Roman Numeral Converter

Convert between whole numbers and Roman numerals from 1 to 3,999.

Worked example

Input: 1994. Method: write the number as thousands, hundreds, tens and units, using the largest symbol that fits each time. Substitution: 1,000 + 900 + 90 + 4 is M + CM + XC + IV. Result: MCMXCIV.

How it works

The seven symbols are I (1), V (5), X (10), L (50), C (100), D (500) and M (1,000). In standard form a smaller symbol before a larger one is subtracted, but only in the six pairs IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900), and no symbol is repeated more than three times in a row. This converter covers 1 to 3,999; zero has no Roman numeral, and larger numbers need an overline to multiply a numeral by 1,000, which cannot be written reliably in plain text. It accepts only standard form: a numeral such as IIII or IL is rejected with a reason rather than silently read as a different number. Lowercase input is accepted and shown in capitals.

What this result means

The converter shows the standard Roman numeral for a whole number, or the number a standard Roman numeral stands for, together with the symbols and their values.

Common mistakes

Writing four of the same symbol in a row. 4 is IV, not IIII, and 9 is IX, not VIIII.
Subtracting the wrong symbols. Only I, X and C are subtracted, and only in the pairs IV, IX, XL, XC, CD and CM: 49 is XLIX, not IL, and 99 is XCIX, not IC.

Assumptions and limitations

Covers 1 to 3,999. Larger numbers need an overline to multiply a numeral by 1,000, which cannot be written reliably in plain text; 0 has no Roman numeral.
Only standard form is accepted. A malformed numeral is rejected with a reason rather than silently read as a different number. Lowercase letters are accepted and shown in capitals.