Decade, Century and Millennium Calculator
Enter a year and see which decade, century and millennium it belongs to — and why 2000 was still the 20th century.
How it works
century = ⌈year ÷ 100⌉ · millennium = ⌈year ÷ 1000⌉ · decade = ⌊year ÷ 10⌋ × 10 · the Nth century runs from (N−1)×100+1 to N×100
Because the calendar has no year zero, centuries and millennia are counted from year 1: the 1st century is years 1 to 100, so the Nth century ends on a round hundred and begins the year after the previous one. That is why 2000 was the last year of the 20th century and 2001 opened the 21st — the ceiling division captures this. Decades, by contrast, are named informally by their tens digit, so "the 2020s" simply means 2020 through 2029.
Worked example
For 2026: 2026 ÷ 100 rounded up is 21, so it is the 21st century (2001–2100); 2026 ÷ 1000 rounded up is 3, the 3rd millennium (2001–3000); and its decade is the 2020s.
Frequently asked questions
Why is 2000 the 20th century, not the 21st?
Counting starts at year 1, so the 1st century is 1–100 and every century ends on a round hundred. The 20th century is 1901–2000, which means 2000 is its final year and 2001 begins the 21st.
How is a decade defined here?
The everyday way: by the tens digit. The 2020s are 2020–2029. Strict "ordinal" decades counted from year 1 would run 2021–2030, but almost no one uses that in speech.
Does this work for BCE years?
No — it is for years in the Common Era (CE/AD), from year 1 onward. BCE numbering runs backwards and would need its own handling.
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This calculator is for educational purposes. Double-check important results before acting on them.