We use the Hepburn System of romanization with macrons. Hepburn is the most widely used and accepted form of romaji.
In most cases with ん we romanize to n, not m.
However, there are exceptions with words that have been popularized with the alternative spelling:
Macrons are the diacritical marks above letters. In Japanese, we’ll be encountering ō most often. Here are some basic macron guidelines:
Use the macron when the word is popularly stylized that way.
Do not use the macron for words already common in English.