A century is a hundred years. Simple enough — and yet it trips up almost everyone: why is 1812 in the 19th century rather than the 18th? Which century is the year 1900? And why do years BC run "backwards"? Let's go through it.
What BC and AD mean
AD stands for Anno Domini, Latin for "in the year of the Lord": years counted from the traditional date of the birth of Jesus. BC means "before Christ". Many historians and scientists prefer the religion-neutral CE ("Common Era") and BCE ("Before Common Era") — the numbers are exactly the same, only the labels differ.
The system was devised in the 6th century by the monk Dionysius Exiguus and became standard in Europe by the 11th century. Its starting point is a convention: most historians today date the Nativity of Jesus to around 6–4 BC.
Why there is no year zero
Dionysius worked before the number zero reached European mathematics, so 1 BC is followed directly by AD 1. That is why historians and astronomers count differently: in astronomical year numbering, 1 BC is year 0 and 2 BC is year −1. It also means that to count the years between a BC date and an AD date, you add them and subtract one: from 44 BC to AD 14 is 44 + 14 − 1 = 57 years.
How to work out the century from a year
Drop the last two digits and add one. If the year ends in 00, don't add one.
- 1812 → 18 + 1 = 19th century
- 1453 → 14 + 1 = 15th century
- 988 → 9 + 1 = 10th century
- 2026 → 20 + 1 = 21st century
- 1900 → ends in 00 → 19th century (its last year)
- 2000 → 20th century; the 21st century began on 1 January 2001
Why? Because counting starts at year 1. The first century is years 1–100, the second is 101–200, and so on — every century ends with a "round" year and begins with a year ending in 01.
How to count centuries BC
Years BC count down: the bigger the number, the older the date. The rule is the same — drop the last two digits and add one:
- 776 BC (First Olympic Games) → 8th century BC
- 331 BC (Battle of Gaugamela) → 4th century BC
- 44 BC (Assassination of Julius Caesar) → 1st century BC
- c. 2560 BC (Great Pyramid of Giza) → 26th century BC
Inside a BC century the years run backwards too: the 1st century BC begins in 100 BC and ends in 1 BC. So "early 5th century BC" means years close to 500 BC, not 401 BC.
Table: which years are in each century
| Century | Years |
|---|---|
| 30th century BC | 2901 BC … 3000 BC |
| 20th century BC | 1901 BC … 2000 BC |
| 10th century BC | 901 BC … 1000 BC |
| 5th century BC | 401 BC … 500 BC |
| 3rd century BC | 201 BC … 300 BC |
| 1st century BC | 1 BC … 100 BC |
| 1st century | 1 – 100 |
| 5th century | 401 – 500 |
| 10th century | 901 – 1000 |
| 15th century | 1401 – 1500 |
| 18th century | 1701 – 1800 |
| 19th century | 1801 – 1900 |
| 20th century | 1901 – 2000 |
| 21st century | 2001 – 2100 |
Millennia
A millennium is 1,000 years and is counted the same way: the first millennium AD is years 1–1000, the second 1001–2000, and the third began in 2001. Before Christ, the 3rd millennium BC runs from 3000 to 2001 BC — when the first states appeared, such as the Sumerian City-States and Egypt.
Centuries on the timeline
On the Panchronia globe, the timeline at the bottom is marked in centuries and millennia. Jump to the 5th century BC to see that Pericles, the Buddha and Confucius were contemporaries, or open the 15th century and compare Europe, Russia and China in the same years. For the big picture, see the World history timeline: major events by century and region.
Questions
What century is the year 1900?
The 19th century — its last year. The 19th century covers 1801–1900; the 20th began on 1 January 1901.
What century are we in?
The 21st century, which began on 1 January 2001 and will end on 31 December 2100.
Is there a year 0?
No. In the BC/AD system 1 BC is followed directly by AD 1. Astronomers use a year 0 (equal to 1 BC) for calculations.
What is the difference between BC/AD and BCE/CE?
Only the labels. BCE ("Before Common Era") equals BC and CE ("Common Era") equals AD; the year numbers are identical.