Decade — Definition, Formula & Examples
A decade is a period of time equal to 10 years. For example, the years 2010 through 2019 make up one decade.
A decade is a unit of time measurement defined as exactly 10 consecutive years. Decades are commonly referenced by their starting year (e.g., "the 1990s" refers to the decade spanning 1990–1999).
Key Formula
Where:
- = Total number of years
- = Number of decades
How It Works
To convert decades to years, multiply the number of decades by 10. To convert years to decades, divide the number of years by 10. Decades are useful for describing historical periods, tracking long-term trends, or talking about someone's age range. Ten decades make up one century (100 years).
Worked Example
Problem: How many years are in 3 decades?
Step 1: Write the conversion formula.
Step 2: Substitute 3 for the number of decades.
Step 3: Multiply to find the answer.
Answer: 3 decades is equal to 30 years.
Another Example
Problem: A tree was planted 50 years ago. How many decades ago was it planted?
Step 1: To convert years to decades, divide by 10.
Step 2: Substitute 50 for the number of years.
Step 3: Divide to get the result.
Answer: The tree was planted 5 decades ago.
Visualization
Why It Matters
Understanding decades helps you make sense of timelines in social studies and history classes, where events are often grouped by decade ("the Roaring Twenties," "the 1960s"). Converting between decades, years, and centuries also builds skill with place value and multiplying or dividing by powers of 10.
Common Mistakes
Mistake: Thinking a decade is 100 years.
Correction: A decade is 10 years. The unit for 100 years is a century. Remember: dec- means ten (like in decimal or decagon).
Mistake: Saying "the 2000s decade" runs from 2000 to 2010.
Correction: A named decade like "the 2000s" covers the ten years 2000 through 2009 — that is, all years sharing the same tens digit.