12-Hour vs 24-Hour Time
The 12-hour clock runs 1–12 twice a day, using AM (midnight to noon) and PM (noon to midnight). The 24-hour clock — also called military time — runs 00:00 through 23:59 once, with no AM/PM. Converting between them is simple once you know two things: the +12 rule and the midnight/noon exceptions.
Minutes never change in a conversion — only the hour and the AM/PM suffix. The only tricky values are 12 AM (midnight) and 12 PM (noon).
The Two Rules That Cover Everything
AM → 24-hour
Keep the same hour and drop the AM — except 12 AM, which becomes 00. So 7:20 AM is 07:20, and 12:30 AM is 00:30.
PM → 24-hour
Add 12 to the hour and drop the PM — except 12 PM, which stays 12. So 3:45 PM is 15:45, and 12:15 PM is 12:15.
Read the suffix
Is the time AM or PM? That decides which rule you apply.
Apply the hour rule
AM: keep the hour (12 → 00). PM: add 12 to the hour (12 → 12).
Keep the minutes
Minutes are identical in both formats — copy them across unchanged.
Drop the AM/PM
24-hour time has no suffix. 3:45 PM becomes simply 15:45.
Full Conversion Chart
| 12-hour (AM/PM) | 24-hour | Spoken (military) |
|---|---|---|
| 12:00 AM (midnight) | 00:00 | zero hundred |
| 1:00 AM | 01:00 | zero one hundred |
| 2:00 AM | 02:00 | zero two hundred |
| 3:00 AM | 03:00 | zero three hundred |
| 4:00 AM | 04:00 | zero four hundred |
| 5:00 AM | 05:00 | zero five hundred |
| 6:00 AM | 06:00 | zero six hundred |
| 7:00 AM | 07:00 | zero seven hundred |
| 8:00 AM | 08:00 | zero eight hundred |
| 9:00 AM | 09:00 | zero nine hundred |
| 10:00 AM | 10:00 | ten hundred |
| 11:00 AM | 11:00 | eleven hundred |
| 12:00 PM (noon) | 12:00 | twelve hundred |
| 1:00 PM | 13:00 | thirteen hundred |
| 2:00 PM | 14:00 | fourteen hundred |
| 3:00 PM | 15:00 | fifteen hundred |
| 4:00 PM | 16:00 | sixteen hundred |
| 5:00 PM | 17:00 | seventeen hundred |
| 6:00 PM | 18:00 | eighteen hundred |
| 7:00 PM | 19:00 | nineteen hundred |
| 8:00 PM | 20:00 | twenty hundred |
| 9:00 PM | 21:00 | twenty-one hundred |
| 10:00 PM | 22:00 | twenty-two hundred |
| 11:00 PM | 23:00 | twenty-three hundred |
The Midnight & Noon Trap
This is where almost every mistake happens:
- 12:00 AM = 00:00 — midnight, the start of the day. It is not 12:00.
- 12:00 PM = 12:00 — noon, the middle of the day.
A memory hook: AM noon doesn’t exist and PM midnight doesn’t exist. “12 AM” is the first minute of the day (00:00); “12 PM” is the only PM hour that keeps its number (12:00).
To avoid ambiguity entirely, many schedules, hospitals, and airlines simply write 00:00 and 12:00 in 24-hour format instead of “12 AM” / “12 PM”.
Worked Examples
| You have | Rule | 24-hour |
|---|---|---|
| 9:05 AM | AM, keep hour | 09:05 |
| 12:40 AM | AM, 12 → 00 | 00:40 |
| 12:40 PM | PM, 12 stays | 12:40 |
| 4:15 PM | PM, +12 | 16:15 |
| 11:59 PM | PM, +12 | 23:59 |
And going the other way (24-hour → 12-hour):
| You have | Rule | 12-hour |
|---|---|---|
| 00:30 | 00 → 12, AM | 12:30 AM |
| 08:00 | keep, AM | 8:00 AM |
| 13:00 | −12, PM | 1:00 PM |
| 18:45 | −12, PM | 6:45 PM |
| 23:10 | −12, PM | 11:10 PM |
Which Countries Use Which?
The 24-hour clock is the written standard across most of Europe, Latin America, and Asia, and in aviation, military, medicine, and computing everywhere. The 12-hour clock dominates everyday speech in the US, Canada, India, Australia, and the UK. Knowing both — and converting cleanly — avoids missed meetings and double-booked calls.
Related Tools
- Time Format Converter — switch a time between 12-hour and 24-hour formats
- Timezone Converter — convert a time across timezones, DST-aware
- Time Duration Calculator — hours and minutes between two times