Why time math trips people up
Clocks are base-60: sixty seconds to a minute, sixty minutes to an hour. Adding 2:45 and 1:30 by normal arithmetic gives the meaningless "3:75"; the correct move is to carry — 75 minutes is 1 hour 15 minutes, so the answer is 4:15. The calculator converts everything to seconds, does plain arithmetic, and converts back, which is also the trick to use in your head:
Decimal hours: the payroll trap
7 hours 30 minutes is 7.5 hours — not 7.30. Timesheets and invoices want minutes divided by 60:
| Minutes | Decimal | Minutes | Decimal |
|---|---|---|---|
| 5 min | 0.08 h | 35 min | 0.58 h |
| 10 min | 0.17 h | 40 min | 0.67 h |
| 15 min | 0.25 h | 45 min | 0.75 h |
| 20 min | 0.33 h | 50 min | 0.83 h |
| 30 min | 0.50 h | 60 min | 1.00 h |
Negative results
Subtracting a larger time from a smaller one gives a negative duration, shown with a minus sign — useful when computing how far behind schedule something ran. If you're timing across midnight (22:30 to 6:15), you want the hours calculator, which handles clock times rather than durations.
Frequently asked questions
How do I add hours and minutes correctly?
Add the parts separately, then carry: minutes over 59 become hours (subtract 60, add 1 hour), seconds over 59 become minutes. 1:45 + 2:30 → minutes 45+30=75 → carry → 4:15.
How do I convert minutes to decimal hours?
Divide by 60. 45 minutes = 45 ÷ 60 = 0.75 hours, so 7 h 45 m = 7.75 in a timesheet. Going the other way, multiply the decimal part by 60: 6.4 h = 6 h 24 m.
What's the difference between a duration and a clock time?
A duration is an amount of time (2 h 30 m of work); a clock time is a position in the day (14:30). This calculator handles durations. For 'from 9:00 to 17:30 minus lunch', use the hours calculator.
How do I subtract a longer time from a shorter one?
The calculator returns a negative duration — e.g. 1:00 − 1:30 = −0:30 — which reads as 'thirty minutes short'. In manual math, subtract the smaller from the larger and attach a minus sign.
How many seconds are in an hour or a day?
An hour has 3,600 seconds (60 × 60); a day has 86,400 (24 × 3,600). These constants are what the calculator uses under the hood for all conversions.