Why time arithmetic goes wrong
Time is base sixty, not base ten, and that mismatch is where mistakes happen. Three quarters of an hour is 45 minutes but 0.75 in decimal, and adding 1:45 to 2:30 gives 4:15 rather than the 3:75 that naive addition suggests.
Spreadsheets make this worse rather than better, because a cell can hold either a time value or a plain number and the two behave completely differently.
Decimal hours
Payroll and invoicing systems almost universally work in decimal hours, not hours and minutes. Eight hours thirty is 8.5, but seven hours twenty is 7.33 recurring, and rounding that inconsistently across a month produces discrepancies that are tedious to reconcile.
Every result here shows both formats for exactly that reason. If you are billing or being paid by the hour, decimal is the figure your system wants.
Shifts crossing midnight
A shift from 22:00 to 06:00 is eight hours, but subtracting the numbers gives minus sixteen. The overnight option adds a day to the end time so the calculation works. It is also applied automatically when the end time is earlier than the start, since that combination almost always means an overnight shift.
Breaks
Unpaid breaks are deducted from the total, which is what most timesheets require. Whether a break is paid varies by jurisdiction and contract — many places mandate a rest break after a certain number of continuous hours, and whether it is paid differs. Check your local rules rather than assuming.
Adding up multiple durations
The third calculator totals a list, which is the common need at the end of a week or month. It accepts both formats, so you can paste hours and minutes as h:mm or decimal values and mix them freely. The average is shown alongside the total, which is useful for spotting an outlier entry.
Where this gets used
Timesheets and billable hours are the most common. Beyond that: calculating shift lengths for rota planning, working out flight durations, timing how long processes take, adding up study or practice hours, and checking that a schedule adds up before committing to it.
A note on timezones
This works with clock times as entered and does not account for timezone changes or daylight saving transitions. A shift spanning a clock change is an hour longer or shorter than the arithmetic suggests, which matters for payroll twice a year.
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Frequently Asked Questions
How do I convert hours and minutes to decimal?
Divide the minutes by 60 and add to the hours. Thirty minutes is 0.5, twenty minutes is 0.33 recurring.
How do I calculate a shift that crosses midnight?
Tick the overnight option, or simply enter an end time earlier than the start — the calculator handles it automatically.
Should I deduct breaks?
Usually yes for unpaid breaks, which is what most timesheets require. Whether a break is paid varies by jurisdiction and contract.
Can I mix formats when adding durations?
Yes. The list accepts both h:mm and decimal hours, and you can mix them freely.
Does it account for daylight saving?
No. It works with clock times as entered. A shift spanning a clock change differs by an hour from the arithmetic.