Roman Numeral Converter

Convert Arabic numbers to Roman numerals and Roman numerals back to numbers, from 1 to 3999. Live two-way conversion with a symbol breakdown and validation.

Two-way conversion between Arabic numbers and Roman numerals. Type in either field and the other updates immediately. The supported range is 1 to 3999, which is everything classical Roman notation can express without overline notation.

Symbol breakdown

Symbol reference

I 1
V 5
X 10
L 50
C 100
D 500
M 1000

How Roman numerals work

Seven symbols, two rules

Roman numerals are built from seven letters: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Everything else follows from two rules. When a smaller symbol appears after a larger one you add it, so VI is 6. When it appears before a larger one you subtract it, so IV is 4.

The repetition rule

I, X, C and M may repeat up to three times in a row. V, L and D never repeat, because two of them would simply be the next symbol up: VV is just X. This is why 4 is written IV rather than IIII, and why 9 is IX rather than VIIII.

Which subtractions are legal

Only six subtractive pairs are valid in the standard form: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). A symbol may only be subtracted from the next two higher symbols in its own power-of-ten family. That rules out IL for 49, which must be written XLIX.

The 3999 ceiling

With no symbol above M, and M limited to three repetitions, the largest expressible value is MMMCMXCIX = 3999. Larger numbers historically used a vinculum, an overline meaning "multiply by 1000", so V with a bar means 5000. That notation has no single standard in plain text, so this converter stops at 3999.

Where you still see them

Clock faces, book chapter headings, film copyright dates, monarch and pope regnal numbers, and the Super Bowl. Note that many clock faces use IIII instead of IV for visual balance, a long-standing convention rather than an error.

Open-source notice: implemented from the standard subtractive-notation rules. No third-party library is used.

FAQs

Why is 4 written IV and not IIII?
Standard notation caps repetition at three and uses subtraction instead. IIII does appear on many clock faces, but that is a decorative convention rather than the canonical form.
Why can't I enter 0 or a negative number?
Roman numerals have no symbol for zero and no concept of negative values. The system was designed for counting and tallying, so it starts at 1.
What is the largest number I can convert?
3999, written MMMCMXCIX. Beyond that Romans used an overline to multiply by 1000, which has no reliable plain-text representation.
Is IL a valid way to write 49?
No. A symbol can only be subtracted from the next two higher symbols in its family, so 49 is XLIX. The tool rejects IL as invalid.
Does the tool accept lowercase input?
Yes. Lowercase Roman numerals are converted automatically, so 'mmxxiv' is read the same as 'MMXXIV'.
What does the symbol breakdown show?
It splits the result into the additive pieces used to build it, so you can see exactly how 2024 becomes M + M + X + X + IV.