What this calculator does
Adding times in hours, minutes and seconds trips people up because the carrying is in sixties rather than tens. The reliable method is to convert both durations to seconds, do the arithmetic there, then divide back into hours, minutes and seconds at the end. That is what this does.
The decimal hours line is the one most often needed. Timesheets, invoices and payroll systems almost always want hours as a decimal rather than as hours and minutes, and the conversion is where errors creep in: 50 minutes is 0.833 hours, not 0.50, and writing the minutes after the point is the single most common timesheet mistake.
The formula
Both durations are converted to seconds, combined according to the operation chosen, and converted back. A negative result is shown with a minus sign rather than being floored at zero, so subtracting a longer duration from a shorter one gives a meaningful answer. The decimal hours figure divides the total seconds by 3,600.
| Term | Meaning |
|---|---|
| Duration | A length of time, as opposed to a clock time. Durations can exceed 24 hours and can be negative. |
| Decimal hours | Hours with the minutes expressed as a fraction, so 1 h 30 m is 1.5 rather than 1.30. |
| Sexagesimal | Counting in sixties, which is why 60 minutes carry into an hour rather than 100. |
The inputs explained
| Field | What to enter |
|---|---|
| Hours | Hours of the first duration. |
| Minutes | Minutes of the first duration. Values over 59 are fine and carry into hours. |
| Seconds | Seconds of the first duration. |
| Operation | Whether to add the second duration to the first or subtract it. |
| Hours | Hours of the second duration. |
| Minutes | Minutes of the second duration. |
| Seconds | Seconds of the second duration. |
When to use it
Totalling a timesheet
Shifts recorded in hours and minutes have to be added and then converted to decimal hours for payroll. Doing both in one step avoids the usual error of treating the minutes as hundredths.
Adding split times or lap times
Sport and music both work in minutes and seconds, where carrying at sixty rather than a hundred is easy to get wrong by hand over several entries.
Finding the gap between two durations
Subtracting gives the difference, and a negative result is reported as negative rather than as an error, which is what you want when comparing an actual against a target.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What are common minute values in decimal hours?
A single duration of a given number of minutes, converted to decimal hours and seconds.
| Minutes | Result | As a decimal clock | In seconds |
|---|---|---|---|
| 15 minutes | 0 h 15 m 0 s | 0.250 h | 900 |
| 30 minutes | 0 h 30 m 0 s | 0.500 h | 1,800 |
| 45 minutes | 0 h 45 m 0 s | 0.750 h | 2,700 |
| 50 minutes | 0 h 50 m 0 s | 0.833 h | 3,000 |
| 90 minutes | 1 h 30 m 0 s | 1.500 h | 5,400 |
Questions
Why is 50 minutes 0.833 hours rather than 0.50?
Because there are 60 minutes in an hour, not 100. To convert minutes to decimal hours, divide by 60: 50 divided by 60 is 0.8333. Writing the minutes straight after the decimal point treats the hour as if it had 100 minutes, and on a 50-minute entry it understates the time by a third.
Can I enter more than 59 minutes or seconds?
Yes. Values above 59 carry correctly, so entering 90 minutes gives 1 hour 30 minutes. This is useful when a duration was recorded in one unit and you would rather not convert it first.
What happens if the result is negative?
It is shown with a minus sign. Subtracting 3 hours from 1 hour gives negative 2 hours rather than zero or an error, which is what you want when measuring an actual against a target or a deficit against a budget.
Is this the same as adding clock times?
No. This adds durations, which can exceed 24 hours and have no notion of a day. Adding clock times wraps around midnight and produces a time of day rather than a length of time.
For hours worked in a shift and what they are worth, see the timesheet and pay calculator. For the gap between two dates rather than two durations, see the days between dates calculator.