goodbye

"Doei" is the most common informal way to say goodbye in Dutch. It carries a friendly, lighthearted tone, similar to saying "bye" or "see ya" in English. It is derived from the older, more formal "dag" (day), which eventually evolved into the diminutive "dagje" and then shortened to "doei." You can use "doei" with friends, family, and colleagues you are on good terms with. It is perfectly acceptable to say it once, or double it up as "doei doei" for a slightly more emphatic or cheerful farewell. Avoid using it in very formal business letters or official government correspondence, where "met vriendelijke groet" or "tot ziens" would be preferred.
In conversation
Leaving a friend's house
Ik ga er weer vandoor, doei!
I'm heading off again, bye!
Doei doei, tot snel!
Bye bye, see you soon!
Doubling 'doei' is very common and sounds extra friendly.
Hanging up the phone
Bedankt voor het bellen, doei!
Thanks for calling, bye!
Geen probleem, doei!
No problem, bye!
It is a standard way to end a casual phone conversation.
Examples
Doei, tot morgen!
Bye, see you tomorrow!
Ik moet nu echt gaan, doei doei!
I really have to go now, bye bye!
Doei, en doe de groeten aan je moeder.
Bye, and say hi to your mother.