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12-Hour to 24-Hour Time: Conversion Rules and Common Slips

By Reeliy Editorial Team · · 3 min read

The 24-hour clock numbers the hours from 0 to 23 and has no AM or PM. It is the standard in transport timetables, hospitals, aviation and the armed forces, and it is how most computers store time. Converting to and from the 12-hour clock is easy once you know where the two systems agree and where they split.

The only tricky moments are the hours around 12, which behave differently from all the others. This guide gives the rules, a worked set of examples, an explanation of midnight and noon, and a short section on how military time is written and spoken.

The two conversion rules

From 12-hour to 24-hour:

  • AM times: keep the hour, except 12 AM becomes 00. Add a leading zero for hours 1 to 9, so 7:05 AM is 07:05.
  • PM times: add 12 to the hour, except 12 PM stays 12. So 1:00 PM is 13:00 and 11:30 PM is 23:30.

From 24-hour to 12-hour, run the rules backwards: 00 becomes 12 AM, 01 to 11 become AM, 12 becomes 12 PM, and 13 to 23 lose 12 and become PM. The minutes never change.

12-hour clock24-hour clock
12:00 AM (midnight)00:00
12:20 AM00:20
7:05 AM07:05
11:59 AM11:59
12:00 PM (noon)12:00
12:45 PM12:45
1:00 PM13:00
6:15 PM18:15
9:30 PM21:30
11:59 PM23:59

Midnight and noon

AM and PM are short for ante meridiem and post meridiem, Latin for before and after midday. Noon itself is neither, which is why "12 AM" and "12 PM" cause so much confusion. The convention is that 12:00 AM is midnight and 12:00 PM is noon, but many people mix them up.

Two habits remove the doubt. Write "noon" or "midnight" in words when it matters, and use the 24-hour form for anything that has to be exact, such as a deadline or a train time.

A related problem is which date a midnight belongs to. 00:00 is the very start of a day, so a deadline of "midnight on Friday" could mean the start of Friday or the end of it. Saying "23:59 on Friday" or "00:00 on Saturday" settles it. Some schedules write 24:00 for the end of a day, but 00:00 is the safer way to write the start of the next one.

Worked examples

  • 7:45 PM is a PM time, so 7 + 12 = 19, giving 19:45.
  • 12:20 AM is just after midnight, so the hour becomes 00, giving 00:20.
  • 15:05 is above 12, so subtract 12 to get 3, giving 3:05 PM.
  • 00:50 has hour 0, which is 12 in the 12-hour clock, giving 12:50 AM.

A mental shortcut for 24-hour to 12-hour: for hours 13 to 23, subtract 10 and then subtract 2 more. For 18, 18 − 10 = 8, and 8 − 2 = 6, so 18:00 is 6 PM.

The 24-hour clock also makes time arithmetic simpler. To add 3 hours 45 minutes to 22:30, add the hours and minutes: 22:30 + 3:45 = 25:75. Minutes cannot exceed 59, so carry one hour to get 26:15. Hours cannot exceed 23, so subtract 24 to get 02:15 the next day.

Reading and saying military time

In the United States, "military time" is the 24-hour clock written as four digits with no colon and spoken in a particular way. The hours part is always two digits, so early times start with a zero.

WrittenSpoken
0000zero hundred
0600zero six hundred
0745zero seven forty-five
1200twelve hundred
1830eighteen thirty
2359twenty-three fifty-nine

Letters after the time often show the zone, with Z meaning UTC, sometimes called Zulu time. So 1830Z is 18:30 UTC.

Common slips

  • Adding 12 to 12 PM and getting 24:00. Noon stays 12:00.
  • Treating 12 AM as 12:00. It is 00:00, the start of the day.
  • Dropping the leading zero when sorting or storing times as text: "7:05" sorts after "13:00" as a string, but "07:05" does not.
  • Inventing times such as 24:30 or 13:75. Hours run 00 to 23 and minutes 00 to 59.

To convert in either direction without thinking, use the 24-hour to 12-hour converter or the military time converter. For adding or subtracting durations, the time calculator handles the carrying.

Try the tools
  • 24 Hour to 12 Hour Time Converter: Choose a direction, type a time such as 17:30 or 5:30 PM, and the converted time appears underneath. The chart below lists all 24 hours for a quick lookup.
  • Military Time Converter: Pick a direction, enter a time such as 2030 or 8:30 PM, and see it converted at once. The chart and rules below explain how every hour maps between the two systems.
  • Time Calculator: Use the clock calculator to see what time it will be after adding or removing hours, minutes and seconds, or the date and time calculator to move a whole timestamp forwards or backwards.

This article is general information, not professional advice. See our Disclaimer and Editorial Policy.

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