How to Use This Tool
Type in either box and the other follows. Invalid numerals are rejected with the specific rule they break, rather than being quietly interpreted.
The rules, in full
- Symbols — I 1, V 5, X 10, L 50, C 100, D 500, M 1000.
- Repetition — I, X, C and M may repeat up to three times. V, L and D may never repeat, because two of them equals the next symbol up.
- Subtraction — only I, X and C may be subtracted, and only from the next two symbols. That gives exactly six pairs: IV, IX, XL, XC, CD, CM.
- One at a time — only a single symbol may be subtracted, so IIX is not 8.
The subtraction rule is where most errors come from. IC looks like a reasonable way to write 99, and it is not permitted, because I may only precede V and X. The correct form builds from the largest part down: 90 then 9, which is XCIX.
Why 3999 is the maximum
M is the largest symbol, and no symbol may appear more than three times, so MMM is 3000 and there is nothing to write 4000 with. The Romans handled larger numbers with a bar above a numeral to multiply it by a thousand, which is not something ordinary text can represent, so 3999 is the practical ceiling.
There is also no zero and no way to write a negative or a fraction of the sort we would recognise. Roman numerals were built for counting and recording, not for arithmetic — which is why the adoption of Hindu-Arabic numerals in Europe mattered so much.
Where you still meet them
- Years on buildings and films — a copyright year in credits is often the only Roman numeral a person reads all year.
- Monarchs and popes — Elizabeth II, Louis XIV.
- Book front matter — prefaces numbered i, ii, iii before page 1.
- Outlines and legal documents — nested numbering, often in lower case.
- Super Bowls — which skipped L for Super Bowl 50, on the grounds that it looked odd alone.
Lower case and the letter J
Lower case numerals are common in book pagination and outlines and mean the same thing. Historically,
a final i in a sequence was sometimes written j — viij for 8
— which you may meet in old manuscripts and prescriptions. This tool accepts either case and does
not accept j.
