When I write in word, in the lower left you should see your word count. If it's not there, I think it's under tools. Then you copy your story into Medium and highlight just the story and in the top left, it will show you the word count. You don't want to highlight the whole page because that would include the title or whatever else you write.

I don't know why but when I wrote short forms, sometimes word would tell me I'm under 150 and when I put it into Medium, it showed it was over so I'd check the word count on Medium, too, and not just word.