How to use the time calculator
- Enter hours, minutes and seconds for the first duration. Leave unused boxes blank.
- Choose Add or Subtract for each row.
- Use Add a duration for more rows, up to 20.
- Read the total in hours:minutes:seconds, total minutes and decimal hours.
Formula
Row seconds = hours x 3600 + minutes x 60 + seconds
Total seconds = sum of (+ or -) row seconds
Hours = floor(total / 3600); minutes and seconds come from the remainder
Decimal hours = total seconds / 3600
Working in seconds avoids carrying errors. You can type 90 in the minutes box and it is understood as 1 hour 30 minutes.
Worked example: adding two work blocks
Add 2 hours 30 minutes and 1 hour 45 minutes. In seconds that is 9,000 + 6,300 = 15,300. Divide by 3,600 to get 4 whole hours with 900 seconds left, which is 15 minutes.
The total is 4:15:00, or 4.25 decimal hours and 255 minutes.
Now subtract 1 hour 30 minutes from 1 hour. The total is negative: -0:30:00. The calculator keeps the sign so you can see you have gone under.
When to use this instead of a clock calculator
This tool adds lengths of time, such as practice sessions, recipe steps or billable blocks. If you have clock times like 9:15 AM and 5:45 PM, use the time duration calculator or the hours calculator.
Common ways time sums go wrong
The most frequent error is treating minutes like decimals. 1 hour 45 minutes plus 1 hour 30 minutes is 3 hours 15 minutes, not 2.75 plus 2.30 = 5.05. The second most common is forgetting to carry: 50 minutes plus 25 minutes is 75 minutes, which is 1 hour 15 minutes.
- Add minutes and seconds first, then carry every 60 into the next unit.
- Keep subtraction for last, so the sign of the total is clear.
- If you need pay, convert the final total to decimal hours once, not each row.
Assumptions and limits
- Inputs are whole numbers. Use minutes and seconds for fractions of an hour.
- Minutes and seconds above 59 are accepted and normalized.
- Totals above 24 hours are also shown as days.
Frequently asked questions
How do I add hours and minutes by hand?
Add the minutes first. If they reach 60 or more, subtract 60 and carry 1 hour. Then add the hours. For example, 2:50 plus 1:25 is 3 hours 75 minutes, which carries to 4:15.
How do I subtract time that crosses an hour?
Convert both to minutes, subtract, then convert back. This calculator does that for you and shows a negative sign if the result is below zero.
Can I use it to add video or song lengths?
Yes. Enter minutes and seconds for each item and leave hours blank. The total shows in hours:minutes:seconds so a long playlist is easy to read.