How this calculator works
Roman numerals represent numbers using combinations of seven letters, each with a fixed value: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Most digits are formed by addition (VI = 5+1 = 6), but six specific combinations use subtraction, where a smaller symbol placed before a larger one is subtracted rather than added (IV = 5−1 = 4).
Converting text to numbers is trickier than it looks, because plenty of strings that follow the value-and-position rules loosely are not actually valid Roman numerals. This converter enforces canonical form: it only accepts the one correct way of writing each number, rejecting common malformed variants such as IIII (which should be IV) or IC (which should be XCIX).
The rules
Symbols and values: I=1, V=5, X=10, L=50, C=100, D=500, M=1000Subtractive pairs (the only six): IV=4, IX=9, XL=40, XC=90, CD=400, CM=900A symbol may repeat up to three times in a row (III=3), but never four (IIII is invalid)A smaller symbol may only precede the next one or two symbols up in value, and only once per subtractionStandard Roman numerals cover 1 through 3999. There is no standard symbol for zero, and numbers of 4000 or more require an extended notation (such as a bar over a symbol to multiply it by 1000) that this converter does not implement.
Worked example: 1994 to Roman and back
- Break 1994 into place values: 1000 + 900 + 90 + 4.
- 1000 → M. 900 → CM (the subtractive pair for 900). 90 → XC. 4 → IV.
- Concatenating in order gives MCMXCIV.
- Checking it back: M(1000) + CM(900) + XC(90) + IV(4) = 1994, and re-encoding 1994 produces the same string MCMXCIV, confirming it is in canonical form.
Frequently asked questions
Why is IIII rejected as 4 instead of IV?
Repeating a symbol four times in a row is never standard Roman numeral practice; the subtractive form IV (5−1) is the canonical way to write 4. This converter checks canonical form by converting a parsed value back to a Roman numeral and requiring an exact match, so IIII fails because the canonical form of 4 is IV, not IIII.
Why is IC not accepted for 99?
Subtraction rules only allow a symbol to precede the next one or two symbols above it in value (I can only precede V or X). Since C (100) is far above I (1) in the sequence, IC is not a valid subtractive pair; 99 is correctly written XCIX (XC=90, IX=9).
What is the largest number Roman numerals can represent?
Under the standard system, 3999 (MMMCMXCIX) is the largest value, because M can only repeat up to three times and there is no single symbol above M (1000). Larger numbers historically used a bar (vinculum) over a numeral to multiply its value by 1000.
Is there a Roman numeral for zero?
Not in the standard positional system used for counting. Ancient Romans occasionally used the word 'nulla' to mean 'none,' but there is no conventional single symbol for zero the way modern numerals use 0.
Are lowercase Roman numerals valid?
This converter accepts lowercase or mixed-case input (mcmxciv) and treats it the same as uppercase, since case doesn't affect the value. Uppercase is the conventional way to display Roman numerals in formal writing.