Time Calculator (add & subtract hours, minutes, seconds)

Understanding the time calculator — concept and principle

This calculator adds, subtracts, multiplies and divides durations written as hours, minutes and seconds. Hours can go past 24, so it suits timesheets, video edits, race splits and cooking schedules.

Convert to one unit, calculate, then convert back with division and remainders.

How it is calculated

Seconds = hours × 3,600 + minutes × 60 + seconds

Result = A + B, A − B, A × number or A ÷ number (in seconds)

The result is turned back into h:mm:ss, and also shown as decimal hours (÷ 3,600), total minutes (÷ 60) and total seconds.
Two parts (1:30) mean hours and minutes; three parts (1:30:15) mean hours, minutes and seconds.
A minus sign in front makes a time negative. Whole days = result ÷ 86,400 s, truncated.

The principle

Time uses base 60 for minutes and seconds, so you cannot add 2:45 and 1:20 as decimals. The cleanest way is to convert everything into one unit, seconds, do the arithmetic, and convert back. Converting back is division with remainders: hours = total ÷ 3,600, the remainder ÷ 60 gives minutes, and what is left is seconds. Multiplying and dividing a duration by a plain number works the same way, for example to find the total of five equal laps or the average of a split.

Worked example

Default inputs: A = 2:45:30, operation A + B, B = 1:20:45.

A = 2 × 3,600 + 45 × 60 + 30 = 9,930 s.
B = 1 × 3,600 + 20 × 60 + 45 = 4,845 s.
A + B = 14,775 s = 14,775 total seconds.
14,775 ÷ 3,600 = 4 h, remainder 375 s = 6 min 15 s, so 4:06:15.
Decimal hours = 14,775 ÷ 3,600 = 4.1042; minutes = 246.25; whole days = 0.

How to read the result

Use h:mm:ss for display and decimal hours for pay or billing, since an hourly rate multiplies decimal hours: 4:06:15 is 4.1042 hours, not 4.06. A negative result from A − B means B is longer than A. When the result is over 24 hours, the "whole days" line tells you how many full days it contains.

Common mistakes

Writing 4:06 for 4.1 hours or 4.06 hours for 4:06.
Entering 90 minutes as 0:90 instead of 1:30 (both may read correctly, but check).
Treating a clock time such as 23:00 + 2:00 as a time of day; this tool works with durations.
Forgetting the seconds part when copying from a stopwatch.

Keep in mind: The tool works with durations, not calendar dates, time zones or daylight saving changes. Use a time difference calculator for those.

Frequently asked questions

How do I convert 2 hours 45 minutes to decimal?

2 + 45 ÷ 60 = 2.75 hours.

What is 1:30:00 × 3?

4:30:00.

How many seconds are in a day?

86,400.

Sources: BIPM, The International System of Units (SI Brochure), 9th ed., Table 8 (minute, hour, day)

Checked 9 Nov 2026. Sexagesimal time units: 1 min = 60 s, 1 h = 60 min = 3,600 s, 1 d = 86,400 s (SI Brochure, 9th ed., Table 8 — non-SI units accepted for use with the SI).

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