Decades and Centuries: Definitions and How to Count Them

Published October 08, 2026 · By the YearsAgoCalculator team

A decade is 10 years and a century is 100 years. The tricky part is where they start.

Calendar decades

In everyday speech "the 1990s" means 1990 to 1999. That is the common way to name decades.

Ordinal centuries

There was no year zero, so the 1st century ran from year 1 to year 100. That's why the 21st century formally began on January 1, 2001.

Counting back

"Three decades ago" simply means 30 years ago, and "two centuries ago" means 200 years ago. Try the decades ago and centuries ago calculators.

Try it in your language

The same tool is available in 20 languages, including Español, Français, Deutsch, 日本語 and العربية. See all calculators.