Roman Numeral Converter
Convert numbers to Roman numerals and Roman numerals back to numbers. Works both ways, checks your numeral is valid, and updates as you type.
- Free, no account
- No watermark
- No usage limit
About the Roman Numeral Converter
Type a number from 1 to 3999 and this Roman Numeral Converter gives you the numeral. Type a numeral and it reads it back as a plain number. Both directions update live as you type, and nothing you enter leaves your browser.
But the reason to pick this one over the dozens of others is that it refuses to guess. Most Roman numeral converters will cheerfully tell you IIII is 4 and IL is 49, because all they do is add the letters up, they never stop to check whether the numeral was written correctly in the first place. This one does, because every answer gets re-encoded and compared back against exactly what you typed, and if the two don't match to the letter, you get a warning instead of a confident wrong number, usually with the numeral you probably meant. So it works as a spell-checker for Roman numerals, which is exactly what you want before you commit one to something you can't easily redo.
That matters most when the numeral is headed somewhere permanent. A birth year going onto a tattoo, or a founding date getting carved into a storefront sign. The dangerous part of a dumb converter is that it fails silently, it hands you a clean-looking answer for a numeral that's actually malformed, and you don't find out until it's set in metal. This one flags the usual offenders the moment you type them: a letter repeated four times (IIII, XXXX), an illegal subtractive pair (IL, IC, VX), or a small symbol wedged somewhere it doesn't belong. When it rejects your input it tells you why, and more often than not it shows the correct spelling and its value.
How to use
- Number to numeral: in the top box, type any whole number from 1 to 3999. The numeral appears underneath as you type, no button to press.
- Numeral to number: in the bottom box, type a Roman numeral. Case doesn't matter, mcmxciv reads the same as MCMXCIV. Its value shows below.
- Copy: the Copy button next to either answer drops it straight onto your clipboard.
- A red box means something's off. Read the message. It names the problem (out of range, not a whole number, or a numeral that breaks the rules) and usually points you at the fix.
Both boxes run on their own, so you can convert a number up top and decode an unrelated numeral below without one clobbering the other. Useful when you're cross-checking a year against a date.
Years are the reason this exists
The single most common thing people convert here is a four-digit year, and it's nearly always for something they're about to make permanent. Years are also where Roman numerals get genuinely hard to eyeball. Take 1994. Written properly it's MCMXCIV, seven letters, because CM covers the 900, XC the 90, and IV the 4. Do it the naive way, just stacking symbols by value, and you'd land on MDCCCCLXXXXIIII, which is both a mouthful and flat wrong by modern rules. The whole reason the subtractive pairs exist (IV, IX, XL, XC, CD, CM) is to keep numbers like that short and readable.
Reading one back cold is the harder direction. Is MCMLXXXVII 1987 or 1897? The letters blur, and the subtractive pairs are easy to miscount when you're squinting at a weathered cornerstone or the copyright line at the end of an old film. Drop it into the bottom box and let the tool sum it for you.
Small stuff you honestly don't need a tool for. A clock hour or a chapter number, you can knock those out in your head, and doing them by hand is how the rules stick. It's the long years and the stuff you can't undo where being sure actually matters.
The seven letters and the one rule that trips everyone
Every numeral is built from seven letters. I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1000. Read left to right and add as you go, except when a smaller letter sits in front of a bigger one, then you subtract it. That's the IV, the IX, the CM.
The rule this tool is strict about is repetition. The same letter shows up at most three times in a row. III is fine, IIII is not, it becomes IV. XXX is 30, XXXX is wrong, it's XL. And V, L, and D never repeat or get subtracted at all, so there's no VV and no IL. Any time you catch yourself writing a fourth identical letter, that's the signal to reach for a subtractive pair instead. The converter enforces every bit of this, which is the reason it can catch a bad numeral at all. It knows there's exactly one correct way to spell each number, so anything that isn't that spelling gets rejected.
Frequently asked questions
Why do so many clocks show IIII instead of IV?
Watchmakers have used IIII at the 4 position for centuries, and it's deliberate. The leading theory is visual balance: IIII (four characters) mirrors the VIII directly opposite it at the 8, so the dial reads evenly around the ring. Some pin the habit on a preference of Louis XIV. Either way it's a design tradition, not a math rule, and everywhere except a clock face the standard 4 is IV. Worth knowing that this converter follows the standard, so it treats IIII as invalid and nudges you to IV.
What will it reject if I mistype a numeral?
Any letter outside the valid seven. Type a Z or a stray J and it stops you, since neither one is a Roman numeral to begin with. Stray spaces around your entry get trimmed for you, so a gap on either end won't throw a false error. And case makes no difference, lowercase or uppercase reads the same.
What's the largest number I can convert?
3999, written MMMCMXCIX. M (1000) is the biggest letter and nothing repeats past three in a row, so the most thousands you can stack is MMM. Add the largest legal remainder, 999, and 3999 is the ceiling. The Romans did go higher by drawing a bar over a letter to multiply it by 1000 (a vinculum), but that was never fully standardized and there's no clean way to type it, so this tool stays inside the 1 to 3999 range where there's only ever one right answer.
Is there a Roman numeral for zero?
None, because the system was built for counting real things and you don't count zero of anything. Zero as a working number reached Europe much later, through Arabic numerals. That's also why there's no way to write a negative, Roman numerals begin at I and only climb.
How do I convert a full date like a birthday?
Convert each part on its own, since Roman numerals have no built-in date format. So 3 July 1994 becomes III, VII, MCMXCIV, and you arrange them however you like, a slash or a dot between the parts is common on engravings. The tool handles one number at a time, which is all a date needs.
Why did it reject a numeral I was sure was right?
Almost always an illegal subtractive pair that looks reasonable. IL feels like it should be 49, but 49 is XLIX. IC looks like 99, but 99 is XCIX. Only six subtractions are legal (IV, IX, XL, XC, CD, CM), the small letter has to be I, X, or C, and it can only sit in front of the next size or two up. Anything outside that, the tool re-encodes what you typed, sees it doesn't match the single correct spelling, and flags it with the numeral you likely meant.