Roman Numeral Converter

The seven Roman numerals

As Encyclopaedia Britannica summarises the system, a symbol placed after one of equal or greater value adds its value, and a symbol placed before one of greater value subtracts it. In the standard modern form only six subtractive pairs are used:

The rules the converter enforces

1. Write from largest to smallest, apart from the six subtractive pairs above.
2. I, X, C and M repeat at most three times in a row. Four is IV, not IIII; forty is XL, not XXXX.
3. V, L and D never repeat (VV would just be X) and are never subtracted (VX is not a way to write 5).
4. Only I, X and C subtract, and only from the next two larger symbols: I from V and X, X from L and C, C from D and M. So 99 is XCIX, not IC.

When you type a numeral that breaks a rule, the converter keeps your input, explains which rule it breaks and, where it can, suggests the standard form. For example, IIII gets: “IIII is not a valid standard Roman numeral: I can repeat at most three times in a row; use subtractive notation instead (IV for 4 or IX for 9). Some clock faces show IIII, but the standard form of 4 is IV. If you meant 4, the standard form is IV.” and VX gets: “VX is not a valid standard Roman numeral: V is never written before a larger numeral; only I, X and C are used for subtraction.”

Worked example: 1994

Split the number into thousands, hundreds, tens and ones, convert each part, then join them:

M + CM + XC + IV = MCMXCIV. Reading it back works the same way: M = 1000, CM = 900, XC = 90, IV = 4, total 1,994.

Years in Roman numerals

Films, buildings and sporting events often carry the year this way. Type any year into the converter, or several numbers at once — every whole number in the box is converted and separators such as slashes and dots are kept, which is how the date box builds dates like X.VII.MMXXVI.

Why stop at 3,999?

Because M can appear only three times, the largest standard numeral is MMMCMXCIX (3,999). Romans and later writers handled bigger numbers with other conventions, most commonly a bar over a numeral (a vinculum) to multiply it by 1,000, but these are not part of the standard form and are hard to type, so the converter reports numbers of 4,000 and above instead of producing a non-standard result. There is also no zero and no negative numbers in the system.

IIII on clock faces

Many clocks and watches show four as IIII rather than IV. Historically the additive forms IIII and VIIII were also used, and the subtractive style became standard later. This converter is deliberately strict — it accepts only the standard form — so it is a reliable check for homework, tattoos and engraving. For a printable list of 1 to 100 and key values up to 3,999, see the Roman numerals chart.

Common mistakes the converter catches

Each “standard form” in this table was produced by the converter itself from the value the mistaken numeral was presumably meant to show.