Roman Numeral Converter

Convert a number into Roman numerals or read a Roman numeral back as a number, both directions at once and updating as you type. The converter covers 1 through 3999 and uses proper subtractive notation, so 4 becomes IV rather than IIII and 900 becomes CM rather than DCCCC.

Number → Roman Numeral

Number (1–3999)

MMXXVI

Roman Numeral → Number

Roman Numeral

2026

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Why use Roman Numeral Converter?

Roman numerals build a number from seven letters: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, and M is 1000. Reading them works left to right, adding each symbol, with one exception. When a smaller symbol sits before a larger one it is subtracted instead, which is how IV becomes 4 and CM becomes 900. Converting a number the other way uses the same rule in reverse. The tool works through the thirteen value symbol pairs from largest to smallest, taking as many of each as will fit, which naturally produces the shortest correct form rather than a long run of repeated letters. The 3999 ceiling is not arbitrary. There is no standard letter above M, so once you pass 3999 the conventional system runs out of symbols. Historical texts solved this with a vinculum, a bar drawn over a letter to multiply it by a thousand, but that is outside what plain text can represent cleanly.

Key benefits

Both directions on one page, with no mode switch to remember
Correct subtractive pairs applied automatically: IV, IX, XL, XC, CD, and CM
Rejects invalid letters instead of guessing at what you meant
Lowercase input is accepted and read the same as uppercase
Runs on your device, so nothing you type is sent anywhere

Features

Greedy conversion against the standard thirteen value symbol pairs
Validation against the pattern of the seven legal Roman letters
Live output as you type, with no button to press
Separate copy buttons for the numeral and the number
Clear message when a value falls outside the supported range

Popular use cases

Reading a copyright year or film credit written in numerals
Numbering appendices, chapters, or outline sections in a document
Checking a clock face, monument inscription, or building date
Homework and teaching material on how the numeral system works

How to use Roman Numeral Converter

1
Type a number
Enter any whole number from 1 to 3999 in the first box. The Roman numeral appears immediately below it.
2
Or type a Roman numeral
Use the second box for the reverse direction. Only the seven valid letters are accepted: I, V, X, L, C, D, and M.
3
Check the result
Both boxes work independently, so you can convert in either direction without clearing the other one.
4
Copy what you need
Each result has its own Copy button, so you can take just the numeral or just the number.

Frequently asked questions

Why does the converter stop at 3999?

Standard Roman numerals have no symbol above M for 1000, so the largest value you can write with the usual seven letters is MMMCMXCIX, which is 3999. Larger numbers historically used a bar over a letter to multiply it by 1000, and that notation is not part of this tool.

What is subtractive notation?

Instead of writing four identical symbols, Roman numerals place a smaller symbol before a larger one to mean subtraction. So 4 is IV rather than IIII and 90 is XC rather than LXXXX. The converter applies these six pairs automatically: IV, IX, XL, XC, CD, and CM.

Can I type lowercase Roman numerals?

Yes. Input is uppercased before it is read, so mmxxvi and MMXXVI both return 2026.

Why is my Roman numeral rejected?

Only the seven letters I, V, X, L, C, D, and M are valid. Any other character, including spaces inside the numeral or a stray zero, will fail validation. There is also no Roman symbol for zero or for negative numbers.

Does it check that a numeral is well formed?

It validates the letters and computes a value using subtractive rules, but it does not reject every unconventional arrangement. A string like IIII will still be read as 4 even though the conventional form is IV.

Last reviewed: July 2026

Number → Roman Numeral

Roman Numeral → Number

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