12-Hour to 24-Hour Clock Conversion: Rules, Table, and the Midnight Trap
October 21, 2026
To convert 12-hour time to 24-hour time, keep AM hours the same (except 12 AM, which becomes 00), and add 12 to PM hours (except 12 PM, which stays 12). So 9:30 AM is 09:30, 3:45 PM is 15:45, 12:15 AM is 00:15, and 12:15 PM is 12:15. To go back, subtract 12 from hours above 12 and add PM, and treat 00 as 12 AM.
Skip the math and get your answer instantly:
Open the Time Calculator →Conversion Table
The table shows key times through the day. Notice that the 24-hour clock avoids the need for AM and PM completely.
| 12-hour | 24-hour | Note |
|---|---|---|
| 12:00 AM | 00:00 | Midnight, start of the day |
| 1:00 AM | 01:00 | |
| 12:00 PM | 12:00 | Noon |
| 1:00 PM | 13:00 | Add 12 |
| 6:30 PM | 18:30 | Add 12 |
| 11:59 PM | 23:59 | Last minute of the day |
The Midnight and Noon Trap
The labels 12 AM and 12 PM confuse many people. Noon is 12 PM, because it is the first moment after the 'morning' (ante meridiem) ends. Midnight is 12 AM, because it is the start of the new day's morning hours. In writing, 'noon' and 'midnight' are clearer, and for midnight you should name the date, since 'midnight on Friday' can mean the start or end of Friday.
The 24-hour clock handles this unambiguously. 00:00 is the start of a day and 24:00 is sometimes used to mean the end of that same day, as in schedules, which is the same instant as 00:00 of the next day.
Where the 24-Hour Clock Is Used
Transport timetables, hospitals, the military, aviation, and computing use 24-hour time to prevent AM/PM errors. Much of Europe and Asia also uses it in everyday speech and writing, while the US and a few other countries mostly use the 12-hour clock.
When calculating durations, convert to 24-hour time first. From 10:45 PM to 6:20 AM, convert to 22:45 and 06:20, then compute: 6:20 + 24:00 − 22:45 = 7 hours 35 minutes.
Frequently Asked Questions
What is 12 AM in 24-hour time?
00:00, the start of the day.
Is 12 PM noon or midnight?
Noon. Midnight is 12 AM.
How do I convert 8:15 PM to 24-hour time?
Add 12 to the hour: 20:15.
What does 24:00 mean?
The end of a day, equivalent to 00:00 of the following day. It is used in some schedules but not in most systems.