How to calculate time
- Choose whether you are adding durations or measuring between two clock times.
- For durations, enter hours, minutes and seconds for each and pick add or subtract.
- For clock times, enter the start and end. A later start is treated as crossing midnight.
- Read the result in hours, minutes and seconds.
- Use the decimal hours figure for timesheets, which almost always want it in that form.
What this calculator does
- Adds and subtracts durations without manual carrying between units.
- Measures between two clock times, including periods running past midnight.
- Reports decimal hours, which is the format most timesheets and invoices require.
- Totals in minutes and seconds for anything measured finely.
- Negative results shown clearly rather than as a confusing wrap-around.
- Instant recalculation as you change any field.
Why time arithmetic goes wrong on paper
Time is base 60, and people are not. Adding 2 hours 45 minutes to 1 hour 20 minutes means carrying at 60 rather than at 10, and the mistake, writing 3 hours 65 minutes, is easy to make and easy to miss.
Subtraction is worse, because borrowing works the same way in reverse. Taking 45 minutes from 2 hours 20 minutes requires borrowing 60 minutes rather than 100, and doing that quickly under pressure is where most timesheet errors come from.
Decimal hours are what timesheets want
Payroll systems, invoicing software and most contracts express time as decimal hours rather than hours and minutes. The conversion is not intuitive: 1 hour 30 minutes is 1.5 hours, but 1 hour 20 minutes is 1.33 hours, not 1.2.
Getting that wrong is a genuine and common billing error. Twenty minutes is a third of an hour, so it is 0.33, writing 1.2 undercharges by roughly eight minutes on every entry, which compounds across a month. The decimal figure here is calculated directly so there is nothing to convert by hand.
Shifts that cross midnight
A shift from 22:30 to 06:15 is seven hours 45 minutes, not minus sixteen hours. Naive subtraction gives the wrong answer because it assumes both times fall on the same day.
When the end time is earlier than the start, this calculator treats it as the following day, which is what night shifts, overnight journeys and long processes actually mean. If you genuinely wanted a negative interval, swap the fields round.
Where this gets used
- Timesheets and freelance invoicing, where decimal hours are required.
- Shift patterns, particularly overnight and rotating shifts.
- Cooking and process timing, where several durations stack up.
- Travel planning, adding journey legs and layovers together.
- Working out overtime by subtracting contracted hours from hours worked.
What this does not handle
Durations only, not calendar dates. A period longer than 24 hours is reported as a total number of hours rather than as days and hours, which is usually what you want for billing and awkward for scheduling. Time zones are not involved at all. Both times are assumed to be in the same zone.
For intervals spanning multiple days, the date calculator works in days and working days. For converting Unix timestamps into readable times, the timestamp converter handles that. Everything runs in your browser.
Rounding on timesheets
Many organisations round recorded time to the nearest six or fifteen minutes. Six-minute rounding is common in professional services because it divides the hour into ten, so each unit is exactly 0.1 hours and the decimal conversion is trivial. Fifteen-minute rounding gives quarters, 0.25, 0.5, 0.75, which are easy to read and coarser.
The thing to check is whether rounding is to the nearest unit or always upward, because the two produce noticeably different totals over a month. Rounding up every entry on a day with eight separate tasks adds up faster than people expect. This calculator gives the exact figure so you can apply whatever rounding rule your contract specifies rather than having one imposed on you.