Roman Numeral Converter with Strict Validation

Convert Roman numerals to numbers and numbers to Roman numerals

Type in either box and the other updates as you type. Enter 1994 and you get MCMXCIV; type MCMXCIV (or mcmxciv — case doesn't matter) and you get 1994 back. Everything runs in your browser, so there is nothing to submit and nothing is sent anywhere.

Roman numerals are built from seven letters: I (1), V (5), X (10), L (50), C (100), D (500) and M (1000). Values are written from largest to smallest and added up, with one exception — subtractive notation. When a smaller numeral sits directly in front of a larger one it is subtracted, which is how 4 becomes IV instead of IIII and 900 becomes CM. 1994 breaks down as M (1000) + CM (900) + XC (90) + IV (4).

The standard system has no zero and no negative numbers, and it tops out at 3999 (MMMCMXCIX), because writing 4000 would need four Ms in a row. For larger values the converter supports the vinculum: an overline over a numeral multiplies it by 1,000, so V̅ is 5,000 and M̅ is 1,000,000. Switch the overline option on to convert anything up to 3,999,999. The overline is written with the Unicode combining overline character (U+0305) placed after each letter, so you can copy the result into documents; how cleanly the line renders depends on the font.

Why the converter rejects IIII, VX and IC

Many converters accept any string of Roman letters and add it up loosely, so IIII comes back as 4 and IC as 99. That is fine for decoding a clock face but useless if you are checking whether a numeral is correct. This tool only accepts the canonical form — the single spelling the modern rules allow — and tells you which rule a bad input breaks:

I, X, C and M may repeat at most three times in a row, so IIII must be IV and XXXX must be XL. V, L and D are never repeated (VV is just X) and never subtracted, which rules out VX and LC. Only I, X and C can be subtracted, and only from the next two higher numerals: I from V and X, X from L and C, C from D and M. That leaves exactly six subtractive pairs — IV, IX, XL, XC, CD and CM — so IC, IL and XM are all invalid; 99 is XCIX. A subtracted numeral cannot itself be doubled, so IIX is not 8; that is VIII. Anything else that only adds up loosely, such as IXI, is reported together with the value it seems to mean and its correct spelling.

One honest caveat: IIII is still used on many clock and watch faces for 4, and medieval texts used plenty of non-standard forms. They are not wrong in those contexts, they are simply not the canonical modern form this tool validates.

Roman numeral date converter and reference chart

Switch to Date mode to turn a date into Roman numerals — popular for tattoos, wedding engravings and anniversary gifts. Enter the date as three numbers in the order you choose (month-day-year, day-month-year or year-month-day) and each part is converted separately: 07.04.1776 becomes VII.IV.MDCCLXXVI. You can pick a dot, slash, dash or middle dot as the separator. The date is checked first, so 02.30.2024 or February 29 in a non-leap year is flagged instead of being converted into a date that never existed. Years must be between 1 and 3999. Double-check the order before anything gets engraved: 04.07 means April 7 in the US and 4 July in most other countries.

The reference chart below the converter lists every basic symbol, the six subtractive pairs and the overlined thousands, so you can verify a result by hand.