AM and PM Capitalization Rules
Updated June 16, 2026
There is no single "correct" way to write AM and PM. It depends on which style guide you follow. Small caps with periods (a.m. / p.m.) is the most common in books and newspapers. Uppercase without periods (AM / PM) is standard in digital interfaces and technical writing.
Direct answer
For most everyday writing, pick one format and stick with it. AM / PM (uppercase, no periods) is the most common choice online. a.m. / p.m. (lowercase, periods) is the traditional print style. Avoid mixing them in the same document. See what AM and PM stand for.
The four common formats
| Format | Example | Used by |
|---|---|---|
| AM / PM | 9:00 AM, 6:00 PM | Websites, apps, digital clocks, technical docs, this website |
| a.m. / p.m. | 9:00 a.m., 6:00 p.m. | AP Style, Chicago Manual, newspapers, books |
| A.M. / P.M. | 9:00 A.M., 6:00 P.M. | Some academic writing, older style guides |
| am / pm | 9:00am, 6:00pm | Casual text messages, some UK publications, train displays |
What each major style guide says
AP Stylebook (Associated Press). Lowercase with periods: a.m. and p.m. Always put a space after the time: "9 a.m." not "9a.m." This is the standard for American newspapers, magazines, and most journalism.
Chicago Manual of Style. Lowercase with periods, same as AP: a.m. and p.m. Chicago also allows small caps in some contexts, but that is a typographic choice, not a rule.
Microsoft Manual of Style. Uppercase without periods: AM and PM. Include a space before them: "9:00 AM." This is the convention for software interfaces, help documentation, and most digital writing.
US Government Printing Office. Uppercase without periods: AM and PM. Federal documents follow this pattern.
British style (The Guardian, BBC). Lowercase without periods or spaces: 9am, 6pm. Some UK publications also use "9am" with no space. Train and bus displays in the UK often show times this way.
Quick reference by context
| If you are writing | Use |
|---|---|
| A website or app | AM / PM |
| A newspaper article | a.m. / p.m. |
| A school essay or academic paper | Check your style guide (usually a.m. / p.m.) |
| A work email | Either, just be consistent |
| A government form | AM / PM |
| A UK publication | am / pm (no periods, no space) |
| A text message | Whatever you want, nobody will judge |
Common mistakes to avoid
Mixing formats in the same document. If you write "9:00 AM" in one paragraph and "5:00 p.m." in the next, it looks sloppy. Pick one and use it everywhere.
Forgetting the space. "9AM" without a space is never correct in formal writing. Always put a space or a non-breaking space between the time and the label.
Using periods without lowering the case. "A.M." and "P.M." (uppercase with periods) is the rarest format. Most style guides that use periods also use lowercase.
Using 12 AM and 12 PM incorrectly. This is not a capitalization issue, but it is the single most common AM/PM mistake. 12 AM is midnight, 12 PM is noon. Read the full explainer.