Roman Numeral Symbols

Roman numerals have been in continuous use for over two thousand years, from the inscriptions of the Roman Republic to the copyright dates on modern films. Unicode provides two approaches to representing Roman numerals: using ordinary Latin letters (I, V, X, L, C, D, M) or using dedicated precomposed Roman numeral characters from the Number Forms block. This guide explains both approaches, catalogs every precomposed Roman numeral character in Unicode, and helps developers choose the right representation for their context.

Quick Copy-Paste Table: Precomposed Roman Numerals

Lowercase Precomposed Roman Numerals

The Number Forms Block (U+2150–U+218F)

All precomposed Roman numerals live in the Number Forms block (U+2150–U+218F), which also contains vulgar fractions (like ½, ⅓, ¼). The block provides:

• Uppercase I–XII (U+2160–U+216B): Composite characters for 1–12
• Uppercase L, C, D, M (U+216C–U+216F): Single-value characters for 50, 100, 500, 1000
• Lowercase i–xii (U+2170–U+217B): Small versions of 1–12
• Lowercase l, c, d, m (U+217C–U+217F): Small versions of 50, 100, 500, 1000

Why 1–12 Are Special

Unicode provides precomposed characters for 1 through 12 specifically because these values appear most frequently in typographic contexts: clock faces (I–XII), book chapter numbers, outline numbering, and list items. The precomposed forms like Ⅲ (U+2162) are single characters that render as a ligature — the three vertical strokes are part of one glyph, with font-specific kerning and spacing.

For numbers beyond 12, you combine the individual characters. For example, "14" would be ⅩⅣ (U+2169 + U+2163) — two precomposed characters — or simply written as XIV using Latin letters.

Precomposed vs Latin Letters: Which to Use

The fundamental question: should you write Roman numerals using precomposed Unicode characters (Ⅳ, U+2163) or ordinary Latin letters (IV)?

Comparison

The Unicode Consortium's Recommendation

The Unicode Standard itself states that these characters exist primarily for compatibility with East Asian encoding standards (like JIS X 0208 and KS X 1001) that included precomposed Roman numerals for vertical text layout. The Standard recommends using ordinary Latin letters for Roman numerals in most contexts.

From the Unicode Standard, Chapter 22:

"For most purposes, it is preferable to compose the Roman numerals from sequences of the appropriate Latin letters."

When Precomposed Characters Make Sense

Despite the general recommendation, precomposed Roman numerals are useful in:

1. CJK vertical text: In Japanese, Chinese, and Korean vertical writing, a precomposed Ⅲ occupies a single character cell and rotates correctly. Writing "III" with three Latin I characters in vertical text creates three separate rotated letters.

2. Clock faces: The sequence Ⅰ through Ⅻ represents the 12 positions on an analog clock. Using precomposed characters ensures consistent glyph design.

3. Semantic markup: When you need software to recognize that a character is a Roman numeral (not a Latin letter), the precomposed form carries that semantic information in its Unicode properties.

Unicode Properties of Roman Numerals

Each precomposed Roman numeral carries a numeric value in Unicode's character database, making programmatic conversion straightforward:

import unicodedata

# Precomposed Roman numeral has numeric value
char = "\u2163"  # Ⅳ
name = unicodedata.name(char)       # "ROMAN NUMERAL FOUR"
value = unicodedata.numeric(char)    # 4.0
category = unicodedata.category(char)  # "Nl" (Number, letter)

# Latin letter "I" has NO numeric value for Roman numeral
latin_i = "I"
category_i = unicodedata.category(latin_i)  # "Lu" (Letter, uppercase)
# unicodedata.numeric(latin_i) raises ValueError

The General Category for precomposed Roman numerals is Nl (Number, letter), while ordinary Latin letters used as Roman numerals have category Lu (Letter, uppercase). This distinction allows programs to identify precomposed Roman numerals programmatically.

