How Long is a Decade? (Meaning, Length, Formula)

A decade is described as a period equal to 10 years. Technically, the term “decade” was derived from the Greek word “Dekas,” which means “a group of ten.”

The Romans commonly used the prefix “dec” to depict collections of items that sum up to 10 in number, such as a decagon (a regular 10-sided polygon) or a decathlon (an athletic competition comprising ten events).

Other time categories derived from Latin include the century and the millennium, which are 100 and 1000 years, respectively.

Decades represent ten years that have allowed humanity to differentiate important historical events easily.

Moreover, decades have made the identification of specific past events easy. For instance, in the 30s or 40s, alternatively as the 1930s and 1940s, respectively.

Every decade has a familiar name derived from important events that happened in that period.

For instance, in the history of the United States (US), there’s an event recognized as the ‘Roaring Twenties,’ which refers to the 1920s period, when many changes took place and improved economic growth.

Today, many of the historical happenings are grouped in decades. These groupings made it easier for people to differentiate events during a particular time in previous years.

How many years make up a decade?

Technically, a decade is a group of ten years. The word “decade” describes periods according to the ten digits of the calendar year. For instance, “1980s” refers to the period between 1980–1989, equivalent to a decade.

Historically, 1 Anno Domini (AD) marks the beginning of the Gregorian calendar (note that the Gregorian calendar has no year 0); therefore, the Gregorian calendar’s first decade depicts the period between 1–10 AD, while the second decade is referred to as the period between 11–20 AD.

Though the nineties (90s) refer to the years that fall between 1990-1999 AD, the 200th decade is referred to the period between 1991–2000 AD.

Any event that repeats every decade is referred to as a decennial event. Likewise, events every 100 and 1000 years are considered centennial and millennial, respectively.

For instance, a census is given by the US government decennially, which means every ten years. Also, many reunions in high schools and colleges take place every ten years as well.

Note that a decade is just a coined name for a specific period on a calendar equivalent to 10 years.

The Gregorian calendar stipulates that timelines are categorized into years:

  • 1 millennium = 10 centuries = 1000 years
  • 1 century = 10 decades = 100 years.
  • 1 decade = 10 years = 3652 days.

A decade and a half are how long?

A decade and a half is equivalent to 15 years; since a decade is ten years, half a decade will equal five years. Therefore, (10+5) years make it a total of 15 years.

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A decade ago is how long?

Officially, in 1950, the decade became a part of the English vocabulary. The significance of a decade is that it refers to 10 years in the previous years. For example, the 1530s typically represent the period between 1530-1539.

A decade and a century are how long?

A decade is called 10 years, while a century is 100 years. The two terms originated from the Latin “Decuria,” meaning a group of 10 men, and were widely used by the Romans in the military. 

On the other hand, “centuria” was used to depict items made up of 100 parts. The origin of “centuria” came from “Saeculum,” meaning 100 years.

More so, it can portray a group of Individuals giving birth at a specific time or a cohort within the same range.

In effect, we refer to generational theories such as Gen Z, Millennials, etc.

At the same time, a century can be thought of as a definition of a long human life that ultimately spans a period of 4 generations.

The formula for Calculating Decade

Decade formula calculation is based on two dimensions of view or two perspectives, of which one view denotes that a new decade kicks off with a year (the year should have zero as the last digit) and ends with a year with nine as the last digit as well.

For example, in the 20th century, the 1980s began in 1980 and ended in 1989.

The same applies to the 21st century; let’s take, for example, the 2000s, a decade that began in 2000 and ended in 2009.

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Another view is that a decade starts with the last number as one and ends the year with the last number as 0.

For instance, the 10th decade of the 20th century began from 1991 to 1999. Likewise, 2030 will mark the last year of this decade.

It is noteworthy that the calculation of 2020 is either the beginning of a new decade or the last year of a decade; this does not affect life and its essence. However, this also depends on the perspective of each Individual.

Formula For Calculating A Century

The century calculation formula is simpler than (n = 100xn – 99). The first century begins from 1/1/1 to 12/31/100, which could be 1 to 100.

Whereas the 2nd century starts from 1/1/101 and stops at 12/31/200, alternatively written as 101 to 200.

Technically, the 21st century is marked from January 1, 2001, and ends on December 31, 2100. Starting from the year 2000, the marking of a new era begins known as the 21st century.

FAQs on How Long A Decade Is

What exactly is a decade?

A decade refers to a period of 10 consecutive years.

Does a decade always start in a year ending with a ‘0’ and finish in a year ending with a ‘9’?

While it’s common to refer to decades in this manner (like the 1980s or 1990s), technically, any period of 10 consecutive years is a decade. So, 2003 to 2012 is also a decade.

Are there specific events or changes that define a decade?

While a decade denotes a time span of 10 years, culturally and historically, certain decades are often associated with specific events, trends, or shifts, like the “Roaring ’20s” or the “Swinging ’60s.”

Is a decade the same across different calendars?

While the concept of a decade as 10 years remains consistent, different calendars might mark years differently. So, the start and end of a decade could differ based on the calendar in use.


Decades are widely known to possess cultural symbols, such as rebirth or a clean slate from which people can start life afresh.

Moreover, a decade is identified as a special period that idolizes our experience, giving the idea of “decade” a lot of symbolism. 

In the history of the United States, decades are often described as a nickname that encompasses important cultural events or is used as a symbol to represent the cultural fundamentality during that time.

For instance, the 1940s are often recognized as “The Fighting Forties,” which refers to the involvement of the United States in both the European and Pacific theatres of World War II (WWII).

Also, a decade can be referenced as the historical component of time, the division of events in their corresponding decades.

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Stella-maris Achumba
Stella-maris Achumba

Hello, I am Stella-maris! For two years, I began actively assisting students in the United States, and Canada in their pursuit of college advice and scholarship prospects. I am a content writer at

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