How Long is a Decade? (Decade, Century, Millennium + more!)

If you’ve ever wondered “how long is a decade,” then the answer is simply that a decade is 10 years long.

If you’re still wondering, don’t worry — most people do.

It’s easy to understand why this question comes up; after all, the average person lives about 70 years, and a decade represents 14% of that time frame.

But the truth of the matter is that while the dictionary definition of the term “decade” means 10 years long, the actual decade label has no fixed length.

It can mean different things to different people, depending on their lifestyle and circumstances. Keep reading to learn more about people’s differences when saying the word decade and more information about time terminology.

How Long Is A Decade

A Decade Is 10 Years Long

A Decade Is 10 Years Long

The word Decade comes from Latin decennium, meaning a group of ten. A decade consists of 10 years, or one-tenth of a century (a century is 100 years long).

In literal terms, ten years are comprised within each Decade.

The first year in any decade is its first year and likewise for every following year.

There are no exceptions to these rules—that is how long a decade lasts!

In other words, a new year can only fall within its corresponding Decade, not before or after.

It’s important to understand that when determining how long a decade is, 10 years do not comprise every single year in any given decade.

Rather, there are nine previous years and one subsequent year. For example:

1991 – 1992 – 1993 – 1994 – 1995 -1996 – 1997 – 1998 – 1999 -2000

Together, these ten years comprise just one Decade.

How Many Decades Existed During All Of Human History?

While much of human history can be categorized into centuries or millennia, decades have only existed in some parts of human history.

The answer to how many decades have existed throughout human history depends on which century we’re referring to.

Suppose you consider that there were only one hundred centuries in all of human history. In that case, you will end up with around 100,000 decades of human history.

However, if we consider hundreds or thousands of millennia (depending on how early or late in human history you start counting), it becomes much more difficult to account for how many decades exist at any given point in time.

Origin Of The Term Decade

Origin Of The Term Decade

A decade comes from an old Latin term, “decennium,” which means ten years.

The Romans divided their year into 10 months: Januarius, Februarius, Martius, Aprilis, Maius, Junius, Quintilis (later renamed Iunius), Sextilis (later renamed Augustus), September, October, and November. So every Decade was represented by each of these months.

The years within each Decade were identified by their ordinal numbers (1st, 2nd, 3rd, 4th, etc.). For example, the fifth year of a decade was known as an annus quintilis.

That term eventually evolved into ‘quintile’ and then in Middle English into ‘quintel,’ which later shortened to ‘quel.’

So from these origins, we get our modern term: quinquennium.

If you didn’t already know, quinquennium means five years.

The general public has condensed it over time, though, so now, when they say Decade, they refer to 10 years instead of any five years.

Which I guess makes sense – ten years is roughly one-tenth of a century after all!

Why Is A Decade 10 Years Long?

It’s easy to think of a decade as simply ten years, but not many know the reason behind it.

The word Decade has Latin roots and means ten years. The Latin term “decennium” means 10 years, so this is why a decade is 10 years long.

Many people make the mistake of considering that a decade is 11 years long. For example, people think that the period from 2010 to 2020 is a decade, but this is not true as it is 11 years long.

A decade would be from 2010 to 2019.

What Is A Century?

What Is A Century

A century is exactly 100 years long. Century has been derived from the Latin word “Centuria.” There is a lot of history behind its root.

The Centuria was a soldiers group that comprised 100 total members in Rome.

This is how we got the modern word Century from Centuria.

The term century also changed meanings over time and was also used to count 80 people.

Now, a century means 100 or 100 years long. The term century is also used in sports to count the score of a game.

What Is A Millennium

A millennium (plural millennia or millennia) is, in both secular and religious contexts, any period consisting of 1000 years.

It comes from the Latin word mille and annus. Mille means thousand while annus means year.

It can also be defined as one hundred consecutive years.

Hence, 2000 to 2999 (1st to 9th century BCE) or 800 to 999 CE (Common Era). It would be shorter than an epoch or an age that both reference 10 000-year periods.

 Millennia are most commonly used for longer periods than great ages but shorter than geologic epochs.

In English, ‘millennium’ commonly refers to a single 1000-year period or the periods within religious thinking that revolve around humans moving into and through different eras.


Following are some of the commonly asked questions associated with how long a decade is.

Why Is Decade Used To Explain History?

You might see the term “decade” used in historical articles or history books.

This is very simple: it is easier to recall times in intervals of 10 years compared to a century or something else.

When we read history, we mostly read history related to the last 100 years.

I’m not saying people don’t read history older than 100 years. Still, you’ll probably only read a history of the 1900s in your school or college books.

For books discussing older history, by older, we mean a few hundred or thousand years back, you’ll see the terms “century” or “millennium” being used.

In simple terms, it is easier to say “a few decades ago” than to say “40-50 years ago”.

How Long Is 20 Years Called?

One Decade is 10 years long, or a single decade equals 10 years.

Another way of saying it would be one Decade equals one-tenth of a century. So, how long are 20 years in terms of decades?

A fancy way of saying 20 years is “vicennial,” which means 20 years or 20 years ago.

Twenty years equals two decades. An easy way to remember when you need to identify how many decades are contained within twenty years is by knowing that there are two digits in 2 decades.

How Long Is 50 Years Called?

50 years equals half a century, which is the easiest way to say how long 50 years are. A person of around 50 years may also be called a “quinquagenarian.” An easy way to say 50 years is to say “half a century.”

Conclusion (How Long Is A Decade)

A decade is exactly 10 years long. You may mistake considering a decade 11 years long when you see the 2010-2020 or 1990-2000.

A decade would look like 2010-2019 or 1990-1999.

The word Decennium comes from Latin roots, which means tenth + year. In English, Decade and decennium are synonyms; both terms mean 10 years.

A century is 100 years long, while a millennium is 1000 years long.

Now that you’ve made it this far, consider reading our other interesting articles as well such as “what does tendered for delivery mean“, “petsmart return policy“, and “who owns target.”


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