I know how to measure length of a path using 'Extensions > Visualize Path > Measure'.
However, it occured that does not work for every path. I couldn't measure length of a gear created in inkscape and also a length of a letter.
May you know what the reason is?

Is the gear a real path? (select it, look into the status line)

Or did you use the path effects dialog to create it? Then first turn it into a path by doing Path -> Object to Path.

Else, please upload an example file, so we can take a look.

Thank you for your answer. Converting into a path helped. That was a silly problem.

No, not silly at all, Maja. Happy drawing!

Hi Guys,

a little help please?
I've followed your advice here, but it doesn't show me distance information anywhere.
I have already changed everything to path.
I require the length of the outer and inner perimeters of the "O".

Please see the attachment.

Maybe this:

Has anyone tried using the measure tool? It's supposed to work with text. Holding Ctrl will limit it to vertical/horizontal measurement.
One Caveat: I just found a bug (didn't look if known or fixed already) with /transformed/ text, which does not get measured correctly.

Not seeing the same thing here:

Doesn't work in your gif... See also: https://bugs.launchpad.net/inkscape/+bug/1738504