Convert Roman Numerals to Numbers and Back Online

You are writing a test fixture and the spec says the copyright year should be MMXXVI. Or you are parsing a Super Bowl number out of a string and need an integer. Roman numerals show up in places that seem archaic until you realize they are still baked into outlines, chapter headings, movie credits, and king lists.

The quick fix is an online converter. The durable fix is understanding the algorithm so you can handle the edge cases that break the cheap ones.

How Roman numerals actually work

Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), M (1000). You build numbers by combining them, and the rule that trips people up is subtractive notation: IV means 4, not 6. IX means 9. XL means 40. CD means 400.

The pattern is consistent. A smaller numeral before a larger one means subtract. Everything else means add. That single rule covers the entire system.

const VALUES = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 };

function romanToInt(roman) {
  let result = 0;
  for (let i = 0; i < roman.length; i++) {
    const current = VALUES[roman[i]];
    const next = VALUES[roman[i + 1]] || 0;
    result += current < next ? -current : current;
  }
  return result;
}

Walk through MCMXCIV with that function: M (1000) + C (100 before M, so subtract 100) + M (1000) + X (10 before C, so subtract 10) + C (100) + I (1 before V, so subtract 1) + V (5). Result: 1994. The loop is O(n) and handles any valid Roman numeral in one pass.

Going the other direction: numbers to Roman numerals

The reverse is less obvious because you are matching against decreasing thresholds. Start with the largest value and work down, appending symbols and subtracting until you reach zero.

function intToRoman(num) {
  const pairs = [
    [1000, 'M'], [900, 'CM'], [500, 'D'], [400, 'CD'],
    [100, 'C'], [90, 'XC'], [50, 'L'], [40, 'XL'],
    [10, 'X'], [9, 'IX'], [5, 'V'], [4, 'IV'], [1, 'I']
  ];
  let result = '';
  for (const [value, symbol] of pairs) {
    while (num >= value) {
      result += symbol;
      num -= value;
    }
  }
  return result;
}

The trick is including the subtractive pairs (900, 400, 90, 40, 9, 4) in the lookup table. Without them, you would need extra conditional logic to handle the IV, IX, XL cases separately. With them, the greedy loop just works.

intToRoman(1994); // MCMXCIV
intToRoman(2026); // MMXXVI
intToRoman(3999); // MMMCMXCIX

That 3999 ceiling is real. Standard Roman numerals have no representation for 4000 or above without extending the system with overlines or special notations. Most converters silently cap at 3999. If you need larger values, you are building a custom system, not using Roman numerals.

Where developers actually encounter Roman numerals

The most common spots are test data, legacy system outputs, and display formatting. A few examples:

• Copyright notices in UI. Many templates render MMXXVI instead of 2026 in the footer. If you are building a CMS or template engine, you need this conversion to validate or parse user input.
• Outline and chapter numbering. Word processors and legal documents use Roman numeral outlines (I, II, III, IV). Parsing these from structured text means handling both uppercase and lowercase (iv vs IV).
• Super Bowl and World Cup numbering. These events are branded with Roman numerals. Any sports API that parses event names needs to handle them.
• King and pope names. Henry VIII, Louis XIV, Pope Benedict XVI. Historical databases and name parsers run into these constantly.

The lowercase variants are worth handling because real input is messy. A quick fix:

function normalizeRoman(input) {
  return input.toUpperCase().replace(/[^IVXLCDM]/g, '');
}

That strips anything that is not a valid Roman numeral character and uppercases the rest. It is not validation (you still need to check the result makes sense), but it cleans up the common "iv" and "ix" inputs.

Validation: catching bad input

A Roman numeral is valid if it follows the subtractive notation rules and does not repeat symbols more than three times in a row. A quick regex check:

function isValidRoman(str) {
  return /^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/.test(str);
}

That regex matches everything from I through MMMCMXCIX (1 through 3999). It rejects things like IIII (should be IV), VV (not valid), and XM (not valid). For most use cases, this is enough to separate valid input from garbage before you attempt conversion.

When you need more than a quick conversion

If you are building something that handles Roman numerals repeatedly (a parser, a formatter, a test generator), the code above is a starting point. You will probably want:

• Case-insensitive input handling
• Validation before conversion
• Error messages that say what went wrong, not just "invalid"
• Support for the (subtractive) notation used in some traditions (like IIX for 8 instead of VIII)

For a quick one-off conversion, a browser-based tool is faster than writing a script. The advantage of running it locally is that the input never leaves your machine. If you are converting a Roman numeral from a legal document, a historical record, or a test fixture that contains real data, that matters.

Roman numerals are a solved problem in terms of the algorithm. The unsolved part is handling messy real-world input without surprising your users. The code here gives you the foundation. The edge cases are where the actual work lives.