Roman numeral converter
Fill in either box. Numbers become numerals in standard subtractive form. 4 is IV, not IIII, and numerals are read back to numbers, with anything malformed rejected rather than guessed at.
How to convert Roman numerals
Two rules do most of the work. A smaller symbol before a larger one subtracts, so IV is four and IX is nine; only I, X and C can subtract, and only from the next two symbols up, which is why 99 is XCIX and never IC. And no symbol repeats more than three times, which is why 4 is IV rather than IIII. The IIII you see on clock faces is a deliberate stylistic exception and not standard. The system also has no zero and no way to write a negative number, and it stops at 3999 because 4000 would need four M’s. Larger numbers used an overbar meaning "times a thousand", which is not what anyone converting a date needs.
Questions
Standard form does not repeat a symbol more than three times. The IIII on clock faces is a stylistic convention, not correct notation.
MMMCMXCIX, 3999. Beyond that you would need four M’s, which standard form disallows.
No. The system has no zero and no negative numbers; it was built for counting, not arithmetic.
Only I, X and C can subtract, and only from the next two symbols up. I can precede V and X, never C.
2026. M is 1000, so MM is 2000, XX is 20 and VI is 6.
Convert the year as a number. Films and buildings usually use the year alone, so 2026 is MMXXVI.