Roman Numeral Converter
Numbers to numerals and back, malformed ones rejected.
Made of
A letter before a bigger one is subtracted, not added — CM is M minus C, which is why it is worth 900 and not 1100.
How this was worked outMCMXCIV
- 1994MCMXCIV
The system stops at 3,999: there is no Roman numeral for four thousand without a bar over the M, which is not a letter.
Guide: The rules, briefly2 min read
The rules, briefly
Seven letters: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1000. Letters are written largest first and added together.
A smaller letter before a larger one is subtracted instead, and only in six specific pairs: IV, IX, XL, XC, CD and CM. That restriction is why IM is not 999 and why it is refused here rather than quietly accepted.
Why it stops at 3999
Because there is no letter for 5000. The largest numeral this system can write is MMMCMXCIX, and anything above it needs an extension — a bar over a letter to multiply it by a thousand — that was never standardised and does not survive well in plain text.
There is also no zero and no way to write a negative number or a fraction. These are not oversights; the system was built for counting and labeling, not for arithmetic.
Clock faces break the rules, on purpose
Many clocks show IIII rather than IV for four. It has been that way for centuries, and the usual explanations are visual balance against the VIII opposite, and the awkwardness of IV as the first letters of a Roman god’s name.
IIII is rejected by this converter because it is not well-formed, however venerable the exception. That is the correct behavior for a converter, even where it is the wrong behavior for a clockmaker.
Nothing is sent anywhere
Every figure on this page is worked out in your own browser. No request is made, nothing is stored between visits, and the page keeps working with the connection switched off.
Frequently asked questions
What year is MCMXCIV?
1994. M is 1000, CM is 900, XC is 90 and IV is 4. Reading it in those groups rather than letter by letter is the whole trick.
Why is IIII rejected?
Because four is written IV in the standard form. The converter checks its answer by converting back and comparing — if what you typed is not what the canonical form would be, it was not a well-formed numeral, whatever it might have meant.
How do I write a year like 2026?
MMXXVI. Two thousands, two tens, a five and a one, in descending order.
Is anything sent anywhere?
No. The conversion happens in this tab and the page works offline.