Time Duration Calculator: The Gap Between Two Times

Time is the one everyday measurement that is not decimal, and that is where the errors come from. Subtract 09:15 from 17:40 the way you would subtract money and you get 8.25, which is wrong by ten minutes. Do that across twenty timesheets and it adds up.

Why subtracting times by hand goes wrong

Minutes count in base 60, so borrowing works differently. To get from 08:50 to 14:20 you cannot just take 50 from 20. You borrow an hour, making it 80 minutes, then 80 minus 50 is 30, and 13 minus 8 is 5. Five hours 30 minutes.

Midnight is the other trap. A night shift from 22:45 to 06:20 produces a negative number if you subtract directly. Split it instead: 22:45 to midnight is 1 hour 15 minutes, then midnight to 06:20 is 6 hours 20 minutes, giving 7 hours 35 minutes. A duration calculator does that split for you as soon as the end time is earlier than the start.

Decimal hours, the version payroll wants

Converting the minutes

Payroll systems multiply a rate by a decimal number, so 7 hours 35 minutes has to become 7.58 hours. Divide the minutes by 60: 35 / 60 = 0.583. At £12.60 an hour that shift is worth £95.55. Type 7.35 instead and you get £92.61, short by nearly three pounds on a single night.

A few conversions are worth knowing cold. 15 minutes is 0.25, 20 minutes is 0.33, 30 is 0.5, 40 is 0.67 and 45 is 0.75. Everything else you divide or look up. There is more on this in our note on converting minutes to decimal hours.

Rounding rules

Many employers round clock times to the nearest quarter hour. Under the long-standing US seven-minute convention, clocking in at 08:53 or later rounds to 09:00, while 08:52 or earlier rounds back to 08:45. It is acceptable only because it rounds both ways and evens out over time. Check which convention your employer uses before arguing about a missing quarter hour, and note that UK employers have no equivalent rule, so any rounding there is purely a matter of contract.

Using the calculator

Enter the start time and the end time, and be careful with AM and PM if the tool uses a 12-hour clock. 12:00 PM is noon and 12:00 AM is midnight, and swapping them is a twelve-hour error. If the gap spans more than one day, add the dates as well as the times, because otherwise a Friday-to-Monday span reads as a few hours. Deduct unpaid breaks separately rather than trying to build them into the end time. For a full week of shifts, the hours worked walkthrough shows how to total them up.

Common questions

How many hours is 9 to 5:30 with a 30 minute break? The raw span is 8 hours 30 minutes, so 8.5 decimal hours. Take off the half-hour break and you are paid for 8.0 hours.

What happens on the night the clocks change? A shift running across the spring change is genuinely an hour shorter in real time, and an hour longer in autumn. UK clocks move on the last Sunday of March and October; US clocks move on the second Sunday of March and the first Sunday of November. Whether payroll pays clock hours or elapsed hours is a policy question, and employers answer it differently.

Why does Excel show 0.354 instead of a time? Excel stores time as a fraction of a day, so 8 hours 30 minutes is 0.354. Multiply by 24 to get 8.5 hours, or format the cell as a duration with square brackets so totals above 24 hours do not wrap back round to zero.

Can I add several durations together? Yes, but convert each one to decimal hours first, add, then convert back if you need hours and minutes. Adding 7:35 and 6:50 as though they were decimals gives 13.85 rather than the correct 14 hours 25 minutes.