Usted tranquilo

Usted tranquilo is a common way to tell someone that everything will be okay. Note the absence of any verb.

• Usted tranquilo. Relax. Everything will be fine. It will all work out in the end.
• Tú tranquilo, que todo está bajo control. Everything's under control. No need to worry yourself. No te preocupes.

This fixed expression can be said with usted even if you're talking to someone you usually use tú with. However, as you can see from the last example, it can be used with tú as well.

If the person you are addressing is female, make the appropriate adjustment: Usted tranquila, que todo está bajo control. Relax, we've got everything under control. And if more than one person is being address, of course you'll want to use: Ustedes tranquilos/as.

Humorously, I've heard: Usted tranquilo ... y los demás preocupados.

Tranquilo

This expression also work in its one-word version: Tranquilo, relax. It is worth mentioning that although tranquil is a somewhat formal word in English, its Spanish equivalent is a perfectly good everyday word.

Cálmate

Tranquilízate is another way to say: relax, don't get upset. Tienes que tranquilizarte = You need to calm down. Cálmate is also heard. I confess that hearing this never seems to have its intended effect on me.