Roman Numeral Converter
Convert numbers to Roman numerals and Roman numerals back to numbers, with validation.
How to Convert Roman Numerals
- Type a number (1–3999) to see its Roman numeral form.
- Or type a Roman numeral to see its numeric value.
- Both fields update instantly and independently.
Frequently Asked Questions
Why is the range limited to 1–3999?
Standard Roman numerals don't have a symbol for values of 5,000 or more without using special notation (like a bar over a numeral to multiply it by 1,000), so 3,999 (MMMCMXCIX) is the largest number that can be written with the standard letters alone.
How does the subtractive notation rule work?
Normally symbols are added together (VI = 5 + 1 = 6), but when a smaller-value symbol appears directly before a larger one, it's subtracted instead (IV = 5 − 1 = 4). This subtractive form is only used for specific pairs: IV, IX, XL, XC, CD, and CM.
Why does the Roman-to-number side sometimes show a warning?
This tool double-checks every Roman numeral you enter by converting the resulting number back into its own standard form and comparing it to what you typed. If they don't match exactly — like "IIII" instead of the standard "IV" — it still shows the computed value but flags that the input isn't in standard canonical form.
Is the data I enter sent anywhere?
No — both conversions happen entirely in your browser using JavaScript, so nothing you type is ever sent to a server.