The Online Graded Reader Text Editor--https://www.er-central.com/ogte/

Above is a link to a website I found useful.  So I'm going to add it to my index of useful TESOL resources. 

As part of my Story Time Listening project, I'm experimenting with creating some of my own graded texts.  (I'll be posting some of those in the next few weeks.)  And I found the above website useful in measuring the language I was using.

...with some complaints...
There's a few bugs in the system.  Initially I was trying to use the NGSL level 1 setting, but after fooling around with this for a while, I realized that the text editor wasn't actually flagging all of the words outside of the NGSL level 1.  (I think maybe someone inaccurately entered the list in the computer program).

So, I switched to the "General" setting.
The General setting seems to work much better, but the problem is that I can't find any outside standard which corresponds to the "General" word-lists.  I think this is just the in-house classification made up by the website.  It looks good, as far as I can tell, but the disadvantage is that if you create graded texts using their in-house word-lists, then you can't link it back to any outside standard.  (i.e., if you tell the students that "89% of these words come from the NGSL level " then that's an outside standard that can be referenced--see word list here.  But it's not as useful to tell your students "89% of these words come from level 3 of a word-list that I can't access, and that is used by some website somewhere."

So, use with caution I guess is what I'm saying.
All that being said, I've still been finding this website useful as a way of highlighting vocabulary that might potentially cause my students' difficulty when I am trying to create graded texts.