions like:
Thank you for your answer! I think I got this now
I have a further question: I need calculate the mean of all this length fields and just want to transform lines that are less than mean value to be a polygon. How can I make this possible? I know there was a function called "features to polygon" but this is only work for whole line. Could you please kindly give me some tips? Thank you again!