Roman Numeral Converter

Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500) and M (1,000). Values are added from left to right, so VI is 6 and XX is 20, but when a smaller numeral sits before a larger one it is subtracted instead, which gives IV = 4 and IX = 9. The subtractive rule is narrower than most people assume: I may only precede V or X, X may only precede L or C, and C may only precede D or M. Those pairings produce XL = 40, XC = 90, CD = 400 and CM = 900, and nothing else.

This converter translates between standard Arabic numbers and Roman numerals for values from 1 to 3,999. Going from decimal to Roman it works greedily, repeatedly subtracting the largest value that still fits, so 2026 breaks into 2000 (MM), 20 (XX) and 6 (VI) to give MMXXVI. The year 1492 becomes 1000 (M) plus 400 (CD) plus 90 (XC) plus 2 (II), or MCDXCII. Reading the other way you total the symbols and subtract wherever a lower value precedes a higher one, which is why MCMXCIX is 1000 + 900 + 90 + 9 = 1999 and MCMXCIV comes out at 1994.

Some strings are worth recognising on sight. Years appear constantly: MCM is 1900, MCMLXXXIV is 1984, MM is 2000, MMXX is 2020 and MMXXVI is 2026. British monarchs carry the numerals as regnal numbers, from Henry VIII and George VI to Elizabeth II and Charles III, and popes do the same, although Francis takes no number at all. Film sequels borrow the convention for Rocky II, Rambo III and Toy Story IV, and the Super Bowl numbers its editions the same way, with LVIII for 58 and LIX for 59 in 2025.

In British typesetting the numerals survive in a handful of specific places. Book chapters are numbered Chapter VII and so on, and formal outlines nest as I, A, 1, a, i, with the lower-case forms often carrying sub-points. Prefaces and front matter are paginated i, ii, iii so that the main text can begin again at page one. Royal Society publications use them, as do tombstones, building cornerstones (MMCC for 2200), film credits and copyright years, and the Olympic Games, where London hosted the XXX Olympiad in 2012. Clock faces are the most visible survivor, from Big Ben at Westminster to traditional watchmaking, where the dial runs I, II, III, V, VI, VII, VIII, IX, X, XI and XII, with IIII often used in place of IV for symmetry.

The system has real limits, which is why it lost out for calculation. There is no zero, because the concept did not exist in Roman mathematics, and no standardised way of writing numbers above 3,999 beyond an overline such as V̄ for 5,000, or simply repeating M. Fractions have no standard form either, since Romans counted in unciae, or twelfths. Arithmetic in the notation is awkward, and Hindu-Arabic numerals (0-9) had displaced it for practical maths by about the 13th century, leaving Roman numerals in ceremonial and decorative use.