Time Calculator

How to use this tool

Pick what you want to work out. Add or subtract times takes two lengths of time, such as 2h 30m and 1h 45m, and can also multiply or divide one by a plain number. A clock time plus or minus answers questions like what time it will be 8 hours after 9:00 AM. Time between two clock times gives the gap from a start to an end, including overnight.

Type times the way you would write them: 2:30, 2h 30m, 150 min or 2 hours 30 minutes, and clock times as 9:00 AM, 9pm or 21:30. The entry people most often get wrong is a bare decimal: 2.5 means two and a half hours, not 2 hours 50 minutes, and 2:45 means 2 hours 45 minutes, not 2 minutes 45 seconds. The line under the boxes says how an entry like that was read, so a slip shows up straight away.

Why time does not add like ordinary numbers

Time is counted in mixed bases. Seconds and minutes roll over at 60, a clock rolls over at 24 hours, and none of those is 10. Adding 2:30 and 1:45 as if they were decimals gives 3.75, which is wrong twice over. The minutes 30 and 45 make 75, which is 1 hour and 15 minutes, so the true answer is 4:15.

The calculator avoids the trap by converting everything to whole seconds, doing the arithmetic there, and converting back to hours, minutes and seconds. The working it shows uses the column method instead, carrying and borrowing 60 at a time, because that is how you would check it with a pencil.

Clock times and durations are different things

A duration is a length, like 8 hours 15 minutes. It can be any size, and two durations can be added. A clock time is a position on a 24 hour dial that goes back to zero at midnight. Adding two clock times makes no sense, since nobody can say what 9:00 AM plus 5:00 PM is, but a clock time plus a duration is another clock time, and the gap between two clock times is a duration.

The wrap at midnight is where most slips happen. 11:00 PM plus 7 hours 30 minutes lands 30 hours 30 minutes after the start of the day, which is 6:30 AM the next morning. Going the other way, 10:30 PM to 6:45 AM looks negative if you subtract directly; adding a day to the end time fixes it and gives 8 hours 15 minutes. The tool does that on its own and says when an answer has moved to another day.

Decimal hours are the bridge to money. Payroll and invoices multiply a rate by hours, and the rate does not understand minutes. 7 hours 45 minutes is 7.75 hours, so at a rate of 20 per hour it pays 155. Typing 7.45 instead pays 149, a small gap that repeats every shift.

Common shift lengths

A worked example you can check on paper

Take the starting example, 2h 30m plus 1h 45m. Write them in columns: hours 2 and 1, minutes 30 and 45. Minutes first: 30 + 45 = 75. That is more than 60, so write 15 and carry 1 hour. Hours: 2 + 1 + 1 = 4. The answer is 4 hours 15 minutes, written 4:15.

For decimal hours, divide the minutes by 60: 15 ÷ 60 = 0.25, so 4.25 hours. As a second route, turn both times into minutes: 150 + 105 = 255, and 255 ÷ 60 = 4 remainder 15. Both routes agree.

An overnight case: 10:30 PM to 6:45 AM. On the 24 hour clock that is 22:30 to 06:45, or 1,350 and 405 minutes after midnight. The end is the smaller number, so add a day of 1,440 minutes: 1,845 − 1,350 = 495 minutes, which is 8 hours 15 minutes, or 8.25 hours.

Mistakes that catch people out

Treating minutes as decimals. 1:30 is 1.5 hours, not 1.3, and 1 hour 50 minutes is 1.83 hours. Convert with ÷ 60 before multiplying by a rate.

Forgetting the day change. A shift from 10:00 PM to 6:00 AM is 8 hours, not minus 16. If the end is earlier than the start, add 24 hours.

Mixing up 12 AM and 12 PM. By the usual convention 12 AM is midnight at the start of the day and 12 PM is noon, but the labels are easy to misread. When it matters, write 12:00 noon or 12:00 midnight, or use the 24 hour clock.

Rounding every line. Rounding each entry on a timesheet to the nearest quarter hour before adding can drift by several minutes over a week. Add the exact times first and round once at the end, if your rules allow rounding at all.

Reading a race time as hours. 2:45 here means 2 hours 45 minutes. For 2 minutes 45 seconds, type 2m 45s.

Popular calculations

Each opens the calculator with those times filled in, and they can be changed once it is open.

Adding and subtracting times

Multiplying and dividing a time

A clock time plus or minus hours

Time between two clock times

How this was checked

The engine works in whole seconds, so there is no rounding drift, and every result was checked two ways: by the column method with carries and borrows, and by converting to seconds and back. Clock answers were checked on the 24 hour clock, including exactly midnight, noon, 24:00 and times that run back past midnight into the previous day.

Every address in the lists above was then opened and checked for a distinct title, the right answer, a timeline and clock face that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.

Common questions

How do you add hours and minutes?

Add the minutes first, then the hours. If the minutes come to 60 or more, take 60 away and add 1 to the hours. For 2h 30m + 1h 45m: 30 + 45 = 75 minutes, which is 1 hour 15 minutes, so the total is 4 hours 15 minutes.

How do you convert minutes to decimal hours?

Divide the minutes by 60. 15 minutes is 0.25 hours, 20 is 0.3333, 30 is 0.5 and 45 is 0.75, so 7 hours 45 minutes is 7.75 hours. Going the other way, multiply the decimal part by 60: 0.4 hours is 24 minutes.

How do I work out hours across midnight?

Subtract the start from the end, and if the answer is negative add 24 hours. From 10:30 PM to 6:45 AM: 6.75 − 22.5 = −15.75, and −15.75 + 24 = 8.25 hours, which is 8 hours 15 minutes.

What time will it be in 8 hours?

Add 8 to the hour on the 24 hour clock and take off 24 if you pass midnight. From 9:00 AM (09:00), 9 + 8 = 17, so 5:00 PM. From 11:00 PM, 23 + 8 = 31, and 31 − 24 = 7, so 7:00 AM the next day.

Is 12 AM midnight or noon?

On clocks, phones and in this calculator, 12 AM is midnight at the start of the day and 12 PM is noon; on the 24 hour clock they are 00:00 and 12:00. Strictly, neither label is correct, since a.m. and p.m. mean before and after noon, which is why NIST recommends writing 12:00 noon, 12:00 midnight or using the 24 hour clock when it matters.

How do you subtract one time from another?

Work from the right and borrow 60, not 10, when the top number is smaller. For 5h 15m − 2h 45m: 15 is less than 45, so borrow an hour to make 75 minutes; 75 − 45 = 30, and 4 − 2 = 2 hours, giving 2 hours 30 minutes.

Related Calculators and Reference

• Timesheet Calculator: a week of shifts with breaks and overtime.
• Date Difference Calculator: spans longer than a day, in days, weeks and months.
• Military Time Converter: switch between the 12 and 24 hour clocks.
• Time Unit Converter: seconds, minutes and hours to days, weeks and years.
• Days From Today Calculator: the date a number of days ahead or back.
• Time Zone Converter: the same clock time in another city or zone.
• Speed Distance Time Calculator: how long a distance takes at a given speed.
• Overtime Calculator: turn hours worked into overtime pay.
• NIST: Times of Day FAQs: why 12 a.m. and 12 p.m. are best replaced by noon, midnight or the 24 hour clock.