The seven symbols
Standard Roman numerals use seven symbols: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, and M is 1,000. You build a number by addition, with a subtraction when a smaller symbol stands before a larger one. VI is 5 + 1 = 6. IV is 5 − 1 = 4. The subtractive pairs in common use are IV (4), IX (9), XL (40), XC (90), CD (400), and CM (900). This converter accepts those and rejects strings that are not the standard form, including IIII for 4 and IC for 99.
The practical upper end of this system, without an overline for thousands, is 3,999, written MMMCMXCIX. Zero has no Roman numeral. Decimals and negatives are not part of the system, so the number box accepts whole numbers from 1 through 3,999 only.
Worked examples
A reliable way to convert a number is to peel off the largest symbol that still fits, including the subtractive pairs, and repeat. For 2026, M fits twice (2,000), XX fits (20), V fits (5), and I fits once. Nothing is left. The same procedure fixes a numeral you are reading: add each symbol, but subtract it when the next symbol is larger. That is why the C in CM is subtracted from the following M.
- 2026 = 1,000 + 1,000 + 10 + 10 + 5 + 1 = MMXXVI.
- 1994 = 1,000 + 900 + 90 + 4 = M + CM + XC + IV = MCMXCIV.
- 58 = 50 + 5 + 3 = LVIII. It is not ILVIII or LVIIII.
- Reading MCMXCIV from the left: M = 1,000, CM = 900, XC = 90, IV = 4, and 1,000 + 900 + 90 + 4 = 1,994.
Forms this page will not invent
Clock faces sometimes use IIII, and some older inscriptions are more additive than the schoolbook form. Those variants are readable to a person and ambiguous to a strict converter, so they are treated as invalid here. The round trip is the test: a numeral is accepted only when converting it to a number and back produces the same string. IIII becomes nothing, because the standard form of 4 is IV. VV is not 10; X is.
Years are the everyday use. 2026 is MMXXVI, 2000 is MM, and 1999 is MCMXCIX. Superbowl-style numerals and book chapters use the same rules. If you need the number spelled in English words instead of Roman symbols, that is the number to words page.