About the Roman Numeral Converter
A two-way converter built around the standard rules — with strict validation and an optional breakdown.
What it is
The Roman Numeral Converter translates numbers to Roman numerals and Roman numerals back to numbers, live, as you type. Change either field and the other updates instantly — no button to press, no page reload needed.
Everything runs in your browser. Nothing is stored or sent to a server.
Features
Two-way conversion
Type a number to get the Roman numeral, or type a Roman numeral to get the number. Both fields update the other live.
Strict validation
Entered numerals are re-encoded and compared to the canonical form — non-standard forms like IIII, VV, IL, or IC are rejected with a clear message.
Breakdown view
Click "Show breakdown" to see each symbol group and its numeric value — for example, MCMXCIV = M (1000) + CM (900) + XC (90) + IV (4).
Full 1–3,999 range
Covers the entire standard Roman numeral range. Values outside this range (0, negatives, decimals, numbers above 3,999) are caught with a helpful message.
How Roman numerals work
Roman numerals are an additive and subtractive numeral system using seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), and M (1000). Symbols are generally written from largest to smallest, left to right, and their values are added together.
The subtractive rule is the key exception: when a smaller symbol appears immediately before a larger one, its value is subtracted instead of added. There are exactly six valid subtractive pairs in standard notation:
IV = 4
5 − 1
IX = 9
10 − 1
XL = 40
50 − 10
XC = 90
100 − 10
CD = 400
500 − 100
CM = 900
1000 − 100
Any numeral that doesn't follow these rules — such as IIII for 4 or IL for 49 — is non-standard. The converter validates by re-encoding the parsed number and checking that it matches the input exactly.
Completely free
No sign-up, no rate limit, no premium tier. Convert a single numeral or run through hundreds of values — it handles both instantly without any limit.