MATH · ROMAN NUMERAL CONVERTER
Roman Numeral Converter
Convert any integer from 1 to 3999 to Roman numerals, or convert a Roman numeral back to an integer. Validates both directions and rejects non-standard forms like IIII or VV.
About This Calculator
Roman numerals use seven letters (I, V, X, L, C, D, M) combined with subtractive notation to represent integers from 1 to 3999. This converter works in both directions: enter a number to get the Roman form, or enter a Roman numeral to get its integer value. Non-canonical forms like IIII (should be IV) are flagged as invalid.
How It Works
In Integer → Roman mode, enter any whole number from 1 to 3999 and the result appears instantly. In Roman → Integer mode, type a Roman numeral using the characters I, V, X, L, C, D, M. The converter validates the input with a roundtrip check — only canonical subtractive forms pass.
The Formula
The standard Roman numeral table lists 13 values from M (1000) down to I (1). Conversion from integer to Roman works by repeatedly subtracting the largest table value that fits and appending its numeral. Conversion from Roman to integer uses the subtractive rule: if a smaller value precedes a larger one, it is subtracted rather than added (e.g. IV = 5 − 1 = 4).
Frequently Asked Questions
- Why does IIII get rejected?
- Modern standard Roman numerals use subtractive notation exclusively. The canonical form of 4 is IV (one before five), not IIII. Some ancient inscriptions and clock faces do use IIII, but it is not standard subtractive notation.
- What is the largest number you can write in Roman numerals?
- The largest standard Roman numeral is MMMCMXCIX = 3999. Beyond that, vinculum (bar) notation would be needed, which is not supported here.
- Why is there no zero in Roman numerals?
- Roman numerals were developed for counting and commerce, where zero as a concept wasn't used. The modern concept of zero was introduced later from Indian mathematics.
- What year is MMXXVI?
- MMXXVI = 2000 + 20 + 6 = 2026.