If you know when something started and when it ended, you can work out how long it lasted. The catch is that clock time does not behave like ordinary numbers: an hour holds 60 minutes, not 100, and the clock restarts at midnight. That is why subtracting two times in your head so often goes wrong by an hour.
The good news is that there is one simple method that handles all of it. Convert both times to minutes since midnight, subtract, and convert the answer back into hours and minutes.
The short answer: the minutes-since-midnight method
Here is the whole method in four steps:
- Convert each time to 24-hour form if it is in 12-hour form (more on the 12 am and 12 pm trap below).
- Turn each time into minutes since midnight by multiplying the hours by 60 and adding the minutes.
- Subtract the start from the end.
- Turn the answer back into hours and minutes by dividing by 60: the whole number is the hours, and the remainder is the minutes.
That is it. Every worked example in this article uses exactly these steps, so if you learn nothing else, learn this: minutes since midnight first, subtract second, convert back last.
Worked example: a normal day, 09:30 to 17:45
Say a work session runs from 09:30 to 17:45. How long is that?
First, both times are already in 24-hour form, so convert each to minutes since midnight:
– Start: 9 × 60 + 30 = 570 minutes
– End: 17 × 60 + 45 = 1,065 minutes
Subtract the start from the end:
– 1,065 − 570 = 495 minutes
Now convert 495 minutes back into hours and minutes. Divide by 60: 495 ÷ 60 = 8, with 15 left over. So the duration is 8 hours 15 minutes.
The step people get wrong: borrowing across the hour
The most common mistake in duration arithmetic is treating minutes like hundredths. A span from 13:10 to 14:05 looks, at a glance, like 95 minutes — 205 minus 110 is 95, after all. But that answer is wrong, and the reason is the same one that makes clock arithmetic awkward: there are only 60 minutes in an hour.
Work it through the minutes-since-midnight method instead:
– Start: 13 × 60 + 10 = 790 minutes
– End: 14 × 60 + 5 = 845 minutes
– Difference: 845 − 790 = 55 minutes
The real duration is 55 minutes, not 95. What went wrong in the quick version is that the minutes column needed to borrow an hour: when the end minutes (5) are smaller than the start minutes (10), you borrow 60 minutes from the hour, so the calculation becomes 65 minus 10 in the minutes column and one fewer hour. The minutes-since-midnight method does this borrowing for you automatically, which is exactly why it is worth using even for short spans.
If you ever catch yourself writing down an answer with more than 59 minutes in it — “1 hour 95 minutes”, say — that is the signal that borrowing was missed.
Convert 12-hour times to 24-hour times first
If your times are in 12-hour form, convert them before you start subtracting. The rule is simple: keep the hour as it is for am times, and add 12 for pm times — with two exceptions that trip people up.
– 12 am is midnight, so it becomes 0:00, not 12:00. A span starting at 12:30 am starts at 30 minutes after midnight.
– 12 pm is midday, so it stays 12:00. A span starting at 12:30 pm starts at 12 hours 30 minutes after midnight.
So 3:40 pm becomes 15:40, and 9:05 am stays 09:05. Once both times are in 24-hour form, the minutes-since-midnight steps work exactly as above. For example, 3:40 pm to 5:05 pm becomes 15:40 to 17:05: that is 940 minutes to 1,025 minutes, a difference of 85 minutes, or 1 hour 25 minutes.
Getting the 12 am and 12 pm conversions right matters more than any other single step, because an error there throws the answer off by a full 12 hours.
What about overnight spans?
Sometimes the end time is earlier in the day than the start time. A night shift from 22:30 to 06:15 is the classic case: subtracting directly gives a negative number, which is clearly not the answer.
The rule is straightforward: if the end time is earlier than the start time, treat the end time as being on the next day — in other words, add 24 hours (1,440 minutes) to the end time before subtracting.
Worked example, 22:30 to 06:15:
– Start: 22 × 60 + 30 = 1,350 minutes
– End, treated as next day: 6 × 60 + 15 = 375 minutes, plus 1,440 = 1,815 minutes
– Difference: 1,815 − 1,350 = 465 minutes
– Convert back: 465 ÷ 60 = 7, with 45 left over — 7 hours 45 minutes
That single rule covers one crossing of midnight. It does not cover spans of several days: if the start and end are days apart, you are no longer measuring a duration between two clock times, and you need a date-based calculation instead (see the “wrong tool” section below).
