Please note that LENGTH is a field that has no significance in respect to the length of a track as determined by the creating program.
The property for the length can be displayed with the variable_LENGTH.
If you create a column with
then it does not show any data.
If you want to see the length as found in the file header, use value: %_length%
Please note, that the propery _LENGTH cannot be written my MP3tag.
if you want to see your user-defined field (which could contain anything), use value: %length%
If you want to also modify this field in the column, then also fill Field: %length%
More about the customization of the file list, see
For the files that you have treated with your action, check the extended tags dialogue Alt+T to see if a field LENGTH has been created.
If you have files where the column displaying the data from _LENGTH stays empty, then you have a file that has to be checked as it apparently has a broken header.
Thank you for your help.
Ahhh, I understand that Mp3tag cannot calculate the length itself: the length must have been created beforehand by the MP3 creation software...
So I'm going to look on this side (maybe with an FFMPEG or FFPROBE command??)
Sorry for not being clear enough: my files are produced by a tool that uses the Google Text to Speech API. And it appears that the files do not include information on their duration.
By the way, all the files are tagged with "ERROR: MP3 header parse error".
This is why indeed, I think that the problem comes from there.
An MP3 without the proper information in the header is a broken file.
The error message indicates that.
In that case it would not make a difference whatever you added as tag data.
Please follow @LyricsLover 's advice and check the files for integrity first.
All further efforts are vain as long as the file is broken.