What this calculator does
It tells you which century and millennium a year belongs to, and the first and last year of each. It handles BC years, and it points out the years that trip people up, like 1900 and 2000.
How centuries are counted
Our calendar starts at AD 1, not year 0, so the 1st century is the years 1 to 100. Each century is 100 years after that:
Millennium = ⌈year ÷ 1,000⌉
The nth century runs from (n − 1) × 100 + 1 to n × 100
So the 20th century is 1901 to 2000, and the 21st century began on January 1, 2001, as the U.S. Naval Observatory explains. The 3rd millennium runs from 2001 to 3000.
BC years count backward with the same rule. The year before AD 1 is 1 BC, and the 1st century BC is 100 BC to 1 BC. Astronomers number years differently: they call 1 BC "year 0", 2 BC "year −1", and so on. So their year number is 1 − the BC year.
Worked examples
- 1066: 1066 ÷ 100 = 10.66, rounded up is 11, so it's the 11th century (1001–1100). 1066 ÷ 1,000 rounds up to 2, so it's the 2nd millennium.
- 2000: 2000 ÷ 100 = 20 exactly, so it's the last year of the 20th century, not the first of the 21st.
- 44 BC: 44 ÷ 100 rounds up to 1, so it's the 1st century BC. Its astronomical year number is −43.
Quick reference
| Century | Years | Often called |
|---|---|---|
| 18th century | 1701–1800 | the 1700s |
| 19th century | 1801–1900 | the 1800s |
| 20th century | 1901–2000 | the 1900s |
| 21st century | 2001–2100 | the 2000s |
| 1st century BC | 100 BC–1 BC | — |
"The 1900s" vs. "the 20th century"
In everyday speech, "the 1900s" means 1900 to 1999. The strict 20th century is 1901 to 2000. The two overlap almost entirely but differ at the ends. The calculator shows both, so you can pick the one you need.
Limitations
- This is about counting. Historians sometimes use "the long 19th century" or similar labels that don't follow the strict 100-year rule.
- Years before AD 1 are counted the historical way, with no year 0.