How to Write Centuries in Roman Numerals

The rule works for every year AD:

• 1492 → 14.92 → 15th century → XV
• 1900 → 19.00 → exactly 19, so the 19th century → XIX
• 1901 → 19.01 → 20th century → XX
• 2026 → 20.26 → 21st century → XXI

The only years that trip people up are the round ones. 1900 ends the 19th century, it doesn't start the 20th. If the year ends in 00, the century is the first two digits; otherwise it is the first two digits plus one.

The calendar has no year zero. NRICH, the mathematics project of the University of Cambridge, explains that because "there is no such thing as year zero", the first century was complete only at the end of AD 100, and the 21st century and the third millennium began together on 1 January 2001 (NRICH).

So every century runs from a year ending in 01 to a year ending in 00:

The reverse works the same way. A century number n starts in year (n − 1) × 100 + 1 and ends in year n × 100:

• XV → 15th century → 1401 to 1500;
• XVIII → 18th century → 1701 to 1800;
• XXI → 21st century → 2001 to 2100.

If you see a century in Roman numerals, read the numeral first (XVIII = 18) and then apply the rule.

Millennia follow the same logic in groups of 1,000. The first millennium runs from AD 1 to 1000, the second from 1001 to 2000 and the third from 2001 to 3000. NRICH points out that the 21st century and the third millennium began on the same day, 1 January 2001. A millennium in Roman numerals is written like a century: the third millennium is the III millennium.

The arithmetic is the same everywhere; the spelling isn't.

• The letter l instead of I. "xvlll" looks like XVIII, but the small l is a different character. A converter won't read it as a numeral, so type the capital I.
• Writing the suffix in the wrong place. In French the ordinal mark goes after the numeral, as in the OQLF examples "IIIe Reich" and "Ve arrondissement": XVIIIe siècle.
• Using the year instead of the century. "Siglo MCM" is the year 1900, not a century. The 20th century is XX.

BC centuries count backwards from year 1 BC, again with no year zero: the 1st century BC runs from 100 BC to 1 BC, the 5th century BC from 500 BC to 401 BC. The Roman numeral is the same as for the matching century AD; only the label changes (V a.C. in Spanish, Ve siècle av. J.-C. in French).