Hour (hr)

The hour is a unit of time equal to 60 minutes, or 3600 seconds. Its symbol is h, though hr is common in English-language informal use. Twenty-four hours make a day, and the unit is accepted for use with the SI without belonging to it.

Its length was once variable. Ancient Egyptian and later Greco-Roman practice divided daylight into twelve parts and darkness into twelve more, which meant an hour of daylight in summer was considerably longer than one in winter. These seasonal or temporal hours persisted in Europe into the medieval period and were still used in monastic timekeeping long after mechanical clocks appeared. Equal hours won out because a clock cannot easily be made to run at a rate that changes with the season.

Twenty-four is the count because twelve is the count. Egyptian astronomers used a set of thirty-six decan stars, of which twelve were visible across a night, and twelve was already a convenient number in a duodecimal counting tradition that divides cleanly by two, three, four and six. Doubling it to cover the day gave twenty-four, and no serious attempt to change it has succeeded since.

The hour organises modern working life more than any other unit. Wages, shifts, opening times, flight durations, parking charges and legal limits on driving are all expressed in hours, and the hour is the base of the kilowatt-hour, which is how virtually all electricity is billed. Timetables are built on hour boundaries, and time zones are defined as whole-hour offsets from Coordinated Universal Time, with a handful of exceptions offset by thirty or forty-five minutes.

In science the hour appears less often than the second but survives wherever a process runs long enough to make seconds unwieldy: reaction times in chemistry, dosing intervals in medicine, and half-lives in the range where hours are the natural scale.

Geography is divided by it. The Earth turns fifteen degrees of longitude in an hour, so the globe was cut into twenty-four zones an hour apart, a scheme adopted at the International Meridian Conference of 1884 after railways made local solar time unworkable. The tidiness is partial: several countries keep offsets of half an hour or forty-five minutes, and China spans five geographic zones on a single one. The hour also lends its name to a unit of energy. A kilowatt-hour is the energy delivered by a kilowatt for an hour, 3.6 megajoules, and it is the quantity on every electricity bill in the world.

One hour equals 60 minutes, 3600 seconds, or about 0.0416667 days.

Hour conversion table