How this instrument works
Sixty came out of Babylonian arithmetic. Astronomers there counted in base sixty, Ptolemy carried that habit into his tables of angles, then medieval Latin translators called each sixtieth of an hour pars minuta prima — first small part — with each sixtieth of that becoming pars minuta secunda. English kept both, shortened, as minute and second. Hours run older still: Egyptian decan stars split night into twelve watches, daylight took twelve to match, so for centuries an hour stretched through summer, shrinking again through winter. Only escapement clocks, spreading across European towns from about 1300, made every hour equal.
Neither unit belongs to SI. Its base unit for time is the second, defined since 1967 by 9 192 631 770 cycles of caesium-133 hyperfine transition; BIPM then lists minute, hour and day in their own table of non-SI units accepted for use alongside SI, purely because civil life is built on them. Minutes are fixed at exactly 60 s, hours at exactly 3600 s, so 3600 ÷ 60 = 60 falls straight out — no measurement, no tolerance, nothing to round.
Whole industries live on decimal hours rather than clock notation. Payroll software stores the shift as 7.75; law firms bill in tenths, each worth six minutes; aircraft Hobbs meters tick over in tenths of engine time; ward nurses turn an infusion set at 125 mL/h into roughly 2.08 mL/min. Each of those handoffs is multiplication or division by sixty, and each fails in exactly the same way — somebody reads decimal points as colons.
- Type your duration into the Hours (h) field — 2 is preloaded, so you can watch the arithmetic before touching anything.
- Use decimals, never clock notation: half an hour is 0.5, three quarters is 0.75, twenty minutes is 0.333333.
- Read Minutes (min) underneath; it recomputes on every keystroke, carrying six decimal places.
- Working from stopwatch time of 7 h 45 min? Divide 45 by 60 first, then enter 7.75.
- Reversing direction: divide any Minutes (min) figure by 60 to recover hours.
Worked example — two-hour rehearsal booking
The rehearsal room takes its two-hour minimum up front, then charges by the minute after that. Enter 2 into Hours (h) and Minutes (min) returns 120.0. Nothing is lost or rounded along that trip, because 2 × 60.0 is closed arithmetic rather than any measured ratio.
Overrun changes only your input. Quarter-hour extra makes it 2.25 h, or 135 min; ten minutes extra makes it 2.166667 h, or 130 min. Read your stopwatch as 2:15, type 2.15 instead, though, and you bill 129 min — six minutes short, an error that survives audit precisely because it looks plausible.
Questions
Is an hour exactly 60 minutes?
Yes, by definition rather than measurement. BIPM fixes the minute at exactly 60 seconds, the hour at exactly 3600 seconds, so their ratio of exactly 60 follows with zero uncertainty. Even leap seconds leave that count alone: inserting 23:59:60 into UTC gives that day 86401 seconds of length, yet its final hour is still sixty minutes — just that one of them runs 61 seconds.
Why sixty minutes rather than one hundred?
Sixty has twelve divisors — 2, 3, 4, 5, 6, 10, 12, 15, 20, 30 among them — so halves, thirds, quarters, fifths, sixths of an hour all land on whole minutes. Base ten manages neither thirds nor sixths. France did legislate decimal time in 1793, with ten hours per day, 100 minutes per hour; watchmakers loathed it, almost nobody adopted it, so it was suspended by 1795.
How do I enter 7 hours 45 minutes?
Divide minutes by 60 first: 45 ÷ 60 = 0.75, so type 7.75, then read 465 min. Typing 7.45 returns 447 min instead — an 18-minute error, easily the most common timesheet mistake anywhere. Decimal notation agrees with clock notation only on whole hours; everywhere between, they part company.
Are hours and minutes SI units?
No. SI carries just one base unit for time, its second, while hours and minutes sit in their own table of non-SI units accepted for use with it, alongside days plus degrees of angle. Symbols are h and min, both upright, both preceded by single spaces after any figure, neither taking plural s — min covers one minute or ninety. Forms such as hr, hrs, mins pass in speech yet fail in technical writing.
How do I get back from minutes to hours?
Divide by 60: 450 min ÷ 60 = 7.5 h. For clock notation, keep whole hours aside, then multiply what remains by 60 again — 100 min gives 1.666667 h, where 0.666667 × 60 = 40, so 1 h 40 min. Rounding at each step is what makes long timesheets drift, so carry full decimals through to your final total.
Why do lawyers and pilots work in tenths of an hour?
Because 0.1 h is exactly six minutes, with every tenth after it landing on whole minutes too: 0.2 h is 12 min, 0.7 h is 42 min. Legal billing has run on those six-minute increments for decades; aircraft Hobbs meters record engine time in tenths for identical reasons. Quarters behave as well, 0.25 h giving 15 min, while thirds of an hour give exactly 20 min — though that third never terminates as decimal digits.