How Time Arithmetic Works
Time uses base 60, a Babylonian inheritance, so adding durations means carrying at 60 instead of 10. The reliable method — the one this calculator uses — is to convert everything to seconds, do ordinary arithmetic, and convert back.
decimal hours = total seconds ÷ 3600
A shift from tasks of 3 h 50 min and 4 h 25 min: (13,800 + 15,900) seconds = 29,700 s = 8 h 15 min = 8.25 decimal hours — the number that goes on the invoice.
Minutes to Decimal Hours
| Minutes | Decimal hours | Minutes | Decimal hours |
|---|---|---|---|
| 5 | 0.08 | 35 | 0.58 |
| 10 | 0.17 | 40 | 0.67 |
| 15 | 0.25 | 45 | 0.75 |
| 20 | 0.33 | 50 | 0.83 |
| 30 | 0.50 | 60 | 1.00 |
Frequently Asked Questions
How do I add hours and minutes?
Add the parts separately, then carry: 60 minutes becomes 1 hour, 60 seconds becomes 1 minute. 2 h 45 min + 1 h 30 min = 3 h 75 min = 4 h 15 min.
How do I convert time to decimal hours for a timesheet?
Divide the minutes by 60 and add to the hours: 7 h 45 min = 7 + 45/60 = 7.75 hours. Payroll systems multiply this decimal by the hourly rate.
Why is 7 hours 30 minutes not 7.3 hours?
Because minutes are sixtieths, not tenths. 30 minutes is half an hour — 0.5 — so 7 h 30 min is 7.5 hours. Mixing up clock notation and decimals is the most common timesheet error.
What happens if I subtract a bigger time from a smaller one?
You get a negative duration, which the calculator shows explicitly (e.g. −1 h 15 min) so you can see the shortfall — useful for comparing planned vs. actual hours.
Cite This Calculator
Using this calculator in an assignment, article or lesson? Copy a ready-made citation — or the link snippet if you're referencing it from a web page.
The CalculatorsGuide Editorial Team. (2026). Time Calculator. CalculatorsGuide. Retrieved from https://www.calculatorsguide.com/everyday/time-calculator/