Time Calculator
Time arithmetic is base 60, not base 10, which is why adding it in a spreadsheet so often goes wrong. This calculator handles durations and conversions correctly.
Formula
Convert both times to a single unit, add or subtract, then convert back
- Hours are base 60 for minutes and seconds, not base 100
- Decimal hours × 60 gives minutes: 0.25 h is 15 minutes
7h 45m + 3h 30m = 11h 15m, because 75 minutes carries an hour
Why Time Does Not Add Normally
Sixty minutes make an hour, not a hundred, so carrying works differently.
2 hours 45 minutes plus 1 hour 30 minutes is 3 hours 75 minutes, which becomes 4 hours 15 minutes.
The reliable method is to convert everything to the smallest unit first — minutes or seconds — do the arithmetic, then convert back.
Decimal Hours
Payroll and timesheets use decimal hours, where 7 hours 30 minutes is 7.5. The conversion is minutes divided by 60:
- 15 minutes = 0.25
- 20 minutes = 0.333
- 45 minutes = 0.75
- 50 minutes = 0.833
The classic error is reading 7.45 as seven hours forty-five minutes. It is 7 hours 27 minutes, and the difference across a month of timesheets is substantial.
Crossing Midnight and Time Zones
A shift from 22:00 to 06:00 is eight hours, but naive subtraction gives minus sixteen. Add 24 hours when the end time is earlier than the start.
Daylight saving adds another trap: in the UK, the clocks going forward in March makes one day 23 hours long, and the October change makes one 25 hours. Shift calculations spanning those nights need adjusting, and a "24 hours later" appointment may not fall at the same clock time.
Quick Reference
| Decimal | Hours and minutes | Minutes |
|---|---|---|
| 0.25 | 0h 15m | 15 |
| 0.5 | 0h 30m | 30 |
| 0.75 | 0h 45m | 45 |
| 1.25 | 1h 15m | 75 |
| 7.5 | 7h 30m | 450 |
| 8.25 | 8h 15m | 495 |
Frequently Asked Questions
- How do I add times together?
- Convert both to minutes, add them, then convert back. Time is base 60, so 45 minutes plus 30 minutes gives 75, which becomes 1 hour 15.
- What is 7.45 in hours and minutes?
- 7 hours 27 minutes. Decimal hours are fractions of 60, so 0.45 is 27 minutes, not 45.
- How do I calculate hours across midnight?
- Add 24 hours to the end time before subtracting. A 22:00 to 06:00 shift is eight hours, not minus sixteen.
- Does daylight saving affect duration calculations?
- Yes. The UK spring change makes a day 23 hours and the autumn change 25, so shifts spanning those nights need adjusting.