Roman Numeral Converter

1987 = MCMLXXXVII

How it works

• Read the decimal value and matched the largest Roman unit repeatedly.
• Confirmed the value lies in the canonical 1–3999 range.
• Reported the converted value.

1987 = MCMLXXXVII

• Standard-form Roman numerals use subtractive pairs (IV, IX, XL, XC, CD, CM) and no symbol repeats more than three times.

Methodology

1. For number → Roman, repeatedly subtract the largest representable unit (including the subtractive pairs CM, CD, XC, XL, IX, IV).
2. For Roman → number, scan left to right matching the same unit table in descending order, so subtractive pairs are consumed before their components.
3. Re-encode the parsed value and require it to equal the input, so only canonical numerals are accepted.
4. Constrain the range to 1–3999, the span the seven standard symbols cover unambiguously.

Usage

1. Choose the direction: number to Roman, or Roman to number.
2. Enter the value.
3. Read the converted value and whether the Roman form is canonical.

Examples

These are examples, not current values: they use fixed inputs, while the interactive tool computes live results in your browser.

1987 to Roman

{
  "mode": "to-roman",
  "value": "1987"
}

4 to Roman

{
  "mode": "to-roman",
  "value": "4"
}

3999 to Roman

{
  "mode": "to-roman",
  "value": "3999"
}

MCMXCIV to decimal

{
  "mode": "to-decimal",
  "value": "MCMXCIV"
}

Limitations

The seven symbols

Roman numerals use seven symbols — I (1), V (5), X (10), L (50), C (100), D (500) and M (1000). Values are formed by adding symbols, except where a smaller symbol precedes a larger one and is subtracted (IV, IX, XL, XC, CD, CM).

That subtractive rule and the no-more-than-three-repeats rule are what make the notation canonical; the tool enforces both so its output round-trips exactly.