The whole conversion in one line
These units are exact powers of ten. Divide years by 10 for decades, by 100 for centuries, by 1000 for millennia; multiply to go the other way. A decade is 10 years, a century is 10 decades, a millennium is 10 centuries.
Quick reference
| Years | Decades | Centuries | Millennia |
|---|---|---|---|
| 10 | 1 | 0.1 | 0.01 |
| 25 | 2.5 | 0.25 | 0.025 |
| 50 | 5 | 0.5 | 0.05 |
| 100 | 10 | 1 | 0.1 |
| 500 | 50 | 5 | 0.5 |
| 1000 | 100 | 10 | 1 |
A human generation is not a round unit: it averages about 25 years, or 2.5 decades, though recent generations have run closer to 15 to 20 years.
How decades and centuries are numbered
- Decades in speech. "The 1960s" means the years 1960 to 1969, grouped by their shared tens digit — the common convention.
- Centuries. Counted with ordinals and no year zero: the 20th century is 1901 to 2000, the 21st is 2001 to 2100.
- Millennia. The 2nd millennium ran 1001 to 2000; the 3rd began in 2001. The same off-by-one rule applies.
Common questions
How many years are in a decade, century and millennium?
A decade is 10 years, a century is 100 years, and a millennium is 1000 years. So one century is 10 decades and one millennium is 10 centuries or 100 decades.
How do I convert years to decades?
Divide the number of years by 10. So 50 years is 5 decades, 120 years is 12 decades, and 25 years is 2.5 decades. To go the other way, multiply decades by 10.
Why did the 21st century start in 2001, not 2000?
The AD/CE calendar has no year zero; it goes straight from 1 BC to 1 AD. So the 1st century ran from year 1 to year 100, which puts the 21st century at 2001 to 2100. The year 2000 was the last year of the 20th century.
