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XLII = 42

Roman Numeral Converter

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Convert integers to Roman numerals and back — with full breakdown. Supports 1 to 3999 (MMMCMXCIX). Auto-detects direction, case-insensitive.

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Full Breakdown
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Conversion Direction

Integers 1 – 3999

Quick examples

Result

Roman numeral

XLII

42 in Roman numerals

Breakdown

XL40
I1
I1

XL=40 + I=1 + I=1 = 42

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How to Use

  1. 1 Type any number (1–3999) or Roman numeral in the input box
  2. 2 The tool auto-detects direction — numbers convert to Roman, letters convert to integers
  3. 3 Or use the mode buttons to force a specific conversion direction
  4. 4 See the full numeral breakdown (M=1000, D=500, C=100…) below the result
  5. 5 Click any example (XIV, MMXXVI, 42, 2026) to load it instantly

Features

  • Bidirectional — integer→Roman and Roman→integer in one tool
  • Auto-detect mode reads your input and picks the correct direction
  • Full component breakdown showing each numeral and its value
  • Round-trip validation — converts back and checks for canonical form
  • Case-insensitive — XIV and xiv both work
  • Supports 1 to 3999 (the maximum standard Roman numeral)
  • 100% offline — no server, no data sent

Why it Matters

Roman numerals appear on clock faces, movie copyright years, chapter headings, Super Bowl names, and historical dates. Being able to convert quickly — and understand the breakdown — is useful for writers, students, and anyone working with historical documents.

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Use Cases

Students & Writers

Decode chapter numbers, historical years, and outline headings in Roman numerals

Film & Media

Read or write movie copyright years and Super Bowl edition numbers

Clock & Watch Fans

Decode clock-face Roman numerals like IV (4) or XII (12) instantly

Developers

Generate Roman numeral output for outlines, legal docs, and version strings

What this tool does

Type any integer from 1 to 3999 or any Roman numeral and get the instant conversion. Auto-detect mode figures out whether you’re entering a number or a numeral. The full breakdown shows which Roman symbols compose the result and their individual values.

How Roman numerals work

Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), M (1000). Values are generally written largest-to-smallest from left to right. The subtractive rule creates the compact forms: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900).

Use the Number to Words tool to also spell out the number in English words.

Frequently Asked Questions

Why does 4 become IV and not IIII?
Standard modern Roman numerals use subtractive notation — a smaller numeral before a larger one means subtraction. IV = 5−1 = 4. The IIII form was used historically and still appears on some clock faces, but IV is the canonical modern form.
What is the largest standard Roman numeral?
3999 — written MMMCMXCIX. The number 4000 would need a symbol for 4×M, which is beyond the standard set. Medieval scribes used a bar over a numeral to indicate ×1000, but that is not part of the standard system.
Are Roman numerals case-sensitive?
No — this tool accepts both uppercase (XIV) and lowercase (xiv) and normalises to uppercase. Traditionally, Roman numerals are written in uppercase.
What does the breakdown show?
The breakdown lists each Roman numeral component from left to right with its decimal value. For XLII: XL=40, II=2. The values sum to the integer result, illustrating how the subtractive rules work.
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Want to understand the conversions?

Browse all guides →
Roman Numerals Explained
roman numerals

Roman Numerals Explained: History, Rules & Conversion Guide

Learn the seven Roman numeral symbols, the additive and subtractive rules, how to convert integers to Roman numerals and back, and where they are still used today.

25 Jun 2026

5 min read

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