Case Mapping

Precomposed Roman numerals support case conversion:

upper = "\u2160"  # Ⅰ (uppercase)
lower = upper.lower()  # ⅰ (U+2170, lowercase)
back = lower.upper()   # Ⅰ (U+2160, uppercase)

# Works for all 16 pairs
roman_12 = "\u216B"  # Ⅻ
roman_12_lower = roman_12.lower()  # ⅻ (U+217B)

This case mapping is correctly defined in Unicode's CaseFolding.txt and SpecialCasing.txt data files.

Compatibility Decomposition

Each precomposed Roman numeral has a compatibility decomposition to its constituent Latin letters. Under NFKD (Compatibility Decomposition) or NFKC (Compatibility Composition) normalization:

import unicodedata

roman = "\u2162"  # Ⅲ
decomposed = unicodedata.normalize("NFKD", roman)
print(decomposed)  # "III" (three Latin I characters)
print(len(roman), len(decomposed))  # 1, 3

This means that NFKC/NFKD normalization will destroy the distinction between precomposed Roman numerals and Latin letters. If your application applies NFKC normalization (common in search indexing), all precomposed Roman numerals will be converted to their Latin letter equivalents.

Roman Numeral Values Beyond the Basic Set

Standard Roman numeral values and their Unicode representations:

For composite values like 14 (XIV), 27 (XXVII), or 2024 (MMXXIV), you can either use Latin letters or combine precomposed characters:

# Using Latin letters (recommended)
year = "MMXXIV"  # 2024

# Using precomposed characters (CJK/vertical text)
year_precomposed = "\u216F\u216F\u2169\u2169\u2163"  # Ⅿ Ⅿ Ⅹ Ⅹ Ⅳ = MMXXIV

Apostrophic and Vinculum Notation

Classical and medieval Roman numeral notation used additional marks for large numbers that are not encoded as dedicated Unicode characters:

• Vinculum (overline): A bar above a numeral multiplies it by 1,000. V-with-overline = 5,000. Unicode has no precomposed "Roman numeral with vinculum," so you must use combining characters: V + U+0305 (COMBINING OVERLINE) = V̅.

• Apostrophic notation: CIↃ for 1,000, CCIↃↃ for 10,000. The reverse C character Ↄ is encoded at U+2183 (ROMAN NUMERAL REVERSED ONE HUNDRED).

# Vinculum (overline) for large Roman numerals
five_thousand = "V\u0305"       # V̅  = 5,000
ten_thousand = "X\u0305"        # X̅  = 10,000
fifty_thousand = "L\u0305"      # L̅  = 50,000
one_million = "M\u0305"         # M̅  = 1,000,000

Practical Conversion Code

def int_to_roman(num: int) -> str:
    # Standard Roman numeral conversion using Latin letters
    val = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
    syms = ["M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"]
    result = ""
    for i, v in enumerate(val):
        while num >= v:
            result += syms[i]
            num -= v
    return result

def int_to_roman_unicode(num: int) -> str:
    # Using precomposed Unicode characters (1-12 only)
    if 1 <= num <= 12:
        return chr(0x215F + num)
    # Fallback to Latin letters for larger values
    return int_to_roman(num)

print(int_to_roman(2024))           # "MMXXIV"
print(int_to_roman_unicode(7))      # "Ⅶ"
print(int_to_roman_unicode(14))     # "XIV" (fallback)

Key Takeaways

• Unicode provides precomposed Roman numerals Ⅰ–Ⅻ (1–12) plus L, C, D, M in both uppercase and lowercase — 32 characters total in the Number Forms block (U+2150–U+218F).
• These exist primarily for CJK compatibility (vertical text, fixed-width cells). The Unicode Consortium recommends using Latin letters (I, V, X) for most contexts.
• Precomposed characters have General Category Nl (Number, letter) and carry numeric values accessible via unicodedata.numeric().
• NFKC/NFKD normalization decomposes precomposed Roman numerals into Latin letters, which can break applications that depend on the distinction.
• For values 13 and above, there are no precomposed multi-value characters — combine individual characters or use Latin letters.
• The combining overline (U+0305) can be used with Latin letters to represent vinculum notation for large numbers (V̅ = 5,000).