What is a Decade?

The decade equals exactly 10 years (3,652.5 days, using the Julian year) and is the standard unit for medium-term historical, cultural, and demographic discussion. Common uses include 'the 1960s,' 'the past decade,' and 'a decade-long study.' Census data, climate trends, generational analysis, and economic cycles are often reported in decade increments. The word derives from the Greek 'dekas' (group of ten), and the concept of grouping years by tens is ancient. The decade relates to the year (10 years = 1 decade), the century (10 decades = 1 century), and the millennium (100 decades = 1 millennium). 'Decade' calendars (the Babylonian and ancient Egyptian decans) used 10-day weeks, but the modern decade is purely a tens-of-years count. Famous historical decades include 'the Roaring Twenties,' 'the Sixties,' and 'the Aughts.'

Visual reference for how the decade relates to other time units. Each row links to the full converter for that pair.

• Historical-period references
• Long-term infrastructure planning
• Cultural and generational discussion

• ✓1 decade = 10 years = 3652.5 days.
• ✓Informally: "decade" often implies a named block (2020s) not a rolling 10-year window.
• ✓Rare in science; use "years" for precision.

• ✗Starting decades at year 0 vs. year 1 — "2020s" starts Jan 2020, but technically the third millennium's first decade began in 2001.
• ✗Using "decade" for financial or scientific precision — use years.
• ✗Assuming exact 10 × 365 days — forgets leap years.