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Time Calculator

Add and subtract durations expressed in hours, minutes and seconds.

Details

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What Time Calculator does

Time arithmetic is base 60, which is exactly why it goes wrong in a spreadsheet or in your head: 45 minutes plus 30 minutes is an hour and a quarter, not 75 of anything useful.

Enter durations in hours, minutes and seconds and get a correctly normalised total.

  • Add and subtract durations
  • Hours, minutes and seconds normalised in the result

How to use Time Calculator

  1. 1

    Enter the durations

    Use the hours, minutes and seconds fields.

  2. 2

    Choose add or subtract

    The total is normalised so minutes and seconds stay under 60.

  3. 3

    Convert to decimal if you are billing

    Invoicing and payroll usually want decimal hours rather than minutes. Divide the minutes by 60: 45 minutes is 0.75 hours, not 0.45.

Example

Where base 60 catches people out

Input

1h 45m + 2h 30m

Output

4h 15m

Treating the minutes as decimals gives 3.75 + 2.30 = 6.05, which is wrong twice over. Minutes carry at 60, not at 100.

When to use it

Totalling a timesheet

Add several task durations without the carrying errors that come from doing it in your head.

Planning a schedule

Add up segment lengths to see whether a session, a journey or a broadcast fits its slot.

Checking a recorded duration

Subtract one duration from another to find the remainder, for example how much of an allowance is left.

Limits and known behaviour

  • This works with durations, not clock times. For hours worked between a start and end time, use the hours calculator.
  • There is no date component, so durations do not roll over into days.
  • Daylight saving and time zones are irrelevant here, because a duration has no location.
  • Only two values are combined per calculation.

Privacy and data handling

Runs entirely in your browser

  • The calculation runs in this page. Nothing you enter is transmitted or stored.
  • The tool works offline once the page has loaded.

Site-wide data handling, including analytics and advertising, is described in the privacy policy.

Frequently asked questions

Why can I not add my clock times here?

Because a clock time is a point and a duration is a length. Adding 09:00 and 17:00 is not meaningful; the gap between them is. Use the hours calculator for that.

Why is time arithmetic so error-prone?

Because it is base 60 while everything else you do is base 10. Decimal habits carry over and produce answers that are wrong by a consistent amount.

How do I turn minutes into decimal hours?

Divide by 60. 15 minutes is 0.25, 20 minutes is 0.333, 30 minutes is 0.5 and 45 minutes is 0.75. The awkward ones are the values that do not divide cleanly, such as 10 minutes at 0.1667, which is why rounding rules matter on an invoice.

Why do my spreadsheet totals come out wrong?

Usually because the cells are formatted as numbers rather than as durations, so 1.45 is read as one and forty-five hundredths of an hour instead of one hour forty-five. Spreadsheets can handle time correctly, but only with the right cell format.