Roman numeral converter — numbers and numerals in both directions
A Roman numeral converter turns everyday Arabic numbers (1, 2, 3…) into Roman letters (I, V, X, L, C, D, M) and decodes Roman strings back to integers. Toolsle's tool supports the standard range used in textbooks and exams: 1 through 3,999. Type in either box and the other field updates live—no button, no page reload. Pair it with our number to words tool for prose spelling, or the hex to decimal converter when you work in other bases.
How Roman numerals work
Seven symbols carry fixed values: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, and M = 1,000. Most values are built by adding symbols left to right (VI = 5 + 1 = 6). Subtractive pairs place a smaller symbol before a larger one to subtract: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, and CM = 900. This keeps strings short and readable compared with repeating ones (IIII vs IV).
To convert a number to Roman form algorithmically, start with the largest value (M = 1000) and work down the standard table—append the symbol while the remainder fits, then move to the next row. Decoding reverses the process: read left to right, adding each symbol unless the next symbol is larger (then subtract the current from the next and skip ahead).
Quick reference table
| Arabic | Roman |
|---|---|
| 1 | I |
| 4 | IV |
| 9 | IX |
| 14 | XIV |
| 49 | XLIX |
| 99 | XCIX |
| 400 | CD |
| 999 | CMXCIX |
| 2026 | MMXXVI |
| 3999 | MMMCMXCIX |
Common uses today
Roman numerals still appear on clock faces, book chapters, movie sequel titles, monument dates, and outline headings in legal and academic documents. Students use converters to check homework; trivia fans decode Super Bowl numbers (e.g. Super Bowl LIX); designers match typographic styles in posters and wedding programs.
Note that some clocks use IIII instead of IV for four—a stylistic tradition, not the subtractive rule taught in school. This converter follows modern educational and Unicode-friendly notation (IV, IX, etc.).
Limits and validity rules
Classical notation without overlines stops at 3,999 (MMMCMXCIX). Larger ancient values used vinculum bars; if you need bigger numbers, extend the system manually or use Arabic digits. Invalid strings—wrong letters, impossible repeats like VV, or out-of-order pairs—won't decode; the tool flags them so you can fix typos.
Practice examples
- XIX → 10 + 9 = 19
- 2026 → MM + XX + VI = MMXXVI
- MCMXC → 1000 + (1000 − 100) + (100 − 10) = 1990
More answers in the Frequently Asked Questions below.