How this instrument works
The second is the SI base unit of time, and since 1967 it has been defined by an atom rather than by the sky: one second is 9 192 631 770 periods of the radiation from the hyperfine transition in the ground state of caesium-133. The hour is not an SI unit at all. The BIPM lists it among the non-SI units accepted for use with the SI, defined administratively as 60 minutes, each minute as 60 seconds — so an hour is exactly 3600 seconds, and no measurement enters into it.
That factor is sexagesimal inheritance. Babylonian astronomers reckoned in base 60; Egyptian decan stars gave the day its twelve-part halves. Medieval Latin then split the hour into pars minuta prima and pars minuta secunda — the first small part and the second small part — which is literally why we say minute and second. Sixty times sixty is 3600, and that number has never been revised, even while the second itself moved from 1⁄86400 of a mean solar day, to a fraction of the tropical year 1900, to caesium.
Hours are the human accounting unit: shifts, flight times, billable work, energy meters. Seconds are the machine unit: cache lifetimes, token expiry, PLC timers, telemetry stamps, Unix epoch counts. Almost every conversion between them happens at that boundary, when a duration somebody quoted has to be handed to something that only counts ticks. The same 3600 is why a kilowatt-hour is exactly 3.6 megajoules.
- Type your duration into the Hours (h) field — 2 is preloaded, and the result recalculates as you type.
- Use decimal hours, not clock notation: half an hour is 0.5, forty-five minutes is 0.75, ninety minutes is 1.5.
- Read the Seconds (s) line, carried to six decimal places so short intervals survive without truncation.
- Working backwards, divide first: 27000 s ÷ 3600 = 7.5, and typing 7.5 here returns the figure you started from.
Worked example — a two-hour render budget
A video encode has a two-hour ceiling and the job scheduler wants that limit in seconds. Enter 2 in the Hours (h) field and the Seconds (s) line reads 7200.0 — sixty minutes twice over, each of sixty seconds. Paste 7200 into the timeout and the two figures describe the identical interval, with nothing lost in translation.
The arithmetic scales without drift because the factor is definitional rather than empirical. An eight-hour shift is 28800 s, a 90-minute film 5400 s, a nominal day 86400 s — though that last one is a calendar convention rather than a guarantee, since a UTC day carrying a leap second runs to 86401.
Questions
Is an hour exactly 3600 seconds?
Yes — exactly, by definition rather than by measurement. An hour is defined as 60 minutes and a minute as 60 seconds, so the factor carries zero uncertainty. Nothing here was ever determined in a laboratory; the numbers are administrative. The BIPM lists the minute, hour and day among the non-SI units accepted for use with the SI precisely because their relationship to the second is fixed and exact.
Why 3600 rather than a round metric number?
Because these divisions predate the metric system by roughly three and a half millennia. Babylonian astronomy counted in base 60, a number with twelve divisors, so thirds, quarters, fifths and sixths of an hour all land on whole minutes. Revolutionary France did try decimal time — a ten-hour day of 100 minutes each — and it survived about seventeen months. Sixty endured because it divides so agreeably.
How do I enter 2 hours 30 minutes?
Turn the minutes into a decimal fraction first: 30 ÷ 60 = 0.5, so enter 2.5 and the result is 9000 s. Typing 2.30 instead is the classic payroll error — it means two and three tenths of an hour and yields 8280 s, some 720 s short. Clock notation and decimal hours look almost identical on paper and are different numbers; only the decimal form belongs in this field.
Does every day contain 86400 seconds?
Not in UTC. A nominal day is 24 × 3600 = 86400 s, but leap seconds are inserted to keep civil time within 0.9 s of Earth's rotation, and a day carrying one runs to 86401 s. Twenty-seven have been added since 1972, and the CGPM resolved in 2022 to end the practice by 2035. None of that touches the factor used here, which stays 3600; it changes only how many seconds separate two calendar dates.
Why is a kilowatt-hour equal to 3.6 megajoules?
Because a joule is a watt-second, and this very factor does the work: 1000 W × 3600 s = 3600000 J = 3.6 MJ. Utility meters bill in kilowatt-hours while physics calculates in joules, so the 3600 reappears whenever a meter reading meets a thermodynamic sum. Multiply the seconds figure by your power in watts and you have the energy in joules directly.
Is a sidereal hour the same length?
No. The hour used here is a fixed 3600 SI seconds. A sidereal day — one rotation measured against the fixed stars instead of the Sun — runs about 86164.09 s, which makes a sidereal hour roughly 3590.17 s, about ten seconds shorter. Astronomers keep sidereal time for telescope pointing; this instrument uses ordinary civil hours, which is what clocks, contracts and schedulers mean.