UsefulTab
Roman Numeral Converter
Convert between Arabic numbers and Roman numerals instantly.
Enter a number or Roman numeral to convert it
Enter a number from 1 to 3999, or a Roman numeral, to see the breakdown into symbol groups — right here in your browser.
Runs entirely in your browserConvert numbers to Roman numerals and back, locally
This converter turns a whole number between 1 and 3999 into a Roman numeral and turns a Roman numeral back into a number. Type in either field — the Arabic number or the Roman numeral — and the other updates as you type. It uses standard subtractive notation, so 4 is IV (not IIII) and 900 is CM (not DCCCC), building each value from the groups M, CM, D, CD, C, XC, L, XL, X, IX, V, IV and I. Roman input is validated for strict well-formedness, so malformed strings like IIII, VV or IC are rejected with a clear message. A breakdown shows exactly how the number splits into symbol groups. Every conversion runs in your browser and nothing you type is uploaded anywhere.
Common questions
How do Roman numerals work?
Roman numerals combine seven letters — I (1), V (5), X (10), L (50), C (100), D (500) and M (1000). You add symbols from largest to smallest, so MMXXIV is 1000 + 1000 + 10 + 10 + 4 = 2024. This tool builds each value from the standard groups M, CM, D, CD, C, XC, L, XL, X, IX, V, IV and I.
What is subtractive notation?
To avoid four repeated symbols, a smaller value placed before a larger one is subtracted: IV is 4 (5 − 1), IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. Only these six subtractive pairs are valid in strict form, which is why the converter produces IV rather than IIII.
Why is the maximum 3999?
Standard Roman numerals have no symbol beyond M (1000) and no zero, so the largest value you can write with plain letters is MMMCMXCIX = 3999. Larger numbers require a vinculum (a bar over a symbol to multiply it by 1000), which this tool does not use.
Is IIII a valid Roman numeral?
Not in strict form. Standard rules cap I, X, C and M at three repeats and use subtractive notation, so 4 is IV, not IIII. (You may see IIII on some clock faces, but that is a stylistic exception.) This converter rejects IIII, VV, IC and similar malformed input.
Are the numbers I type sent anywhere?
No. All conversion and validation run entirely in your browser with JavaScript. Nothing you enter is uploaded or stored on a server.