About the Roman Numerals to Decimal Converter
This tool reads Roman numerals and returns their value. It validates the numeral against the standard subtractive rules, so a malformed numeral such as "IIII" or "VX" is reported rather than misread. Convert several at once, one per line, in any case.
It is handy for reading a copyright date, a chapter number or an inscription. To write numbers as Roman numerals, use Decimal to Roman.
Reading works left to right by comparing neighbours: if a letter is worth less than the one after it, subtract it, otherwise add it. MCMXCIV runs 1000, then C before M takes 100 off 1000, then X before C takes 10 off 100, then I before V gives 4, totalling 1994. Only I, X and C may be subtracted, and only from the next two values up, which is why IC is not 99 and VL is not 45.
Validation is the part that earns its keep, because a lenient parser will happily hand back a number for nonsense. I, X, C and M may repeat at most three times; V, L and D never repeat at all. Film credits and building inscriptions are the usual reason to look a numeral up, and long strings of Ms are exactly where a dropped letter goes unnoticed. To spell the result out in English, use Number to Words.
How to use
- Paste Roman numerals, one per line.
- Copy the numbers.
- Invalid numerals are reported.
Common questions
- Does it reject invalid numerals?
- Yes. Non-standard forms like IIII or IC are reported as invalid.
- Is it case-sensitive?
- No, mmxxvi and MMXXVI both work.
- What is the maximum value?
- 3999 (MMMCMXCIX), the range of standard Roman numerals.