How Long Is a Decade? 10 Years, 120 Months and About 3,652 Days

A decade is exactly 10 years. That works out to 120 months, about 521.8 weeks, and either 3,652 or 3,653 days, depending on whether the 10 years include two or three leap days. On average, a decade lasts 3,652.425 days, or about 87,658 hours, using the Gregorian calendar.

Convert Decades to Years, Days and Seconds

Enter any amount below. The converter then shows the same span in years, months, weeks, days, hours, minutes and seconds. It uses the average Gregorian year of 365.2425 days, so the day count comes out as 3,652.425 instead of a whole number.

The word comes from the Greek “dekas”, meaning a group of ten. Merriam-Webster’s definition of decade lists “a period of 10 years” as the main sense, and it also gives a group or set of 10 as a meaning. So the year count is never in doubt. Instead, what changes is the number of days, because some years are 365 days long and leap years are 366.

You meet the word in two forms. First, there is any run of 10 years, as in “she has worked here for a decade.” Second, there are named calendar decades such as the 1990s or the 2020s. In addition, those named spans raise a question people still argue about: does the span start in a year ending in 0 or in 1? This guide answers that too, after the quick numbers.

Quick Conversion Chart: 10 Years in Other Units

First, these values cover the units people ask about most. Where the answer depends on leap years, the table shows both the shortest common case and the longest one, plus the long-run average.

Printable tip: save or print this chart, because the numbers never change. Also, for the small end of the scale, see how many seconds are in a day (86,400 on a normal day).

How Many Days in a Decade, and Why It Varies

A common year has 365 days, so 10 of them would give 3,650 days. However, the Gregorian calendar adds a leap day (February 29) in every year divisible by 4. The main exception, however, is century years, which are leap years only when they divide evenly by 400. That is why 2000 was a leap year, while 1900 was not and 2100 will not be.

Any 10 consecutive years therefore contain either 2 or 3 leap years in almost every case. For example, 2020 to 2029 holds 2020, 2024 and 2028, so it has 3,653 days. By contrast, 2021 to 2030 holds only 2024 and 2028, so it has 3,652 days. Similarly, the 1990s (1990 to 1999) had 1992 and 1996, for a total of 3,652 days.

The average figure of 3,652.425 days comes from the Gregorian rule itself. Specifically, over a full 400-year cycle there are 97 leap days, so the average year is 365.2425 days long. Multiply by 10 and you get the average that the converter above uses.

When Does a Decade Start: 2020 or 2021?

Both answers are in use, and each has a reason behind it. First, the everyday convention groups years by their tens digit. In that sense, the 2020s run from January 1, 2020, to December 31, 2029. Merriam-Webster’s entry uses that style in its own example, describing the 1920s as running from January 1, 1920, through December 31, 1929.

Second, the other convention counts decades from year 1, because the historical calendar has no year 0. The US Naval Observatory explains that AD 1 directly follows 1 BC, so the 21st century began on January 1, 2001, not in 2000. Following the same logic, the third set of 10 years in this century is 2021 to 2030.

How to Count the Days in Any Decade

You can work out the exact length of any 10-year span in under a minute. For example, here is the method:

1. Write down the first and last year. For a calendar decade such as the 2030s, that is 2030 and 2039.
2. List the years divisible by 4. In the 2030s, those are 2032 and 2036.
3. Remove century years not divisible by 400. 1900 and 2100 drop out; 2000 stays in.
4. Add the leap days to 3,650. Two leap days give 3,652 days, and three give 3,653.
5. Convert if you need other units. Multiply the days by 24 for hours, or by 86,400 for seconds, or use the converter above.

If your span starts mid-year, check whether February 29 of each leap year actually falls inside your dates. In short, that detail decides whether a leap day counts.

Decade Compared With Other Time Spans

It sits in a family of time words built on multiples of ten and other round counts. This table, for example, puts it in context, using the same 365.2425-day average year.

For the next step up, read how long a century is. At the other extreme, the medieval moment, about 90 seconds, shows how old time words once had exact lengths too.

Do and Don’t for Counting 10-Year Spans

Honest Limits of the Numbers

The figures on this page describe calendar time, and a few small effects sit outside them. For one thing, leap seconds have been added to official clocks now and then since 1972, so a real 10-year span can run a second or two longer than the table shows. Even so, that matters only to timekeeping labs and satellite systems. Also, the converter treats a month as one-twelfth of an average year, about 30.44 days. However, real months run from 28 to 31 days, so “120 months” is exact while a day count for, say, 37 months is only an estimate. Finally, the average of 3,652.425 days is never an actual span length; any real 10-year span has a whole number of days.

When Accuracy Really Matters

For conversation, “10 years” is all anyone needs. Still, a few situations call for the exact day count:

• Interest and finance. Daily interest over 10 years depends on whether the span has 3,652 or 3,653 days.
• Legal terms. Similarly, statutes of limitation and lease terms end on specific dates, not after an average number of days.
• Science and data. In addition, climate records and other long datasets often compare 10-year blocks, so researchers state the exact years used.

Frequently Asked Questions

How long is a decade in years?

A decade is exactly 10 years. In addition, the word comes from the Greek dekas, meaning a group of ten.

How many days are in a decade?

A decade has 3,652 or 3,653 days, depending on whether it includes two or three leap days. As a result, the Gregorian average is 3,652.425 days.

How many weeks are in a decade?

A decade has about 521.8 weeks: 521 weeks plus 5 or 6 extra days.

How many months are in a decade?

There are exactly 120 months in a decade, since each year has 12 months.

How many hours are in a decade?

A decade has 87,648 hours with two leap days or 87,672 hours with three. On average, that comes to about 87,658 hours.

How many seconds are in a decade?

A 3,652-day decade has 315,532,800 seconds, and a 3,653-day decade has 315,619,200 seconds, ignoring rare leap seconds.

Does a decade start in a year ending in 0 or 1?

Both conventions exist. For example, named decades such as the 2020s run from 2020 to 2029. Meanwhile, counting from year 1, the third decade of the 21st century runs from 2021 to 2030.

How many leap years are in a decade?

Most 10-year spans contain two or three leap years. However, a span that crosses a skipped century leap year, such as 1900 or 2100, can contain only one.

How many 10-year periods are in a century?

A century has 10 decades, because it is 100 years long.

What is a five-year period called?

Half of 10 years is five. Also, an older word for a five-year period is lustrum, from ancient Rome.

A Decade in Short: The Bottom Line

To sum up, a decade always means 10 years and 120 months. The day count, however, depends on leap years: 3,652 days with two leap days, 3,653 with three, and 3,652.425 on average. So when someone asks how long a decade is, the honest answer is “10 years, or about 3,652 days.”

Finally, remember the naming split. For instance, the 2020s are 2020 to 2029, while counting from year 1 gives 2021 to 2030. So when dates matter, name the exact years and count the real calendar days.