About Roman Numeral Converter
Convert numbers to Roman numerals and back. Bidirectional, validates inputs, supports 1–3999. Free, no sign-up.
How to use
- Type a number in the top input — the Roman numeral updates live as you type. Valid range is 1 to 3999. The converter rejects fractions, negatives, and zero with helpful error messages, since classical Roman numerals had no concept of zero or decimals.
- Or type a Roman numeral in the bottom input — the Arabic number updates live. Letters are case-insensitive (mcmlxxxiv works the same as MCMLXXXIV). Invalid forms like IIII, VX, or IC are caught and flagged so you can fix them.
- Try a few common conversions to learn the pattern: 2026 → MMXXVI, 1776 → MDCCLXXVI, 49 → XLIX, 99 → XCIX. Each output uses the canonical subtractive pairs (IV, IX, XL, XC, CD, CM) — never IIII, VIIII, or LL.
- Use the quick-reference grid at the bottom to check your work. The seven base symbols are M=1000, D=500, C=100, L=50, X=10, V=5, I=1 — every other Roman numeral is built by adding or subtracting these in left-to-right order.
- Hit the copy button next to either field to send the result to your clipboard — perfect for pasting into a Word document, a watch face design, or a movie sequel poster mockup. Both fields stay synchronized so you always see both representations.
- The tool enforces canonical form on reverse conversion: typing IIII won't return 4 because IIII isn't standard. This makes it useful for verifying numerals on monuments, book chapters, Super Bowl logos, and movie copyright dates where exact convention matters.
- For values above 3999, the converter stops because there's no agreed-upon ASCII representation — classical Romans used overlines (V̄ = 5,000) which aren't standard letters. If you need MMMM or higher, you've probably hit the practical limit of where Roman numerals are still useful.
Frequently asked questions
How do Roman numerals work?
Roman numerals use seven letters: I (1), V (5), X (10), L (50), C (100), D (500), M (1000). Numbers are formed by adding values from left to right (VI = 6) except when a smaller numeral precedes a larger one — then it's subtracted (IV = 4). Only six subtractive pairs are valid: IV, IX, XL, XC, CD, CM. Repeating a symbol up to three times multiplies it (XXX = 30, but not XXXX = 40 — that's XL).
What's the largest standard Roman numeral?
3999 = MMMCMXCIX. To represent larger numbers, classical Romans used overlines: V̄ meant 5,000 (V × 1000), and there were also vinculum and apostrophus systems for very large values. None of those are part of the standard ASCII letter set, so most modern converters — including this one — cap at 3999. If you need to convert something like a year written 2024, MMXXIV is well within range.
Why is 1990 written as MCMXC instead of MXM?
Subtractive notation only allows specific pairs: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900). MXM would skip from M (1000) to a non-canonical form. The rule: the subtractive numeral must be one or two orders of magnitude smaller (I before V or X; X before L or C; C before D or M). So 1990 = M (1000) + CM (900) + XC (90) = MCMXC.
Why do clock faces use IIII instead of IV?
Tradition. Many clockmakers prefer IIII for visual balance — it pairs symmetrically with VIII on the opposite side of the dial. Both forms are technically readable but IV is the standard mathematical notation. This converter rejects IIII because it's verifying canonical Roman numerals, not clock-face conventions.
Are Roman numerals still used today?
Yes — for movie sequels (Rocky IV), book chapters, monarchs (Elizabeth II), Super Bowls (LVIII), copyright dates on TV shows (MMXXIV), Olympic Games numbering, watch dials, and outline numbering in legal documents. They're never used for arithmetic anymore, but they signal formality, tradition, or a numbered series.
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