Type a number to get its Roman numeral, or type a Roman numeral to get the number — the tool works out which you entered and shows both. It validates the numeral against the standard rules, so malformed input like IIII is flagged rather than silently accepted. Everything runs in your browser.
Type a number (1–3999) or a Roman numeral — the tool detects which.
Enter a number or a Roman numeral.
How it works
Roman numerals combine seven letters — I, V, X, L, C, D, M — largest to smallest, with six subtractive pairs (IV, IX, XL, XC, CD, CM) to avoid four of the same symbol in a row. To convert a number, the tool greedily subtracts the largest value that fits; to read a numeral, it adds symbol values while enforcing the canonical form.
The supported range is 1 to 3999, the classical limit of standard Roman numerals — there’s no zero and no single-symbol way to write 4000 or beyond (that needs overlines the notation doesn’t include). Numbers outside the range, and numerals that break the rules, return a clear error.
Practical examples
A year
2024 becomes MMXXIV: two thousands (MM), twenty (XX), and four as IV. Handy for copyright lines, clock faces and film credits.
Reading a numeral
Type MCMLXXXIV and the tool returns 1984 — decoding the subtractive CM (900) and LXXXIV (84) for you.
Catching a mistake
IIII looks like four but isn’t valid standard notation; the correct form is IV. The tool rejects IIII so you don’t propagate the error.
Frequently asked questions
What is the largest number I can convert?
3999, written MMMCMXCIX. Standard Roman numerals stop there because 4000 would need four M’s or an overline (a bar meaning ×1000) that isn’t part of the basic alphabet this tool uses.
Is there a Roman numeral for zero?
No. Romans had no symbol for zero; the system starts at I (one). The word “nulla” was sometimes used in text, but there’s no numeral, so this converter accepts values from 1 upward.
Why is IIII rejected when clocks use it?
Some clock faces use IIII for four as a stylistic tradition, but standard notation uses the subtractive IV. This tool validates against the standard rules, so it treats IIII as invalid. Enter IV for four.
What are the subtractive pairs?
Six combinations put a smaller symbol before a larger one to subtract: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900). They prevent four identical symbols in a row and are required for canonical numerals.
Do I have to pick a direction?
No. If your input is all digits it’s treated as a number to convert to Roman; otherwise it’s parsed as a Roman numeral. You always see both the numeral and the number in the result.
Is it case-sensitive?
No — lowercase input like “mcmlxxxiv” is accepted and normalised to uppercase. Roman numerals are conventionally written in capitals, so the result is always shown that way.
How are the numerals validated?
Against the canonical pattern: no more than three of the same repeatable symbol in a row, and only the six legal subtractive pairs. Anything else — IC, VX, IL — is reported as invalid rather than guessed at.
Is anything I type sent anywhere?
No. The conversion runs entirely in your browser; nothing is uploaded and analytics never receives what you enter.
Related tools
Prime Number Checker
Check whether a number is prime, with its prime factorisation and the nearest primes. Runs in your browser.
Math & education
Number Base Converter
Convert whole numbers between binary, octal, decimal and hexadecimal at once. BigInt-precise, in your browser.
Math & education
GCD and LCM Calculator
Find the greatest common divisor and least common multiple of two or more whole numbers. Runs in your browser.
Math & education
Add Line Numbers
Add a number to the start of every line, with a custom start, step, separator and zero-padding. In your browser.
Text tools
Week Number Calculator
Find the ISO 8601 week number for any date, with the week’s Monday–Sunday range. Runs in your browser.
Date & time
Percentage Calculator
Calculate X% of a number, what percent one number is of another, and percentage change.
Everyday calculators