How the time calculator works
Last updated 10 November 2026
Adding time is awkward because hours and minutes do not work in tens. Sixty seconds make a minute and sixty minutes make an hour, so a column of numbers has to carry at sixty instead of ten. This calculator does the carrying for you, for as many lines as you need, and shows the running total after every line.
It adds and subtracts lengths of time such as a shift, a task or a journey. It does not work with clock times, so it will not tell you what time it is after a certain number of hours.
How it works
Every line is first turned into seconds: hours times 3 600, plus minutes times 60, plus seconds. The lines are then added or subtracted in order, starting with the first. The total in seconds is turned back into hours, minutes and seconds by dividing by 3 600 and then by 60 and keeping the remainders.
You can type more than 59 in any box. Ninety minutes is accepted and treated as 1 hour 30 minutes.
Worked example
A week of daily hours for a timesheet:
- Monday to Friday: 8 h 15 min, 7 h 50 min, 8 h 40 min, 6 h 55 min, 9 h 5 min.
- Added together that is 40 h 45 min, written 40:45:00.
- As decimal hours that is 40.7500, which is the figure to multiply by an hourly rate.
Carrying over 60
Seconds that pass 60 move into the minutes, and minutes that pass 60 move into the hours. For example, 50 min 40 s plus 25 min 35 s is 1 h 16 min 15 s. The 75 seconds became 1 minute 15 seconds, and the extra minute carried over to give 1 hour 16 minutes.
Taking time away
Choose Subtract on a line to take its time off the running total. A 9 h 30 min day with a 45 min unpaid lunch leaves 8 h 45 min.
If you subtract more than you have, the answer goes below zero and shows a minus sign. One hour minus 1 h 15 min is -0:15:00, which is 15 minutes short.
Decimal hours and hours and minutes
Many payroll and invoicing systems want hours as a decimal. To convert, divide the minutes by 60 and add them to the hours. 1 h 45 min is 1.75 hours, because 45 divided by 60 is 0.75. 7 h 30 min is 7.50 hours.
Going the other way, multiply the decimal part by 60. 7.75 hours is 7 h 45 min, and 6.4 hours is 6 h 24 min, because 0.4 of an hour is 24 minutes. Decimals that do not divide neatly, such as a third of an hour, are rounded to the nearest second.
Using it for timesheets
Enter each day or task as a line, read the total as hours and minutes for a human reader, and use the decimal figure for pay or billing. Keep a note of how your employer or client rounds time, because some round to the nearest quarter hour and some do not. This tool does not round for you.
- Add each day as its own line so you can check the running total.
- Subtract unpaid breaks on separate lines.
- Copy the decimal hours into your invoice or timesheet.
Related tools
To count the days between two dates, use the date difference calculator. To count only working days, use the working days calculator. For a clock that counts live, try the timer or the stopwatch.
Privacy
The calculation runs in your browser. Nothing you enter is stored, sent anywhere or added to the web address.
Things to watch
- The calculator adds lengths of time. It does not know about time zones or clock changes.
- Check how your employer rounds time before you submit a timesheet.
Like every tool on MoneyMzansi, this one works in your browser with no account and no upload.