Roman Numeral Converter

Seven symbols and the rules that combine them

Roman numerals use seven letters: I for 1, V for 5, X for 10, L for 50, C for 100, D for 500 and M for 1,000. A numeral is read from left to right with the largest symbols first, and the values are added: MDCLXVI is 1,000 + 500 + 100 + 50 + 10 + 5 + 1, which is 1,666.

The subtractive rule shortens the long forms. A smaller symbol placed before a larger one is subtracted, so 4 is IV rather than IIII and 900 is CM rather than DCCCC. In the standard form only six such pairs exist — IV, IX, XL, XC, CD and CM — which means only I, X and C are ever subtracted, and each only from the next two symbols up. V, L and D are never subtracted and never repeated, and no symbol appears more than three times in a row.

Those rules give every number from 1 to 3,999 exactly one correct spelling, built one decimal place at a time. 1987 is M (1,000) + CM (900) + LXXX (80) + VII (7), which gives MCMLXXXVII; 2026 is MMXXVI; the largest, 3,999, is MMMCMXCIX. The breakdown under every result shows that construction.

Why the converter rejects some numerals you have seen

Type IIII and you are told it is not standard. That is true, but it is not the whole story. Clock and watch faces very often show four as IIII, a watchmakers' tradition with several competing explanations, and the Romans themselves wrote IIII, VIIII and similar additive forms freely; the strict subtractive rules are a later standardisation. The converter is strict because the modern form is what is expected in a copyright date, an outline, a monarch's regnal number or an engraving, where a non-standard spelling reads as a mistake.

Each rejected numeral gets a specific reason and, where one exists, the correct spelling. IC for 99 breaks the rule that I may only be subtracted from V and X; 99 is ninety plus nine, XC followed by IX, giving XCIX. VV is never correct, because it would simply be X. IIX for 8 subtracts two numerals at once, which is not allowed; 8 is VIII. MMMM would be 4,000, beyond the standard range altogether.

Lowercase numerals, used for the page numbers of a book's preface and for list items, are read exactly like capitals, and the Unicode Roman numeral characters such as Ⅻ are understood too. Spaces are ignored.

Numbers above 3,999

Standard notation stops at MMMCMXCIX because the next number would need a fourth M. For larger numbers the convention most often taught today is the vinculum: a bar over a numeral multiplies it by 1,000. V̅ is 5,000, X̅ is 10,000 and M̅ is a million, so 4,000 is I̅V̅ and 12,345 is X̅I̅I̅CCCXLV. Turn on vinculum notation to convert numbers up to 3,999,999 this way.

Treat it as a convention rather than a rule. The Romans wrote large numbers in several ways — the bar, a frame of lines around a numeral, and a system of nested Cs and reversed Cs such as CIↃ for 1,000 — and none of them was ever universal. The bar is also awkward to type: here it is the Unicode combining overline placed after each letter, which most fonts draw correctly but some older software shows as a separate mark.

Roman numerals have no zero and no negative numbers. Medieval scholars working out the date of Easter wrote N, for nulla, where their tables needed a zero; this converter simply tells you there is no numeral for it.

Writing a date in Roman numerals

Dates are one of the most popular uses — for wedding and anniversary engravings, the backs of watches, tattoos and memorials. Each part of the date is converted on its own: 12 April 2020 becomes XII for the day, IV for the month and MMXX for the year, joined by the separator you choose, as in XII.IV.MMXX.

The order matters more than people expect. Most of the world writes day, month, year; the United States writes month, day, year, which turns the same date into IV.XII.MMXX; and year-first order sorts correctly and is the least ambiguous. Choose the order your reader will assume, because IV.XII could mean 4 December or 12 April to someone guessing.

Full stops and middle dots are the traditional separators — Roman inscriptions divided words with a raised dot, the interpunct — while spaces and bullets suit a cleaner modern look. Shortening the year to two digits gives a compact result, 1995 as XCV, but fails for years ending in 00, because there is no numeral for zero; the converter says so rather than inventing one.

Before anything permanent, check every part against the breakdown shown with the result. A misplaced I is the classic tattoo mistake, and it is far easier to catch on a screen than on skin.