Is a decade 12?

Whether a decade is 12 years is a question that many people have. At first glance, it may seem obvious that a decade must be 12 years. After all, the word “decade” comes from the Greek word for ten. However, once we dig deeper into the meaning and usage of the word over time, the answer becomes more nuanced.

The Dictionary Definition

According to the Merriam-Webster dictionary, a decade is defined as:

“a period of 10 years”

So by the strict dictionary definition, a decade is precisely 10 years, neither more nor less. This would imply that a decade cannot be 12 years.

Common Usage

In common usage, however, the meaning of decade is somewhat flexible. It is often used to refer to an approximate 10 year period rather than precisely 10 years.

For example, when people refer to “the 60s” or “the 90s” as a decade, they are not usually referring to the exact 10 year period from 1960-1969 or 1990-1999. Rather, they are referring to the general cultural era surrounding those years.

In this looser sense, it is not unreasonable to think of a 12 year period as a decade. For example, describing the “decade from 2011 to 2022” would be completely understandable even though it is technically a 12 year period.

Decades Based on Calendar

Part of the flexibility of the term decade stems from the fact that calendar years do not align perfectly to 10 year intervals. The Gregorian calendar that is widely used today has 365 days in normal years, and 366 days in leap years.

So a period from January 1st 2011 to December 31st 2020, for example, would span 10 whole calendar years but be slightly longer than 10 precise revolutions of the Earth around the Sun due to the leap years.

This means that when we refer to decades based on calendar years, such as “the 20s” or “the 90s”, we are inherently talking about a period slightly longer than 10 astronomical years. So in casual usage, allowing decades to be 12 calendar years aligns with this convention.

Academic Usage

In more technical or academic contexts, decades are usually defined more precisely as 10 year periods. So a historical study of population trends, for example, would define its decades as 1810-1819, 1820-1829, etc.

Here, allowing decades to be 12 years would introduce inconsistencies and ambiguities. So for clarity, decades are kept to precise 10 year spans.

Starting Points for Decades

There is also some flexibility around the starting and ending points for decades. While we often start decades at years ending in 0, such as 1980-1989, this is not universal.

Some people also talk about decades running from years ending in 3 to years ending in 2, such as 1983-1992. This reflects the fact that culturally defining “eras” does not always perfectly align with 10 year intervals.

If we move the goalposts for the start and end of decades, then defining a decade as 12 years becomes more reasonable.


In summary, while a decade is technically defined as a period of 10 years, the meaning is somewhat flexible in common usage.

Due to the conventions of our calendar system and the looseness of defining cultural eras, decades are often closer to 12 years in practice. So it is not unreasonable to think of a decade as approximating 12 years in many everyday contexts.

However, for academic work and technical definitions, it is important to maintain decades as precise 10 year periods to avoid ambiguity.

So in the end, whether a decade is defined as 10 years or 12 years depends significantly on the context and usage.

Frequently Asked Questions

Why are there 12 years in a decade?

There are 12 years in a decade because of the mismatch between the Gregorian calendar which has 365 days in a normal year and 366 days in a leap year, and the precise astronomical definition of a year as the time for one revolution of the Earth around the Sun. This mismatch adds up over time so that a period of 10 calendar years ends up being slightly longer than 10 precise astronomical years.

Who decides when a decade starts and ends?

There is no official body that defines when decades start and end. By convention, decades are often defined as starting in years ending in 0. But the boundaries are fluid and some people prefer decades aligned to other years. In practice, the start and end points of decades emerge through popular consensus and common usage.

Can a decade be any 10 year period?

In general usage, a decade can refer to any period of 10 years. However, when discussing history, it is common to use decades aligned to calendar decades for consistency. An exception is in academic writing, where decades often deliberately span exact 10 year periods, regardless of calendar years.

What is special about the year 2020?

2020 is viewed as the end point of a decade mainly because the digits change from 2XXX to 3XXX, which reinforces the sense of the passing of an era. The year 2020 also coincided with significant world events, like the coronavirus pandemic, which made it feel like the end of a definable period. But there is nothing mathematically significant about 2020 itself.

How long is a decade in years?

By definition, a decade is a period of precisely 10 years. However, decades are often measured loosely based on calendar years, in which case they tend to be 10 years plus a fraction, typically 1 or 2 additional years depending on when leap years fall. So in casual usage, decades are often thought of as 10, 11, or 12 years in length.

Comparison of 10 Year and 12 Year Decades

Decade Length 10 years 12 years
Definition Precisely 10 revolutions of the Earth around the Sun 10 calendar years plus 2 additional years to adjust for leap years
Advantage Avoids ambiguity for technical/academic work Aligns with calendar decades and cultural eras
Disadvantage Mismatches calendar decades Not an exact 10 year period

As the table illustrates, both 10 year and 12 year decades have advantages and disadvantages. Ten year decades are technically correct but misaligned with the calendar, while 12 year decades match our calendar and cultural tendencies but are numerically imprecise.

Timeline of Past Decades

Here is a visual timeline of some past decades using both 10 year and 12 year definitions:

10 year decades: 12 year decades:
  • 1980-1989
  • 1970-1979
  • 1960-1969
  • 1950-1959
  • 1984-1995
  • 1972-1983
  • 1960-1971
  • 1948-1959

This illustrates how the 10 year definition results in decades neatly lining up with 0 years, while the 12 year definition is offset and matches better to cultural associations.

When Does the Current Decade End?

Under the 10 year definition, the current decade would end on December 31st, 2029. But culturally, most people associate the current decade with the “2020s”, just as we refer to the previous decade as the “2010s”.