If you need to subtract a break, do it by hand
A duration between two clock times is the full elapsed span, breaks included. If you clock in at 09:00, clock out at 17:30, and take a 30-minute unpaid break in the middle, the span itself is 8 hours 30 minutes — but the time actually worked is 8 hours.
The way to handle that is to work out the full span first, then subtract the break yourself:
– Full span: 09:00 to 17:30 = 540 to 1,050 minutes = 510 minutes = 8 hours 30 minutes
– Minus the break: 510 − 30 = 480 minutes = 8 hours exactly
This matters because many duration tools, including the one on this site, report the full elapsed time and do not deduct breaks for you. If you need a net figure after breaks, subtract the break length from the reported duration.
Reading the answer another way: total minutes and decimal hours
Hours and minutes are the natural way to read a duration, but sometimes another form is handier. Two conversions cover most cases, and both are simple hand arithmetic:
– Total minutes: multiply the hours by 60 and add the minutes. So 8 hours 30 minutes is 8 × 60 + 30 = 510 minutes.
– Decimal hours: divide the total minutes by 60. So 510 minutes is 510 ÷ 60 = 8.5 hours, and 465 minutes is 465 ÷ 60 = 7.75 hours.
Decimal hours are the form that timesheets and billing systems usually ask for, but they are a way of writing the same duration, not a different duration. Note that 8 hours 30 minutes is 8.5 hours, not 8.30 hours.
When a duration tool is the wrong tool
Clock times and calendar dates answer different questions, and mixing them up is a common source of wrong answers:
– For spans across calendar dates — how many days between 3 March and 17 March, for example — you need a date difference calculation, not a clock-time duration. Clock times alone cannot tell you how many days passed.
– For converting between units — how many minutes are in 3 hours, or how many seconds are in 45 minutes — you need a unit conversion, not a duration between two times.
– For spans of several days, or for payroll rules like rounding, overtime, or break policies, a simple duration between two clock times is not enough on its own. Work out the elapsed time first, then apply whatever rules your situation requires, separately and explicitly.
A related but different question is what counts as a working day in the first place. If that is the problem you are actually solving, read our guide to what counts as a business day before counting days.
Try it: the Time Duration Calculator on Manage My Tools
If you would rather not do the arithmetic by hand, the Time Duration Calculator on Manage My Tools does exactly the method in this article: enter a start time and an end time, and it reports the duration in hours and minutes. It runs in your browser, needs no sign-up, and is free.
It measures one continuous stretch between two clock times. If the end time is earlier than the start time, it treats the end as the next day, so overnight spans work. It does not deduct breaks, split a span into shifts, or apply rounding or payroll rules.
You can find it alongside the rest of the site’s calculators in the Calculators category or browse all free tools if a different job needs a different tool.
FAQ
Can the end time be earlier than the start time?
Yes — that means the span crossed midnight. Treat the end time as being on the next day: add 24 hours (1,440 minutes) to it before subtracting. A span from 22:30 to 06:15 is 7 hours 45 minutes, not a negative number.
Does the Time Duration Calculator deduct lunch breaks?
No. It reports the full elapsed time between the two clock times, with nothing removed. Measure any unpaid break separately and subtract its length from the duration shown.
What if the span covers several days?
Then two clock times are not enough information on their own — you need the dates as well. Use a date-based calculation for multi-day spans rather than stretching a clock-time duration to cover them.
How do I get the answer in minutes only?
Multiply the hours by 60 and add the minutes. A duration of 8 hours 15 minutes is 8 × 60 + 15 = 495 minutes.
How do I convert hours and minutes to decimal hours?
Divide the total minutes by 60. For 8 hours 30 minutes, that is 510 ÷ 60 = 8.5 hours. Remember that 8 hours 30 minutes is 8.5 hours, not 8.30 hours.
The method, in one line
Convert both times to minutes since midnight, subtract the start from the end — adding 24 hours to the end time if it falls earlier in the day — and convert the difference back into hours and minutes. That one habit handles the borrowing, the midday crossing, and the overnight span, and it is the same calculation the Time Duration Calculator performs for you when you would rather skip the arithmetic.