Number to Roman Numeral Converter

How Roman numerals encode a number

Roman numerals are an additive-subtractive system built from seven letters: I (1), V (5), X (10), L (50), C (100), D (500), and M (1000). Most of the time you simply add symbols from largest to smallest — MMXVI is 1000 + 1000 + 10 + 5 + 1 = 2016. The clever part is the subtractive rule: to avoid writing four identical symbols in a row, a smaller symbol placed immediately before a larger one is subtracted. So 4 is IV (5 − 1), not IIII, and 9 is IX (10 − 1). Only six subtractive pairs are valid in standard notation: IV, IX, XL, XC, CD, and CM.

This converter walks both directions. To turn a number into Roman, it greedily subtracts the largest value it can at each step from a fixed table and appends that symbol, repeating until nothing remains — which is exactly how the standard form is constructed. To read a Roman numeral back into a number, it scans left to right and, for each symbol, checks the one after it: if the current value is smaller than the next, it subtracts; otherwise it adds.

A worked example: 1994

Enter 1994 and watch the greedy algorithm work. The largest value it can take is 1000, so it writes M and 994 remains. Next comes the subtractive pair 900, written CM, leaving 94. Then 90 is XC, leaving 4. Finally 4 is IV. Concatenated, that is MCMXCIV. Reading it back, the scanner sees M (1000, add), C before M so subtract 100, M add 1000, X before C subtract 10, C add 100, I before V subtract 1, V add 5 → 1000 − 100 + 1000 − 10 + 100 − 1 + 5 = 1994. The two processes are mirror images.

Beyond 3,999: the vinculum

Classic Roman numerals stop at 3,999 (MMMCMXCIX) because there is no single symbol for 5,000 or 10,000. To go higher, ancient and medieval scribes drew a vinculum — a bar over a numeral that multiplies its value by 1,000. This tool supports that extended notation when converting numbers up to 3,999,999, producing barred symbols such as V̄ for 5,000 and X̄ for 10,000. Note an important asymmetry: the number-to-Roman direction handles the full extended range, but the Roman-to-number direction only parses the seven standard letters and will reject barred input, so it is best used for conversions in the classic 1–3,999 range.

Genuine use cases

• Copyright and film dates. Movie credits and TV broadcasts traditionally print the year in Roman numerals. Decoding MMXXIII tells you it is 2023.
• Book and document structure. Front matter (prefaces, tables of contents) is conventionally paginated in lowercase Roman numerals, and chapters or appendices are often numbered I, II, III.
• Clock faces, monuments, and cornerstones. Many clocks and engraved buildings record dates in Roman numerals; this tool lets you read them.
• Regnal and event names. Monarchs (Louis XIV), Super Bowls, and Olympiads are numbered in Roman, and converting helps you place them in sequence.

Common mistakes the rules prevent

The most frequent error is repeating a symbol more than three times: IIII, XXXX, or CCCC are non-standard, which is precisely why the subtractive pairs exist. Another is illegal subtraction — only I, X, and C may be subtracted, and each only from the next one or two larger symbols, so IL for 49 is wrong (the correct form is XLIX). The symbols V, L, and D are never subtracted or repeated. Because the generator builds output from the canonical table, it always produces the single correct standard form, making it a reliable check when you suspect a date or numeral you have read elsewhere was written incorrectly.

A note on history and privacy

Each conversion is saved to a short history list (the last 20) so you can copy earlier results, and that list is stored only in your browser's local storage — it never leaves your device and there is no server involved in any conversion. You can clear the history at any time, and turning off your network changes nothing, because all the arithmetic runs locally.