Roman Numeral Converter

Convert Roman numerals ↔ numbers

Instant conversion and copy, all in your browser (1–3999)

Last updated: January 25, 2026

CreatorFrank Zhao

Arabic to roman

Enter a number from 1 to 3999

Number

Roman numeral

XLII

Roman to arabic

Valid range follows standard Roman numerals (I–MMMCMXCIX)

Roman numeral

Number

Introduction / overview

The Roman Numeral Converter helps you convert in both directions: a standard Roman numeral (like $\mathrm{XLII}$ ) into an Arabic number (like $42$ ), or a number in the range $1\text{–}3999$ into its Roman form.

What problems does it solve?

Reading dates and chapter numbers in books, outlines, and documents.
Checking whether a Roman numeral string is valid (standard subtractive notation).
Generating canonical Roman numerals for UI labels, headings, or filenames.

Why the results are consistent

This calculator uses the standard Roman numeral rules (including subtractive pairs like $IV$ and $IX$ ) and produces a canonical output (no alternate spellings).

If you’re working with other number formats, you may also like our Integer Base Converter.

How to use / quick start

In Arabic to roman, type an integer between $1$ and $3999$ .
Copy the Roman numeral output using the copy icon (the output is generated instantly).
In Roman to arabic, paste a Roman numeral like $\mathrm{MCMXCIV}$ .
If the input is not valid under standard rules, you’ll see a validation message and the number result will stay empty.

Tip: Keep it canonical

Many numbers can be written in “non-standard” ways. This tool expects and outputs the canonical Roman form. For example, $4$ becomes $IV$ , not $IIII$ .

Worked examples (step-by-step)

Use these as templates: the converter applies the same rules instantly. You can copy the result from the output box.

Example 1: Convert 42 to Roman

The canonical decomposition is $42 = 40 + 2$ . In Roman numerals, $40 = XL$ and $2 = II$ .

42

=

40 + 2

\Rightarrow

XL + II

=

\mathrm{XLII}

Result: $42 \mapsto \mathrm{XLII}$ .

Example 2: Convert MCMXCIV to 1994

Split it into standard blocks: $M$ $CM$ $XC$ $IV$ .

\mathrm{MCMXCIV}

=

1000 + 900 + 90 + 4

=

1994

Result: $\mathrm{MCMXCIV} \mapsto 1994$ .

Real-world examples / use cases

Formatting headings and outlines

Input $12$ and get $\mathrm{XII}$ for “Chapter XII”.

If you also need other bases (hex, base64), pair this with Integer Base Converter.

Homework and quick checks

You see $\mathrm{XL}$ on a worksheet and want to confirm it means $40$ .

\mathrm{XL}

=

50 - 10

=

40

Canonical output for UI labels

If your app shows “Version $\mathrm{IV}$ ”, you can generate the canonical numeral from $4$ .

4 \mapsto \mathrm{IV}

Preparing data for other tools

Convert a Roman numeral to a number, then feed that number into a different workflow. For example, you can hash a string that includes the numeric value using Hash Text.

Common scenarios / when to use

Reading document sections

Convert a label like XLII into its numeric section index.

Validating input

Quickly check whether a Roman numeral follows standard rules.

Generating canonical numerals

Always output IV, IX, XL, XC, CD, and CM when appropriate.

Learning subtractive notation

Understand why IV means 4 and not 6.

Interoperability with other converters

Convert to a number first, then use base conversion or hashing tools.

Quick reference

Copy results in one click for notes, code, or a spreadsheet.

When it may not apply

If you need non-standard forms used in some contexts (like $\mathrm{IIII}$ for $4$ on clock faces), this converter will still normalize to the canonical form $\mathrm{IV}$ .

Tips & best practices

Use canonical pairs

Memorize the subtractive pairs: $IV$ , $IX$ , $XL$ , $XC$ , $CD$ , and $CM$ .

Keep inputs uppercase

Roman numerals are typically written in uppercase. If you paste lowercase text, convert it to uppercase first.

Convert first, then transform

Need a different representation? Convert Roman to an Arabic number first, then use Integer Base Converter.

Calculation method / formula explanation

Roman numerals are built from symbol values. A common value set is:

I=1

,

V=5

,

X=10

,

L=50

,

C=100

,

D=500

,

M=1000

Roman → Arabic (scan rule)

Read left to right. If a symbol is followed by a larger symbol, subtract it; otherwise add it.

\mathrm{IV}

=

-1 + 5

=

4

Arabic → Roman (greedy rule)

Repeatedly take the largest Roman token that fits the remaining value (including subtractive tokens like $CM=900$ ), append it, and subtract its value.

1994

=

1000 + 900 + 90 + 4

\Rightarrow

\mathrm{MCMXCIV}

Related concepts / background info

Subtractive notation

When a smaller symbol appears before a larger one, it indicates subtraction. Common cases include $IV$ , $IX$ , $XL$ , $XC$ , $CD$ , and $CM$ .

Why 1–3999?

Standard Roman numerals do not have a universally agreed modern notation for larger values without additional marks (like overlines). This tool focuses on the common “no-overline” range.

Canonical formatting

Canonical output avoids ambiguous or uncommon forms. For example, $90$ is $XC$ , not $LXXXX$ .

Frequently asked questions (FAQs)

Why does the converter reject some strings?

The Roman-to-Arabic input is validated against standard Roman rules. Some strings that look plausible are not valid in canonical notation.

Is IIII valid for 4?

In some contexts (like clock faces) you may see $\mathrm{IIII}$ , but the canonical form for $4$ is $\mathrm{IV}$ .

What is the largest number I can convert?

This converter supports $1\text{–}3999$ . For larger values, you’d typically need overlines or other conventions.

Do you normalize the Roman output?

Yes. Arabic-to-Roman always produces a canonical Roman numeral using standard subtractive pairs.

Can I use this for base conversions like hex?

Roman numerals are a separate numeral system. If you need base conversions, use our Integer Base Converter.

Limitations / disclaimers

Scope of supported notation

The tool targets standard Roman numerals in the range $1\text{–}3999$ . It does not implement extended conventions for values above 3999.

Not a substitute for style guides

Some publishers or fields adopt special rules (or accept non-canonical forms). If you’re formatting a formal document, follow the relevant style guide.

External references / sources

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