Roman Numeral Converter

Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000). Numbers are formed by combining these symbols according to two rules: when a smaller value appears before a larger one, it is subtracted (IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900); otherwise, values are added left to right (XVII = 17, CLXIII = 163). The largest standard Roman numeral is MMMCMXCIX (3,999). For larger numbers, a bar over a symbol multiplies its value by 1,000 — V̅ = 5,000, X̅ = 10,000. This converter handles bidirectional conversion: enter a decimal number to get the Roman numeral, or enter Roman numerals to get the decimal equivalent. Common modern uses include clock faces (which often use IIII instead of IV for aesthetic balance), movie copyright years (MMXXIV = 2024), Super Bowl numbering (Super Bowl LVIII = 58), and chapter numbering in books. Validation catches invalid combinations like VV (should be X) or IC (not standard — 99 is XCIX).

Despite being over 2,000 years old, Roman numerals appear everywhere in modern life. Monarchs and popes use them for regnal numbers (King Charles III, Pope Francis I). Outlines in legal documents and academic papers use Roman numeral hierarchies (I, II, III for major sections; i, ii, iii for subsections). Chemistry uses Roman numerals in compound names to indicate oxidation states: iron(III) oxide (Fe₂O₃) versus iron(II) oxide (FeO). Music theory denotes chord functions with Roman numerals — a I-V-vi-IV progression in the key of C means C major, G major, A minor, F major. Building cornerstones often display the construction year in Roman numerals. The converter is particularly useful for students studying Latin, history, or music theory, and for anyone who encounters these numbers in film credits, legal documents, or architectural inscriptions. All conversions run through a straightforward algorithm in your browser — no external APIs needed for what is fundamentally a mapping between two number systems.