No matter how calmly you say it, telling someone to “calm down” has never done anything but make a person less calm.
One time when I was arguing with my my mom as a kid, I remember telling her to “calm down.” She did not react well. After that argument I had a lot of calming alone time in my room.
If you tell someone to “calm down,” the standard reaction is to frantically deny and accuse the opposite. “I need to calm down?! I’m COMPLETELY calm – I’m not the one who needs to calm down. YOU calm down! I’M PERFECTLY CALM!”
It’s tricky; you have to get someone to calm down without letting them know that you know they’re not calm. “Look. We’re both a little amped up. Why don’t we soothe things down with some Miles Davis.”
Otherwise, you’ve entered a yelling match about who is the most calm and you’re both going to look insane if you do that…
“If you were calm, we wouldn’t be arguing in the first place!”
“So I’m always the crazy one! Look at me, everybody! I’m so NOT calm! That’s me: the crazy not calm person!”
“All I said was to please calm down!”
“I was calm until YOU said that!”