How this calculator works
Times are converted to seconds, added or subtracted, and converted back. That avoids the classic mistake of adding minutes as if they were decimals — 1 h 45 min plus 30 min is 2 h 15 min, not 2.75 h. Decimal hours (for timesheets and billing) are the total minutes divided by 60.
For clock times, the calculator wraps around midnight and tells you if the answer lands on the next or previous day. For the time between two clock times, an end time earlier than the start is read as the next day.
Assumptions and tips
- A bare number in the duration list is read as minutes.
- Clock-time mode ignores daylight-saving changes.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| What do you need? | Add or subtract durations |
| Result | Value |
|---|---|
| Total time | 4 h 40 min 30 s |
| As h:mm:ss | 4:40:30 |
| Decimal hours | 4.675 hours |
| Total minutes | 280.5 min |
| Total seconds | 16,830 s |
| Lines added | 4 |
Step by step:
- Convert each line to seconds: hours × 3,600 + minutes × 60 + seconds
- Add the lines (lines starting with − are subtracted)
- Convert back: ÷ 3,600 for hours, remainder ÷ 60 for minutes
- Decimal hours = total seconds ÷ 3,600
Common mistakes to avoid
- Adding times like decimals — 1:45 + 0:30 is 2:15, not 1:75 or 2.15.
- Writing 7 h 30 min as 7.3 hours on a timesheet; it is 7.5.
Frequently asked questions
How do I add 1 hour 45 minutes and 2 hours 30 minutes?
Add hours and minutes separately: 3 h 75 min = 4 h 15 min. Enter each on its own line above.
What time will it be 8 hours 45 minutes after 9:30 AM?
6:15 PM, the same day.
How do I convert 7 h 30 min to decimal hours?
Divide the minutes by 60: 30 ÷ 60 = 0.5, so 7.5 hours.
How many hours between 9:30 AM and 5:15 PM?
7 hours 45 minutes, or 7.75 decimal hours.
Built and reviewed by Anirban Kundu, Creator & Editor · M.Sc Physics, 20 years teaching science