One day is exactly 24 hours by definition. The Earth's actual rotation is not quite that regular, which is why the definition is a convention rather than a measurement — the day used in timekeeping is fixed at 86,400 seconds regardless of what the planet does.
About the units
The twenty-four hour day comes from ancient Egypt, where the night was divided into twelve parts by the rising of twelve stars called decans and the day was given twelve to match. The two sets of twelve were of unequal length for most of history, since daylight hours were longer in summer, and only became equal when mechanical clocks made uniform hours practical in the fourteenth century. The mean solar day — the average time for the Sun to return to the same position — stays within a couple of milliseconds of 86,400 seconds and wanders either side of it. Tidal friction lengthens it by roughly 1.8 ms per century over the long run, but faster changes in the Earth's core and atmosphere dominate on human timescales: the planet has been running slightly fast since around 2020, the shortest days ever measured fell in 2025, and a negative leap second is now under discussion. The sidereal day, measured against the stars rather than the Sun, is about four minutes shorter at 23 hours 56 minutes.
The exact factor
The factor is exactly 24, and a day is exactly 86,400 seconds in every context except one: a day containing a leap second has 86,401. Related figures worth having: a week is 168 hours, a 30-day month is 720 hours, and a common year is 8,760 hours against a leap year's 8,784. Those last two matter in energy calculations, where annual consumption is often computed as an average power multiplied by 8,760.
Where you meet this conversion
Project planning, shift scheduling and any calculation of availability. Uptime commitments are expressed as percentages of a year, so 99.9 per cent allows 8.76 hours of downtime annually and 99.99 per cent allows 52.6 minutes. Battery life, medication intervals and travel durations all cross the boundary. In energy work, a kilowatt sustained for a day is 24 kilowatt-hours, which is the conversion behind every daily consumption estimate.