12 Hour to 24 Hour Converter Calculator

What a 12 hour to 24 hour conversion changes

The 12 hour clock names each hour twice, once before noon and once after, and leans on the AM and PM tags to tell the two apart. The 24 hour clock gives every hour of the day its own number, running from 00 at midnight through 23 for the last hour before the next midnight, so no tag is needed. A conversion between them only touches the hour number. Minutes and seconds pass through untouched, which is why 7:45 PM becomes 19:45 rather than 19:45 plus some adjustment.

That single fact removes most of the confusion. You are not recalculating a time, you are relabeling the same instant. The 24 hour form is a different name for the same moment, the way 19:45 and 7:45 PM describe one point on a Tuesday evening. Once the hour is right, paste the minutes back and the job is done.

The page below converts in both directions and shows the arithmetic for the hour. It also flags the two inputs that trip people up, noon and midnight, because those are the only places where the simple add 12 or keep the number rules bend.

The two rules, and the two exceptions

For AM times, with one exception, the 24 hour hour equals the 12 hour hour, padded to two digits: 9:20 AM is 09:20 and 1:05 AM is 01:05. The exception is 12 AM, which is midnight and becomes 00, so 12:30 AM is 00:30.

For PM times, with one exception, add 12 to the hour: 3:10 PM becomes 15:10, 7:45 PM becomes 19:45, and 11:59 PM becomes 23:59. The exception is 12 PM, which is noon and stays 12, so 12:05 PM is 12:05 and not 24:05.

The reverse trip uses the same logic backwards. Hours 00 through 11 are AM, with 00 written as 12 AM; hour 12 is noon and written 12 PM; hours 13 through 23 are PM after subtracting 12. So 23:45 is 11:45 PM, 00:15 is 12:15 AM, and 12:00 is 12:00 PM.

Noon and midnight: what the standards say

Midnight and noon are the only two times where the labels are shaky, and the shaky part is worth knowing before you sign a lease or catch a flight. NIST's Times of Day FAQs states the problem plainly: a.m. means before noon and p.m. means after noon, noon is neither, and midnight is both twelve hours before and twelve hours after noon, so 12 a.m. and 12 p.m. are ambiguous and should not be used. NIST suggests writing 11:59 p.m. or 12:01 a.m. with the intended date when precision matters.

Everyday convention still assigns the labels: 12:00 AM means midnight and 12:00 PM means noon, which is what the converter on this page follows. That convention is what lets 12:30 AM become 00:30 and 12:30 PM become 12:30. But the NIST caution explains why careful writers use noon and midnight in words, and why the 24 hour clock is the safer choice in schedules, contracts and systems that run around the clock.

The 24 hour clock sidesteps the argument entirely: 00:00 is the first instant of a day and 12:00 is the middle of it. time.gov, the official US time service run by NIST and the US Naval Observatory, displays its clock reading in 24 hour form for exactly this reason. Some timetables do write 24:00 to mark the end of a day, but clocks and most software roll over to 00:00 of the next date, and this converter accepts hours 0 through 23 only.

Three worked conversions

Example 1, evening time. Take 7:45 PM. The hour is PM and not 12, so add 12: 7 + 12 = 19. Keep the minutes at 45. Result, 19:45. Read aloud in 24 hour style that is nineteen forty-five.

Example 2, just after midnight. Take 12:30 AM. The hour is 12 and the tag is AM, which is the midnight exception, so the hour becomes 0 rather than 12. Keep the minutes at 30 and pad the hour. Result, 00:30, which is thirty minutes into the new day.

Example 3, just after noon. Take 12:05 PM. The hour is 12 and the tag is PM, the noon exception, so no 12 is added and the hour stays 12. Result, 12:05, five minutes after midday. Add 12 here by reflex and you would write 24:05, a time that does not exist on a 24 hour clock.

A duration example ties the two clocks together. A shift starts at 9:30 AM and ends at 5:15 PM. In 24 hour time that is 09:30 to 17:15. From 09:30 to 17:30 would be 8 hours, so subtract the 15 minute difference: the shift lasts 7 hours 45 minutes, or 465 minutes. The same span comes out of the 12 hour clock, since both endpoints name the same instants, which is the practical argument for converting before you subtract.

Common mistakes, and how the converter catches them

The mistakes cluster in four places, and each one is easy to spot once you know the shape. First, adding 12 to 12 PM and producing 24:05, a time outside the 0 to 23 range. Second, treating 12 AM as noon and writing 12:00 instead of 00:00, which moves an appointment by half a day. Third, dropping the leading zero and writing 9:05 for what should be 09:05, which breaks sorting in logs and spreadsheets because 9 sorts after 10 as text. Fourth, converting the minutes as well as the hour, when minutes never change.

The converter on this page validates the hour ranges and reports a plain message instead of a wrong number, so an out of range value shows up as an error rather than a plausible looking time. It also prints the hour arithmetic in words, which makes the noon and midnight exceptions visible instead of hidden inside the result.

Where the 24 hour clock is the working standard

Aviation, shipping, railways and military planning all run on the 24 hour clock because a schedule that spans midnight cannot afford an ambiguous 12. Hospital charts and lab systems use it for the same reason: a medication time of 00:30 is unambiguous, while 12:30 AM invites a double check. Server logs, database timestamps and the ISO 8601 date format write hours from 00 to 23, which is why anyone reading logs learns the format whether they want to or not.

The United States still uses the 12 hour clock in daily life, and most US style guides prefer it in prose, so the conversion runs in both directions many times a day: a phone showing 7:45 PM against a ticket that says 19:45, a work roster in 24 hour form against a calendar in 12. Once the two exceptions are memorized, the conversion takes a second and needs no tool, but the tool helps on the days when 12:00 shows up and the brain stalls.

For times that cross a date boundary, convert first and then do the arithmetic. A flight leaving at 11:40 PM and landing at 1:25 AM next day is 23:40 to 01:25, a span of 1 hour 45 minutes once you add the 24 hours that the second leg borrows. In 12 hour form the same subtraction invites an off by 12 hours error.

How to use the converter

Type the 12 hour time in the left column: hour from 1 to 12, minutes from 0 to 59, and the AM or PM tag. The 24 hour result appears immediately, along with the sentence describing how the hour was found, which is the part worth reading while you learn the rules. Type a 24 hour time in the right column and the 12 hour form appears with the AM or PM tag attached.

Limits are deliberate. Hours on the 24 hour side stop at 23, since 24:00 is a bookkeeping convention for the end of a day rather than a time a clock displays. Seconds are not part of the conversion; the same rules apply if you append them unchanged. Dates are not handled, so a time near midnight does not tell you which day it belongs to, and that is the one thing the 24 hour clock cannot fix for you.