One hour is exactly 3,600 seconds. The number is 60 × 60, and it appears throughout physics and engineering as the bridge between rates expressed per hour and quantities expressed in SI.
About the units
The second is the SI base unit, defined since 1967 by the caesium-133 hyperfine transition. The hour is a conventional multiple: 60 minutes of 60 seconds each, both inherited from Babylonian sexagesimal counting. Neither the hour nor the minute is an SI unit, but both are accepted for use with SI because abandoning them was never realistic. The 3,600 turns up constantly because most practical rates are quoted per hour — kilometres per hour, kilowatt-hours, litres per hour — while the underlying physics works in seconds.
The exact factor
The factor is exactly 3,600. Its most common use is converting rates rather than durations. A speed in kilometres per hour becomes metres per second by dividing by 3.6, which is 3,600 metres divided by 1,000. A power in kilowatts sustained for an hour gives 3,600 kilojoules, which is why a kilowatt-hour is exactly 3.6 megajoules. A flow of a litre per hour is 0.278 millilitres per second. Every one of those relationships is the same 3,600 in a different costume.
Where you meet this conversion
Physics and engineering calculations that start from an everyday figure. Energy is the biggest case: any consumption in kilowatt-hours converted to joules passes through 3,600. Fluid flow, heat transfer rates and ventilation are specified per hour and computed per second. In astronomy, right ascension is measured in hours, minutes and seconds of arc-time, where one hour of right ascension is 15 degrees — a different use of the same words that catches people out.