Half an hour

How it works

How it works is a cut, then a spelling. An hour holds 60 minutes. Half of 60 is 30. An hour holds 3,600 seconds. Half of 3,600 is 1,800. Those two halves are one duration. The formats are several. 30 min, 0.5 h, 0:30 when the last field is minutes, 00:30:00 as hh:mm:ss of elapsed time, and PT30M are the same length in different clothes.

The cut is exact in civil time. There is no 50-minute hour hiding in the phrase. Decimal 0.5 is 30 minutes, not 50. Colon 0:30 is 30 minutes only when the minute field is minutes. Colon 0:30 as mm:ss is 30 seconds, which is 1/60 of this page. ISO PT0.5H is the decimal hour inside a duration string; PT30M is the minute form. Prefer PT30M if a parser is picky about fractions.

Write the unit once per task and do not switch mid-list. A recipe that says “half an hour” and a log that says “0.5” and a timer that says “30:00” can still agree if someone named the fields. They fight when 30:00 is read as thirty hours, or when 0.5 is dropped into a minute column and becomes half a minute.

Prose: half an hour, or 30 minutes. Decimal hours: 0.5 h. Minute-second elapsed: 30:00 if the first field is minutes. Hour-minute-second elapsed: 00:30:00. ISO duration: PT30M. Half an hour after 3:00 is 3:30 on a civil clock that is not crossing a daylight-saving fold in that half hour. That is addition of a duration onto a time of day. The duration is still 30 minutes. The labels 3:00 and 3:30 are clock faces, not proof that “half” means 30 on every face: 3:00 as a duration is three hours.

Half hour, half of an hour, and the other spellings

Half hour drops the article. Half of an hour puts “of” in. Half and hour is a missed word. Hour half is the two roots swapped. The search strings are sloppy. The length is not. Each of them is 30 minutes unless the writer clearly meant half of some other hour-sized thing, such as a 45-minute class period. A “half hour” TV slot is a scheduling label; the programme inside it is still measured in minutes, and the slot is still built as 30 minutes of clock unless that network defined a different slot. This page treats the duration, not a broadcast grid.

Hyphens are format, not time. Half-hour as a modifier (“a half-hour meeting”) is the same 30 minutes as “half an hour” in running text. Hour-and-a-half is the other direction: 90 minutes, 1.5 h, PT1H30M. Do not file that under this heading. Half an hour is the 30-minute cut. An hour and a half is three of those cuts.

0.30 hours is 18 minutes, as on the decimal-minutes page next door in spirit: copying “30” after a point does not perform a half. 30 hours is sixty times too long. 30 seconds is sixty times too short. The word “half” is doing the 1/2. The word “hour” is doing the 60 minutes. Lose either word and you have to put the numbers back by hand.

Clock face versus duration

6:30 can be half past six, a time of day. It can also be six hours thirty minutes of elapsed time. Half an hour is neither of those until you say so. “Meet in half an hour” adds 30 minutes to now. “The meeting is half an hour” names a duration. “The meeting is at half past” names a face. Three sentences, one pile of English, three jobs.

Half an hour is thirty minutes in every spelling, including the ones that dropped the “an” or put “hour” first. A daylight-saving spring-forward can skip a labelled local hour. If your “half an hour from now” lands inside a skipped civil hour, local clock faces jump. The SI duration is still 1,800 seconds. For ordinary kitchen and meeting use, add 30 minutes on the local clock you are actually watching, and if the jump is tonight, look at a zone-aware clock instead of doing it in your head.