Roman Numeral Converter

Examples

• 2026 → MMXXVI
• 44 → XLIV
• 1999 → MCMXCIX

Strict in one direction, forgiving in the other

Going from a number to a numeral, this tool follows the standard subtractive rules and refuses anything outside 1 to 3999. Going the other way it is deliberately tolerant: it discards every character that is not one of M D C L X V I before it looks at what is left.

That tolerance is useful when you paste a numeral out of a sentence, and it is also the one thing on this page that can hand you a confidently wrong answer. It comes first.

A typo can produce a valid number instead of an error

Because unrecognised characters are removed rather than rejected, a mistake that leaves a legal numeral behind is accepted silently. Measured, with the status reading OK in green:

• X1V gives 15. The digit 1 is deleted, leaving XV. You almost certainly meant XIV, which is 14.
• 1X gives 10, the 1 having vanished.
• ABC gives 100. The A and the B are not numeral letters, so they go, and the surviving C is a perfectly good hundred.
• MC gives 1100, which is correct and may still surprise you if you typed it as initials.

Typing 1 where you meant I is the common version of this, because the two sit next to each other in most people's typing and look alike in many fonts. There is no warning: the output is a number and the status is green.

Check that the number you get back is roughly the size you expected. The tolerance is genuinely helpful for the ordinary case, where it lets XIV (14) and XIV!! both give 14, and I love XIV is correctly refused because the stripped letters do not form a legal numeral.

Only the strict subtractive form is accepted

After stripping, the remaining letters must match the canonical pattern, and anything else is refused outright. Measured:

• IIII is refused. Only three identical letters may run together, so four is one too many. This is the form that appears on the dials of a great many clocks, where IIII is traditional in place of IV, and it will not convert here.
• IM and VX are refused. Only I before V or X, X before L or C, and C before D or M are legal subtractions, so there is no shortcut for 999 as IM.
• MMMM is refused, which is the 3999 ceiling showing itself from the other side.
• Lower case is fine: mmxxvi gives 2026, the same as MMXXVI.

Each number in range has exactly one accepted spelling, so the conversion is a true round trip in both directions for every value from 1 to 3999. The longest numeral in that range is MMMDCCCLXXXVIII, fifteen characters, which is 3888.

The number side truncates rather than refusing

Converting a number to a numeral reads the value as a whole number and stops at the first character that is not part of one. Measured:

• 12.7 gives XII. The decimal part is discarded, not rounded, so 12.7 does not become XIII.
• 1e3 gives I, not MMM, because the reading stops at the letter.
• 2026xyz gives MMXXVI, the trailing text being ignored.
• Surrounding spaces are fine, and abc or an empty box give “Enter a whole number” in red.
• 0, -5 and 4000 all give “Enter an integer from 1 to 3999” in red.

The 3999 ceiling is not an arbitrary choice. The basic alphabet has no single letter worth more than 1000, and the rule allowing at most three identical letters in a row caps the thousands at MMM. Larger values historically used a vinculum, a bar drawn over a letter to multiply it by a thousand, and that notation is not supported here.

There is no zero either, in the tool or in the system. Roman numerals have no symbol for nothing, which is why the lower bound is 1 rather than 0.

What this tool deliberately does not do

• It does not warn you when it discards characters. A stray digit or letter is removed and whatever remains is converted.
• It does not go above 3999 or below 1, and has no vinculum notation for larger values.
• It does not accept IIII, the clock-dial form, nor any other non-canonical spelling.
• It does not accept irregular subtractions such as IM for 999.
• It does not handle a year range or a list. One value at a time.
• It does not output the dedicated Unicode numeral characters, only ordinary capital letters, which is what you want for almost every purpose.
• It does not do arithmetic on numerals.
• Nothing is uploaded. The conversion runs in your browser.

For a year difference rather than a conversion, use Days Between Dates. To convert between other bases, Number Base Converter handles binary, octal and hexadecimal with exact integer arithmetic.

Sources

• Why 1e3 is read as 1 and 12.7 as 12: the value is parsed as an integer and the reading stops at the first character that is not part of one. developer.mozilla.org
• The accepted spellings, the 1 to 3999 range and the round trip were established by measurement on this page rather than from a reference. There is no single standards body for Roman numerals, so this page describes what this tool accepts and does not claim to describe universal practice.

Every behaviour and every value on this page was measured against this tool as it is published, in a browser, on 14 October 2026. The claim that every value from 1 to 3999 has exactly one spelling was checked by generating all 3,999 numerals.