18 in Roman numerals at a glance
- Roman numeral
- XVIII
- Lowercase
- xviii
- Letters
- 5
- Parts
- X · V · I · I · I
- Subtractive pairs
- None
- In words
- eighteen
- Latin name
- duodeviginti
How XVIII is built
Roman numerals are written one decimal place at a time, from the largest place to the smallest. Converting each digit of 18 separately and joining the pieces gives XVIII:
| Place | Digit | Value | Numeral |
|---|---|---|---|
| Tens | 1 | 10 | X |
| Ones | 8 | 8 | VIII |
Reading it back, left to right:
- X = 10 → running total 10
- V = 5 → running total 15
- I = 1 → running total 16
- I = 1 → running total 17
- I = 1 → running total 18
The rules XVIII follows
- Largest to smallest. XVIII uses the symbols X, V and I (5 characters in all), and the values never increase from left to right except inside a subtractive pair.
- No subtraction needed. None of the digits of 18 is a 4 or a 9, so XVIII is purely additive: just add the symbol values.
- Repetition. III repeats I three times. I, X, C and M may repeat up to three times in a row; V, L and D never repeat.
Where you will see XVIII
- The U.S. Constitution. Amendments are often cited in Roman numerals: Amendment XVIII established Prohibition (ratified in 1919).
- Monarchs and popes. Regnal numbers use Roman numerals, as in Louis XVIII, restored to the French throne after Napoleon.
- The modern Summer Games. They are officially numbered by Olympiad; the Games of the XVIII Olympiad belong to 1964: Tokyo.
- Championship games. Pro football numbers its championship game with Roman numerals, starting from game I in January 1967. Game XVIII was played in early 1984.
- Lists and outlines. Book chapters, acts of a play, outline headings and the preface pages of many books (in lowercase: xviii) are numbered this way.
Latin itself counted backwards here: duodeviginti means “two from twenty,” the same subtractive idea that Roman numerals use in pairs such as IX and XC — although the numeral for 18 is written XVIII, not by subtracting from 20.
Numbers near 18 in Roman numerals
Frequently asked questions
How do you write 18 in Roman numerals?
18 is written XVIII. Break it into X (10), V (5), I (1), I (1), I (1) and add the parts: 10 + 5 + 1 + 1 + 1 = 18.
What is 18 in Latin?
The Latin cardinal number 18 is “duodeviginti.” The Romans wrote it as XVIII.
What is XVIII plus one?
XVIII + I = XIX (19). Adding one can change the whole ending of a numeral, which is why it helps to convert to digits, add, and convert back.