Turn numbers (1–3999) into Roman numerals and Roman numerals back into numbers, instantly. It also catches wrongly-written numerals (like IIII instead of IV). Free, instant and private.
About Roman numerals
Roman numerals combine seven letters — I, V, X, L, C, D and M — using an additive and subtractive system, so 4 is written IV and 1994 is MCMXCIV. They still appear on clock faces, in book chapters, on monuments and in movie copyright years. This converter goes both ways across the standard 1–3999 range and validates Roman input, rejecting non-canonical forms like IIII. Everything runs locally in your browser.
Frequently asked questions
- How do I convert a number to a Roman numeral?
- Enter a whole number from 1 to 3999 — the Roman numeral appears instantly.
- What is the largest standard Roman numeral?
- 3999 (MMMCMXCIX); larger values need overlines outside the standard character set.
- Does it reject invalid numerals?
- Yes — non-standard forms such as IIII or VX are flagged as invalid.
Understanding the Roman Numeral System
Roman numerals rely on a positional system where the values of specific letters are added or subtracted based on their placement. The seven primary symbols represent the values one, five, ten, fifty, one hundred, five hundred, and one thousand respectively. When a symbol of lower value appears before a symbol of higher value, the smaller amount is subtracted from the larger total. Conversely, symbols of equal or decreasing value placed in a sequence are simply added together to reach the final sum.
The modern standard for writing these numerals follows a strict subtractive rule that prevents the repetition of a single character more than three times in a row. For instance, the value four is represented by placing the symbol for one before the symbol for five rather than repeating the symbol for one four times. This convention ensures that every number has one unique, canonical representation. Following this logic, the system effectively covers all integers up to three thousand nine hundred and ninety-nine, which is the maximum limit before the notation requires special symbols for thousands.
The most common mistake users make involves violating the subtractive rules by attempting to subtract a symbol from one that is more than ten times its value. For example, one should not represent ninety-nine by subtracting one from one hundred, as the standard requires breaking the number into tens and ones separately. Additionally, many people incorrectly use non-standard characters or attempt to represent zero, which does not exist in the traditional Roman system. Adhering to these strict grouping and subtraction rules is necessary to maintain the integrity of the numeral sequence.