This association is driven by the changing digits of the calendar years. So even though we are still technically in the 2010s decade until the end of 2029, the cultural perception is that the decade ended in 2020.

Does It Really Matter?

In casual conversation, there is no “right” answer to whether decades should be 10 or 12 years. The usage is flexible enough that either can be justified. The best practice is to be clear about the definition if precision is important.

For historical analysis, it is common to use calendar decades for convenience. But academic work would specify exact 10 year periods to avoid ambiguity.

In the end, the decision comes down to aligning decades to either a precise definition of 10 years, or the messier calendar decades that emerge through popular consensus. Both approaches have merit in different contexts.

The Purpose of Decades

It is worth considering why we even define decades at all. Decades provide convenient goalposts for discussing history and current events. They allow us to bundle years into definable eras for comparison.

But the divisions are inherently arbitrary. The passage of time is continuous, so any discrete decade is just an artificial construct. The question of 10 years versus 12 years only matters if we want decades to mean something consistent.

In reality, decades are blurry periods whose boundaries shift based on perspective. So agonizing over a precise definition may be missing the point of decades altogether.

Decades in Other Calendars

The Gregorian calendar is not used universally around the world. Some other calendar systems have different methods of handling leap years that may avoid the mismatch between calendar years and astronomical years.

For example, the Revised Julian calendar omits leap years in years divisible by 100, unless also divisible by 900. This keeps the calendar from drifting as much relative to the solar year. Under such a calendar, decades would hew closer to being precise 10 year periods.

So the ambiguity around decade length is essentially a quirk of the Gregorian system. Other calendar models could potentially define decades more consistently as 10 years.

Should We Change the Calendar?

Given the imprecision of decades in the Gregorian calendar, should we consider changing the calendar system? An alternative like the International Fixed calendar could redefine months and years to properly align with astronomical events.

While this may resolve the ambiguity around decades, major calendar reform has historically proved extremely difficult to implement globally. The Gregorian system is deeply entrenched worldwide.

Ultimately, the inconsistent decades are a minor inconvenience compared to the massive disruption of uprooting the current calendar. So while imperfect, the Gregorian system is likely here to stay, along with its fuzzy decade definitions.

The Decade Generational Identity

Although decades do not have precise 10 year boundaries, they still profoundly shape generational identity. People powerfully self-identify with the decade they came of age in.

For example, those who grew up in the cultural eras we call the “the 60s”, “the 70s” or “the 80s” feel a connection and nostalgia to that time. It shapes their sense of belonging and shared experiences.

This is true even though the decades do not mathematically align. So despite the temporal imprecision, decades absolutely hold significance in societal terms.

Decades in Retrospect

Looking back historically, decades take on important meaning, separate from their numerical spans. Whether 12 years or 10, we associate memorable events, trends, styles, and cultural moods with past decades.

For instance, “the Roaring 20s” evokes images of flappers, prohibition, and wild parties, while “the 60s” brings to mind civil rights, Vietnam, hippies, and psychedelic rock.

The colors and flavors of decades are clearer in hindsight than when living through them. But the cultural associations are real and meaningful, even if the exact years are debatable.

The Blurring of Decades

While we talk about decades as distinct eras, the reality is that cultural trends bleed across decade boundaries. Fashions, music genres, attitudes, and events flow continuously over time, with no clean breaks.

For example, the Swing music era spilled from the 1930s into the 1940s. The hippie movement emerged in the mid-60s and continued into the 70s.

So decades are ultimately fuzzy categories. This further demonstrates the futility of agonizing over defining decades as precisely 10 years versus 12.

Cyclical Nature of Generations

Generational cycles and archetypes tend to repeat themselves over time. For example, the “Millennials” of today share many behaviors and values with the “Baby Boomers” of the 1960s and 70s.

This generational recycling underscores that the events and ethos of any given decade are not unique. Each era mirrors previous ones in broad strokes. Defining decades as fixed periods thus conflicts with their cyclical nature.

Dangers of Decade Stereotyping

While decades may appear to have distinct cultural traits in retrospect, it is important not to stereotype people based on the decade they were born in.

Individuals have complex identities that cross generational lines. Reducing people to caricatures of a decade denies their individuality. And decade stereotypes can be deployed for ageist purposes.

So while loosely discussing decades as cultural eras can be useful, blindly applying decade-based assumptions to people is simplistic and harmful.

How Events Define Decades

Major events play a key role in carving society into definable decades. The Great Depression clearly delineates the 1920s from the 1930s. World War 2 similarly demarcates the 1930s and 40s.

Likewise, the coronavirus pandemic has been era-defining, separating the pre-2020 “old normal” from the post-2020 “new normal.” Even though decades do not have fixed lengths, pivotal events help give them meaning.

Envisioning the Future

As we look ahead, how should we define decades going forward? Should we continue using the traditional calendar decades, or realign based on more precise 10 year intervals?

There are reasonable arguments on both sides. However, given the weight of history and culture, calendar decades seem likely to prevail into the foreseeable future.

The human mind craves clean segmentation of the past and future. So despite their imprecision, decades fill an important psychological role. That role seems unlikely to change, even if the decades themselves are fluid.

Beyond Decades

Stepping back, decades are just one arbitrary way we carve time into definable chunks. We could also segment history into broader “ages” and “eras” based on major historical pivots.

Or we could abandon discrete blocks of time altogether, recognizing that the passage of time is continuous and does not neatly fit into containers. While divisive, such a radically fluid view of time may be the most philosophically pure.

Ultimately, decades represent a compromise between mathematical precision and the messy unfolding of real events over time. By balancing rigor and flexibility, decades provide insights without pretending more accuracy than warranted.

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