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Roman Numeral Converter

Convert numbers between Roman numerals and decimal values.

Roman Numeral Converter

Roman:

Decimal:

What This Tool Does

  • Roman Numeral Converter translates between Arabic numerals (1, 2, 3...) and Roman numerals (I, II, III...).
  • Convert numbers for documents, lists, historical data, and classic numbering systems.

Usage

  1. Enter a decimal number (e.g., 42) or Roman numeral (e.g., XLII).
  2. The converter automatically detects input type and converts to the opposite.
  3. Copy converted output for use in documents, outlines, or legacy systems.

Examples

  • Convert chapter numbers (1 → I, 2 → II) for book outlines and document headers.
  • Translate Roman numeral dates or centuries into decimal for analysis.
  • Validate Roman numeral parsing from historical datasets and documents.
  • Normalize movie release years (2024 → MMXXIV) for film database entries.

Limitations

  • Results should be validated in your target runtime before production use.
  • Extremely large input payloads may be constrained by browser memory and performance limits.

Common Mistakes

  • Invalid subtractive pairs: IL (49) is invalid; use XLIX. LC (=lost), XM (=invalid). Only I, X, C can be subtractive.
  • Repeating subtractive symbols: IIV (3 via subtraction) is invalid; use III. Only single symbols can be subtracted.
  • Numerals > 3999 without vinculum: 4000 as MMMM is uncommon/invalid. Use vinculum (IV̄) or express as 4 separate thousands.
  • Case sensitivity issues: Roman numerals use uppercase only (standard). Lowercase i, v, x should convert to uppercase I, V, X.
  • Confusing additive/subtractive: VV = invalid (cannot repeat V). IX = 9 is correct; not IXI.
  • Whitespace or symbols in input: Spaces, punctuation, or symbols break parsing. Strip them before entering.

Technical Reference Guide

  • I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000.
  • Additive: III = 3 (1+1+1), XX = 20 (10+10), MM = 2000 (1000+1000).
  • Subtractive: IV = 4 (5-1), IX = 9 (10-1), XL = 40 (50-10), CM = 900 (1000-100).
  • Subtractive rules: I can precede V, X (not L, C, D, M). X can precede L, C. C can precede D, M only.
  • Range: Standard conversions support 1–3999 (I–MMMCMXCIX). Values > 3999 use vinculum (overline) for × 1000 multiplication.
  • No zero: Roman numerals have no symbol for zero; system is not positional.
  • Strict format: Symbols must be arranged in descending value order, with subtractive pairs only in valid combinations.

Specifications & Standards

Roman Numeral System (Wikipedia)Roman Numeral Rules and Examples

FAQ

  • What range is supported?

    Standard range is 1–3999. Values outside this range require vinculum (overline bar) notation or may not be supported.

  • Does the converter accept invalid Roman syntax?

    Malformed patterns are flagged with error messages so you can correct the input.

  • Can I input Roman numerals in lowercase?

    Standard Roman numerals are uppercase. Lowercase input may be normalized to uppercase before conversion.

  • What happens with zero?

    Roman numerals have no symbol for zero. If you enter 0, it cannot be converted. Use 1 as the minimum.

  • How are large numbers (> 3999) handled?

    Standard notation tops out at 3999. Larger numbers require vinculum or are not supported in simple converters.

  • Why is IIII (4) invalid?

    Subtractive notation (IV) is the modern standard for 4. Additive IIII is historical but not in modern usage.

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