A third of an hour is exactly 20 minutes. An hour contains 60 minutes, so dividing it into three equal parts gives 60 ÷ 3 = 20. Two thirds of an hour is 40 minutes, while a quarter of an hour is 15 minutes. These fractions describe different lengths of time.
This guide explains how to convert fractional hours, add them to whole hours, and avoid decimal mistakes in schedules and time logs. The key is to keep the unit visible: 20 minutes, 0.20 hours, and 00:20 do not always mean the same thing.
To check a duration, open the hours and minutes converter. For an exact third of an hour, select minutes to hours and enter 20 rather than typing a rounded decimal such as 0.33.
A third of an hour in minutes: the calculation
A fraction describes equal parts of a whole. In 1/3, the bottom number tells you to divide the whole into three equal parts, and the top number tells you to take one of those parts. With an hour as the whole, each part contains 20 minutes. Three such parts add back up to 60 minutes.
The general formula is minutes = 60 × numerator ÷ denominator. For 1/3 hour, calculate 60 × 1 ÷ 3 = 20 minutes. For 2/3 hour, calculate 60 × 2 ÷ 3 = 40 minutes. Keep the fraction until you finish the calculation if you want to avoid rounding errors.
Fractional hours to minutes: reference table
| Duration | Fraction of an hour | Minutes | Decimal hours |
|---|---|---|---|
| A sixth of an hour | 1/6 | 10 | ≈ 0.1667 |
| A quarter of an hour | 1/4 | 15 | 0.25 |
| A third of an hour | 1/3 | 20 | ≈ 0.3333 |
| Half an hour | 1/2 | 30 | 0.5 |
| Two thirds of an hour | 2/3 | 40 | ≈ 0.6667 |
| Three quarters of an hour | 3/4 | 45 | 0.75 |
| A whole hour | 1 | 60 | 1 |
A quarter and a third differ by five minutes. A third and a half differ by ten minutes. If you are writing instructions for someone else, spelling out the number of minutes is often clearer than asking them to interpret a fraction.
One hour and a third is 80 minutes
A mixed duration combines whole hours with a fraction of an hour. Convert the whole hours first, then add the fractional part. One hour and a third is 60 + 20 = 80 minutes. Two hours and a third is 120 + 20 = 140 minutes.
Be careful with “a third of two hours.” That expression means divide the entire two-hour period into three parts: 120 ÷ 3 = 40 minutes. It is different from “two hours and a third,” which adds a third of one hour to two whole hours. Writing the arithmetic beside the words makes the distinction easy to check.
8 worked examples for everyday schedules
- A break lasting a third of an hour: it lasts 20 minutes. A break beginning at 10:00 a.m. ends at 10:20 a.m.
- A study session lasting two thirds of an hour: 20 × 2 = 40 minutes. Starting at 4:10 p.m. gives an end time of 4:50 p.m.
- A meeting lasting one hour and a third: 60 + 20 = 80 minutes. A 9:00 a.m. start gives a 10:20 a.m. finish.
- A task lasting two hours and a third: 120 + 20 = 140 minutes. Convert the whole hours separately before adding the remaining 20 minutes.
- A lesson lasting three quarters of an hour: 60 × 3 ÷ 4 = 45 minutes. The fraction 3/4 does not mean 0.34 hours.
- Three consecutive tasks lasting a third of an hour each: 3 × 20 = 60 minutes. Any gaps between the tasks must be added separately.
- A third of an hour plus a quarter of an hour: 20 + 15 = 35 minutes. Converting both fractions to minutes makes addition straightforward.
- A third of an hour divided into four equal stages: 20 ÷ 4 = 5 minutes per stage. Divide the 20-minute duration, rather than the original whole hour.
The start and finish examples assume the same time zone and no clock change during the interval. For longer periods or an overnight schedule, see how to calculate hours between two times. A unit conversion alone does not deduct breaks or account for daylight-saving transitions.
Why 0.33 hours is not exactly 20 minutes
The exact decimal representation of 1/3 is 0.3333… with the threes continuing indefinitely. A display may round that value to four or six decimal places. That is a display choice; it does not change the exact fraction.
If you type 0.33 hours as an exact input, the calculation is 0.33 × 60 = 19.8 minutes, which is 19 minutes 48 seconds. Each entry is 12 seconds short of a full third of an hour. Repeating that rounded input across many entries can accumulate a noticeable difference.
Another common error is writing 0.20 hours when you mean 20 minutes. Decimal hours use a base of one whole hour, so 0.20 × 60 = 12 minutes. To express 20 minutes as hours, divide 20 by 60 and keep 1/3 as the exact value.
Writing the duration as hours and minutes
In a duration field labelled HH:MM, a third of an hour can be written as 00:20: zero whole hours and twenty minutes. In a time-of-day field, however, 00:20 usually means twenty minutes after midnight. Check what the field expects before copying a value into a timetable.
For a simple conversion, choose the minutes-to-hours mode in the converter and enter 20. Review the decimal-hours and hours-and-minutes outputs. If you need to follow a 20-minute session while working, use the countdown timer with the required duration.
Frequently asked questions
Is a third of an hour 30 minutes?
No. Thirty minutes is half an hour because 60 ÷ 2 = 30. A third is 20 minutes because 60 ÷ 3 = 20.
How many minutes are in two thirds of an hour?
There are 40 minutes. Multiply the 20-minute third by two, or calculate 60 × 2 ÷ 3 directly.
How many 20-minute sessions fit into two hours?
Six sessions fit exactly, because 120 ÷ 20 = 6. That total leaves no time for breaks, setup, or transitions between sessions.
Which reference supports the conversion?
The National Institute of Standards and Technology, SP 330 Section 4, lists an hour as 60 minutes and a minute as 60 seconds. The fractions and worked examples in this guide apply that unit relationship.
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