Convert Roman Numerals Online
Convert a whole number to Roman numerals or decode a Roman numeral back to a number. The converter rejects nonstandard forms and shows how each value contributes.
Written and checked by TheToolNet Editorial Team · Last reviewed: September 19, 2026 · Editorial method
Roman numeral converter
Roman numeral chart
| Roman | Number | Roman | Number |
|---|---|---|---|
| I | 1 | V | 5 |
| X | 10 | L | 50 |
| C | 100 | D | 500 |
| M | 1000 | MMMCMXCIX | 3999 |
How Roman numeral conversion works
Usually values add from left to right. A smaller allowed symbol before a larger one subtracts: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400, and CM is 900. This page uses canonical modern notation, so forms such as IIII are rejected in favor of IV.
Popular Roman numeral examples
XVII is 10 + 5 + 1 + 1 = 17. LXVII is 50 + 10 + 5 + 1 + 1 = 67. XL is 50 − 10 = 40. For calendar formatting, use the date-to-Roman-numerals converter.
Frequently asked questions
What range does the converter support?
It supports the common modern system from 1 through 3999. Larger values require an overline convention that plain text cannot represent reliably.
Are lowercase letters allowed?
Yes. They are normalized to uppercase before validation.
Why was my Roman numeral rejected?
It may use an invalid subtractive pair or a noncanonical repetition. The tool converts the decoded value back to canonical form as a check.
Reference
The conversion follows the standard seven symbols and conventional subtractive pairs used in modern educational notation.