Adding up times sounds trivial until you actually try it — because the clock runs in 60s and 24s, not tens. Get the carrying wrong, or confuse decimal hours with minutes, and your answer is off. This guide shows you how to add, subtract, multiply, and convert time correctly, with worked examples. For instant answers, use our free Time Calculator.

How to Add Hours, Minutes and Seconds

The reliable method is to convert everything to one unit — seconds — add, then convert back:

Total seconds = (Hours × 3600) + (Minutes × 60) + Seconds

Worked example: add 2 h 45 m and 1 h 30 m.

• First time: 2×3600 + 45×60 = 9,900 seconds
• Second time: 1×3600 + 30×60 = 5,400 seconds
• Total: 15,300 seconds = 4 h 15 m

The hard part by hand is “carrying” — 45 minutes plus 30 minutes is 75 minutes, which becomes 1 hour 15 minutes. Our Time Calculator handles that carrying automatically across as many time values as you add.

How to Subtract Time

Subtraction works the same way — convert to seconds, subtract, convert back. For example, 5 h minus 1 h 30 m is 18,000 − 5,400 = 12,600 seconds = 3 h 30 m. If the result is negative (you subtracted a larger time), the calculator shows it as a negative duration so you can see the gap either way.

The Decimal-Hours Trap (1.5 Hours Is Not 1:50)

This is the single most common time mistake. Decimal hours and “hours and minutes” are not the same:

• 1.5 hours = 1 hour 30 minutes (because 0.5 × 60 = 30), not 1 hour 50 minutes.
• 1.25 hours = 1 hour 15 minutes.
• 2.75 hours = 2 hours 45 minutes.

To convert decimal hours to minutes, multiply the decimal part by 60. To go the other way, divide the minutes by 60. The Time Calculator’s convert mode does both instantly.

How to Find the Duration Between Two Times

Convert both clock times to seconds since midnight and subtract. From 9:00 to 17:30 is 8 hours 30 minutes. If the end time is earlier than the start — like a shift from 22:00 to 06:00 — it crosses midnight, so add 24 hours to get 8 hours. The “duration between” mode handles the overnight case for you.

How to Multiply or Divide a Time

Need a multiple of a duration? Convert to seconds, multiply or divide, then convert back:

• 8 laps at 1:45 each: 105 seconds × 8 = 840 seconds = 14 minutes.
• A 3-hour task split 4 ways: 10,800 ÷ 4 = 2,700 seconds = 45 minutes each.

This is handy for workouts, music and video editing, cooking stages, and dividing project time.

Converting Time Units

Some reference points worth knowing: a minute is 60 seconds, an hour is 3,600 seconds, a day is 86,400 seconds, and a week is 604,800 seconds. So 150 minutes is 2 hours 30 minutes, 2.5 decimal hours, or 9,000 seconds. The convert mode switches between days, hours, minutes, and seconds in one step.

Quick Reference