18 in Roman Numerals = XVIII
18 in Roman numerals is XVIII. At five characters it is the longest numeral anywhere in the teens.
What is 18 in Roman numerals?
XVIII is X (10) + V (5) + I + I + I, with the three strokes at the repetition limit.
The tail is VIII, the numeral for eight, which is itself the longest single-digit form. Stacking the two gives the teens their longest member.
How XVIII is formed
Reading left to right, 18 breaks into these additive parts. Each Roman symbol is added in turn to reach the total.
X + V + I + I + I = 10 + 5 + 1 + 1 + 1 = 18.
Why 18 is written XVIII
Eighteen shows why Roman numerals are not compact. Nineteen, a larger number, is written in only three characters as XIX because it can borrow a subtraction, while eighteen has to spell every unit out. Length in this system tracks the shape of the number, not its size.
Where 18 in Roman numerals is used
- United States constitutional amendments: Amendment XVIII, ratified in 1919, established Prohibition, and it is the only amendment later repealed outright, by Amendment XXI in 1933.
- Milestone birthdays: Eighteen is the age of majority in much of the world, which makes XVIII one of the most requested numerals for eighteenth-birthday gifts and engraving.
- Golf: A full round is eighteen holes, and course signage and clubhouse boards often set the number in Roman form.
- Super Bowl XVIII: Played in January 1984, won by the Los Angeles Raiders.
Common mistakes writing 18 in Roman numerals
- IIXX is wrong. An attempt to subtract two from twenty. Subtraction only works with a single symbol and only in the six standard pairs, so this is not valid.
- XIIX is wrong. Forms like this do turn up in ancient inscriptions, which were never fully standardised, but the modern standard numeral for eighteen is XVIII.
Good to know about 18
- Eighteen, XVIII, is the age of majority in many countries, a milestone-birthday numeral.
Convert another number
18 is written XVIII in Roman numerals.
Frequently asked questions
How do you write 18 in Roman numerals?
Write 18 as XVIII, combining X + V + I + I + I. Start from the largest value that fits and work down until nothing is left over.
Which Super Bowl is XVIII?
Super Bowl XVIII is the 18th Super Bowl, played in 1984. The NFL sets its championship number in Roman numerals, which is why so many people look up values like XVIII.
How do you say XVIII in words?
XVIII is 18, spoken as "eighteen". The numeral is read as the single value 18, not as its separate letters.
What are 17 and 19 in Roman numerals?
17 is XVII and 19 is XIX. Neighbouring numbers often change length in surprising ways, which is why each has its own page.
Related conversions
- 36 in Roman numerals is XXXVI, double 18.
- 19 in Roman numerals is XIX, one more than 18.
- 17 in Roman numerals is XVII, one less than 18.