Roman numeral converter
Convert numbers to Roman numerals and back, in the strict subtractive form that is actually correct.
Both directions, one box
Type a number and get a numeral; type a numeral and get a number. One per line to do a batch — useful for a list of chapter headings, film credits, monarch names or an outline that has to be renumbered.
The subtractive rule, applied strictly
Four is IV, not IIII. Nine is IX. Forty is XL, ninety XC, four hundred CD, nine hundred CM. Those six pairs are the complete set — no others are allowed.
That last point is where most converters are too permissive. IC for 99 looks like it follows the pattern and is not a Roman numeral; 99 is XCIX. Subtraction only works from the next one or two denominations up, which is why IV and IX exist but IL and IC do not.
So a numeral that is readable but malformed is refused with the correct form rather than quietly accepted. Give it IIII and it tells you that reads as 4 and is written IV. That matters if you are checking someone else's numerals rather than writing your own.
You will still see IIII on clock faces. That is a genuine typographic tradition — it balances the VIII opposite it — and it is not correct arithmetic notation.
1 to 3999, and why it stops there
There is no zero. The Romans had no symbol for it and no need of one in a system for counting things.
There are no negatives, and no fractions in the standard letters.
And 4000 has no agreed form. The convention is a bar over a letter to multiply it by a thousand — so IV for four thousand — but that is a typographic mark rather than a letter, it cannot be written in plain text, and it was never standard in Roman use anyway. Anything above 3999 is refused rather than invented.
Where this is still used
Film and television copyright dates, which is the single commonest reason to need it. Book front matter, where the pages before chapter one are numbered i, ii, iii. Monarchs and popes. Legal documents and outlines. Clock faces. Super Bowl numbering, which famously skipped the numeral for the fiftieth because L on its own looked wrong.
Reading the letters
I is 1, V 5, X 10, L 50, C 100, D 500, M 1000. Add them left to right, except that a smaller letter before a larger one is subtracted. MCMXCIV is 1000 + (1000−100) + (100−10) + (5−1) = 1994.
Common questions
Why is IIII rejected?
Because four is IV. A letter cannot repeat more than three times in correct notation. The clock-face IIII is a typographic tradition, chosen to balance the VIII opposite it.
Why can I not write 99 as IC?
Subtraction only works from the next one or two denominations up, which gives exactly six valid pairs: IV IX XL XC CD CM. 99 is XCIX.
What is zero in Roman numerals?
There is none. The system had no symbol for it, which is also why the numerals cannot express negatives or fractions.
Why does it stop at 3999?
Because 4000 has no agreed written form. The bar-over-a-letter convention for thousands is a typographic mark rather than a letter and cannot be written in plain text.
How do I read MCMXCIV?
1000 + (1000−100) + (100−10) + (5−1) = 1994. Add left to right, subtracting any smaller letter that sits before a larger one.
Can I convert a whole list?
Yes — one per line, in either direction, and mixed. Numbers become numerals and numerals become numbers in the same pass.