Common knowledge varies based on the author and the audience.
A fact may be common knowledge to nurses, but not common knowledge for philosophy majors. Your common knowledge will change as your education and experience change. If you have to look up a fact, then it is not common knowledge for you and you need to cite it.
Common knowledge can vary based on age, culture, or geography. A fact may be common knowledge in one country but not in another country.
If you suspect a fact is common knowledge to you, but not your audience, you should consider citing it. Remember, one reason to cite information is so your audience can verify facts and learn more.
Common knowledge does not need to be cited. For example, if you were to ask three random people the name of the current President of the United States, they would probably know because this is common knowledge.
A fact must be common knowledge to you. If a fact is common knowledge to engineers, but you had to learn it through research, then it is not common knowledge to you and you should cite it.
