Hour (h)

Metric (SI)Current unit

60 minutes or 3,600 seconds, one twenty-fourth of a mean solar day.

1 hour = 3,600 seconds (exactly).

About the hour

Egyptian astronomers gave us the twelve. Coffin lids from about 2150 BC show the night divided into exactly 12 intervals, tied to the rising of star groups called decans, and the day was later split into 12 hours with sundials. These were seasonal hours, a twelfth of the daylight, so a summer hour was longer than a winter one. The word itself comes from Greek hora, originally any span of time, through Latin and Old French into English in the 13th century.

Equal hours of 1/24 of a day were probably first proposed by Hipparchus, and mechanical clocks made them the practical choice, since a clock showing seasonal hours had to be readjusted every morning and evening. Today the hour is exactly 3,600 seconds and is a non-SI unit accepted for use with the SI. Civil time still bends it a little: UTC hours have lasted 3,601 seconds 27 times since 1972 to keep clocks within 0.9 seconds of the Earth's rotation. In practice we use it for working shifts, travel times and electricity bills measured in kilowatt-hours.

Sources: Wikipedia: Hour; BIPM: SI Brochure, 9th edition

Hour conversion chart

1 hour expressed in every other time unit:

More Metric (SI) units

• second (time)
• millisecond (time)
• microsecond (time)
• nanosecond (time)
• minute (time)
• day (time)
• meter (length)
• kilometer (length)
• centimeter (length)
• millimeter (length)
• micrometer (length)
• nanometer (length)
• picometer (length)
• decimeter (length)
• hectometer (length)
• decameter (length)

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