Converting Time: Decimal Hours, Seconds and Clock Formats

Two ways to write the same amount of time

A duration can be written in hours and minutes (1 h 45 min) or as decimal hours (1.75 h). They mean exactly the same amount of time. The confusion comes from the fact that minutes are counted in sixties, while decimals are counted in tens. So 1.45 hours is not 1 hour 45 minutes; it is 1 hour 27 minutes.

Decimal hours are handy whenever you need to multiply or divide, for example to find a speed or an average. Hours and minutes are easier to read on a clock. Knowing how to switch between them avoids a whole family of mistakes. The basics of adding and subtracting times are in the time calculation guide; this guide focuses on conversions.

Hours and minutes to decimal hours

Decimal hours = hours + minutes ÷ 60

1 h 30 min = 1 + 30 ÷ 60 = 1.5 h.
1 h 45 min = 1 + 45 ÷ 60 = 1.75 h.
3 h 51 min = 3 + 51 ÷ 60 = 3.85 h.
40 min = 40 ÷ 60 = 0.6667 h (two thirds of an hour).

The table covers every 5 minutes. Values are rounded to four decimal places where the decimal repeats.

Minutes	Decimal hours	Minutes	Decimal hours
5	0.0833	35	0.5833
10	0.1667	40	0.6667
15	0.25	45	0.75
20	0.3333	50	0.8333
25	0.4167	55	0.9167
30	0.5	60	1

A useful pattern: every 6 minutes is exactly 0.1 hour. So 12 minutes is 0.2 h, 18 minutes is 0.3 h, and 54 minutes is 0.9 h. Minutes that are multiples of 6 always give a clean one-place decimal.

Decimal hours back to hours and minutes

Keep the whole number as hours, then multiply the decimal part by 60 to get minutes.

2.2 h: 0.2 × 60 = 12, so 2 h 12 min.
3.85 h: 0.85 × 60 = 51, so 3 h 51 min.
0.1 h: 0.1 × 60 = 6 min.
1.45 h: 0.45 × 60 = 27, so 1 h 27 min, not 1 h 45 min.

If the minutes come out with a decimal, multiply that decimal by 60 again to get seconds. For 1.3958 h: 0.3958 × 60 ≈ 23.75 minutes, and 0.75 × 60 = 45 seconds, so about 1 h 23 min 45 s.

Minutes and seconds

Exactly the same rules apply one level down, because a minute has 60 seconds.

4.25 min = 4 min + 0.25 × 60 s = 4 min 15 s.
2.6 min = 2 min + 0.6 × 60 s = 2 min 36 s.
500 s = 500 ÷ 60 = 8 remainder 20, so 8 min 20 s, or 8.333 minutes.
1 h 23 min 45 s = 3,600 + 23 × 60 + 45 = 5,025 s, or 1 + 23/60 + 45/3,600 ≈ 1.3958 h.

Period	Hours	Minutes	Seconds
1 hour	1	60	3,600
1 day	24	1,440	86,400
1 week	168	10,080	604,800

A running example: a 5 km run takes 27 min 30 s, which is 27.5 minutes. The pace is 27.5 ÷ 5 = 5.5 minutes per km, which is 5 min 30 s per km. The speed is 5 km ÷ (27.5 ÷ 60) h ≈ 10.9 km/h. Notice that a pace of 5.5 min/km is written 5:30, not 5:50.

The 24-hour and 12-hour clocks

The 24-hour clock numbers the hours of the day from 00 to 23. The 12-hour clock runs from 12 to 11 twice, with AM (from the Latin ante meridiem, "before midday") and PM (post meridiem, "after midday").

24-hour	12-hour	Note
00:00	12:00 AM	Midnight, start of the day
00:30	12:30 AM	Half an hour after midnight
06:15	6:15 AM
12:00	12:00 PM	Noon
12:45	12:45 PM	Just after noon
13:00	1:00 PM	Subtract 12
18:30	6:30 PM
23:59	11:59 PM	Last minute of the day

From 24-hour to 12-hour: for hours 13 to 23, subtract 12 and add PM. Hour 12 stays 12 and gets PM. Hour 00 becomes 12 and gets AM. Hours 1 to 11 stay the same with AM.

From 12-hour to 24-hour: for PM times from 1 PM to 11 PM, add 12. 12 PM stays 12. For 12 AM, write 00. All other AM times stay the same.

The midnight and noon problem

"12 AM" and "12 PM" confuse many people, because noon is neither before nor after midday. The convention used by most clocks and phones is 12 AM = midnight and 12 PM = noon, but printed schedules are not always consistent. To avoid any doubt:

Write 12 noon or 12 midnight in words.
Use the 24-hour clock: 12:00 is always noon and 00:00 is always midnight.
For a deadline at the end of a day, write 23:59 on that date. "Midnight on Friday" can be read as the start or the end of Friday. Some timetables write 24:00 to mean the end of a day, which is the same moment as 00:00 the next day.

Adding up several durations

When you add durations, either convert everything to minutes or everything to decimal hours. Never add clock-style numbers as if they were decimals.

Example: four activities take 1 h 20 min, 45 min, 2 h 35 min and 50 min.

In minutes: 80 + 45 + 155 + 50 = 330 minutes = 330 ÷ 60 = 5.5 h = 5 h 30 min.
In decimal hours: 1.3333 + 0.75 + 2.5833 + 0.8333 = 5.4999, which is 5.5 h once the rounded table values are tidied up. Same answer. (Using exact fractions such as 4/3 instead of 1.3333 gives exactly 5.5.)
The wrong way: typing 1.20 + 0.45 + 2.35 + 0.50 = 4.50 and reading it as 4 h 50 min. That is 40 minutes short.

Averaging durations: three runs of 24:10, 23:45 and 25:05 (minutes:seconds) are 1,450, 1,425 and 1,505 seconds. The total is 4,380 s and the average is 4,380 ÷ 3 = 1,460 s = 24 min 20 s.

Multiplying durations: 12 episodes of 42 minutes each last 12 × 42 = 504 minutes = 8 h 24 min = 8.4 h.

For start and end times on a clock, the time calculator does the carrying for you. For durations longer than a day, divide the hours by 24: 50 hours is 2 days and 2 hours.

How spreadsheets store time

Many spreadsheet programs store a time as a fraction of a day. 0.5 is 12:00 (half a day), 0.75 is 18:00, and one hour is 1 ÷ 24 ≈ 0.041667. If a total of hours suddenly shows as a strange decimal or wraps back to zero after 24 hours, the cell is probably formatted as a clock time instead of a duration. Multiplying by 24 turns the stored value into decimal hours.

Common mistakes

Reading 1.45 h as 1 h 45 min. It is 1 h 27 min.
Adding times like decimals. 0.50 + 0.50 is not "1 h 00"; 50 min + 50 min is 1 h 40 min.
Rounding minutes too early. 20 min is 0.3333 h; writing 0.3 loses 2 minutes.
Mixing up 12 AM and 12 PM. Use the 24-hour clock or the words noon and midnight.
Typing 5.30 for 5:30. In decimal that is 5 h 18 min.

FAQ

What is 0.75 of an hour?
0.75 × 60 = 45 minutes.

How do I convert 2 h 40 min to decimal?
2 + 40 ÷ 60 = 2.6667 hours.

Is 12:30 AM in the afternoon?
No. 12:30 AM is half an hour after midnight, written 00:30 on a 24-hour clock. Half an hour after noon is 12:30 PM.

How many seconds are in a day?
24 × 60 × 60 = 86,400 seconds.

Open the calculator: Time Calculator →