Roman Numeral Converter Online — Arabic to Roman and Back

What Are Roman Numerals?

Roman numerals are a numeral system originating in ancient Rome, using letters from the Latin alphabet: I, V, X, L, C, D, and M. Still used today for clock faces, ordinal titles (Super Bowl LVIII), movie sequels, book chapters, and formal document numbering, Roman numerals are a common knowledge requirement and a classic programming interview problem.

How to Use a Roman Numeral Converter Online

1. Enter a number or Roman numeral in the input field.
2. The converter detects the input type automatically — digits vs. Roman letters.
3. View the converted result instantly.
4. Valid range is typically 1–3999 for standard Roman numerals.

Roman Numeral Reference Table

I  = 1       VI   = 6       XI  = 11    XL  = 40
II = 2       VII  = 7       XII = 12    L   = 50
III= 3       VIII = 8       XX  = 20    XC  = 90
IV = 4       IX   = 9       XXX = 30    C   = 100
V  = 5       X    = 10             D   = 500    M  = 1000

Subtractive notation:
IV = 4 (5-1)     IX = 9  (10-1)
XL = 40 (50-10)  XC = 90 (100-10)
CD = 400         CM = 900

Roman Numeral Rules

• Additive notation — values are generally added left to right (LX = 60).
• Subtractive notation — a smaller value before a larger value is subtracted (IV = 4).
• Repetition limit — symbols can repeat up to 3 times (III = 3, but IIII is not standard).
• Subtractive pairs — only I, X, and C can be subtracted, and only from specific values.
• Maximum standard value — 3999 (MMMCMXCIX).

Use Cases

Typography and Design

Roman numerals appear on clock faces, book front matter (preface page numbers), architectural cornerstones, and formal invitations. A quick converter is handy when designing these materials.

Programming Practice

Converting to and from Roman numerals is a classic coding exercise that appears in interviews and coding challenges. Implementing the algorithm reinforces greedy algorithm thinking and string manipulation.

Frequently Asked Questions

What is the largest Roman numeral?

In standard notation, the largest Roman numeral is MMMCMXCIX = 3999. For larger numbers, historical manuscripts used bars over letters to multiply by 1000 (V̄ = 5000, M̄ = 1,000,000), but these aren't standard in modern usage.

Why does the Super Bowl use Roman numerals?

The NFL adopted Roman numerals for Super Bowl numbering in 1971 (Super Bowl V) to avoid confusion with calendar years and to give the event a sense of gravitas and tradition. Super Bowl 50 (2016) was a notable exception where Arabic numerals were used temporarily.

How do I write the year 2026 in Roman numerals?

2026 in Roman numerals is MMXXVI: MM = 2000, XX = 20, VI = 6. Breaking it down: M = 1000 × 2, X = 10 × 2, V = 5, I = 1.

Is zero represented in Roman numerals?

No. The Roman numeral system has no representation for zero, which was one of its major limitations for arithmetic. The concept of zero was later adopted from Hindu-Arabic numeral traditions. This is why Roman numerals aren't used for mathematics.

How do I convert Roman numerals to Arabic in programming?

Create a mapping of Roman values to integers. Process the string from right to left: if the current value is less than the previous value, subtract it; otherwise add it. This elegantly handles subtractive notation without special-casing each pair.