Last updated 26 August 2026
The subtractive rule
Roman numerals normally add symbols left to right — VI is 5+1=6. But four is written IV, not IIII, because a smaller symbol placed before a larger one means subtract instead: 5-1=4. The same rule gives IX for 9, XL for 40, and CM for 900. This tool handles both directions and, when converting from Roman numerals, checks that the input is actually a valid, correctly-formed sequence rather than just guessing at a number.
Common questions
Why is there an upper limit of 3999?
Standard Roman numerals don't have a symbol for numbers at or above 4000 without extra notation (like a bar over a letter to mean times-1000), which isn't part of everyday usage. 1 through 3999 covers what the classic seven symbols (I, V, X, L, C, D, M) can represent directly.
Why is 4 written as IV instead of IIII?
Roman numerals use subtractive notation for four specific cases — a smaller symbol placed before a larger one means subtract it. IV (5-1=4) and IX (10-1=9) are shorter and the standard modern form, though IIII does appear historically, especially on clock faces.
What happens if I type an invalid Roman numeral?
It's flagged rather than guessed at — the converter checks that converting back to a number and then back to Roman numerals produces the exact same text you typed, which catches malformed sequences that use the right letters in an invalid order.