Addition, with six exceptions
Roman numerals are read left to right and added up: MMXXVI is 1000 + 1000 + 10 + 10 + 5 + 1 = 2026. The complication is subtraction. When a smaller symbol sits immediately before a larger one, it is subtracted instead — IV is 4, not 6. Only six such pairs are valid: IV, IX, XL, XC, CD and CM.
The rules that make a numeral canonical
I, X, C and M may repeat up to three times; V, L and D may not repeat at all, since two of them would simply make the next symbol up. Subtraction is limited to one step: you may write IX for 9 but not IC for 99, which must be XCIX. These constraints are why 3,999 — MMMCMXCIX — is the largest number expressible without extra notation.
Plenty of real inscriptions break these rules. Clock faces routinely show IIII rather than IV, and medieval documents are inconsistent throughout. This converter reads non-standard forms and tells you the value, but also shows the canonical spelling.
Beyond 3,999
The classical workaround is the vinculum: a bar over a numeral multiplies it by a thousand. So an overlined V is 5,000 and an overlined X is 10,000, which extends the range to 3,999,999. Because an overline is awkward to type, the same idea is often written with brackets — (IV) for 4,000 — and this converter accepts that form as input while displaying the proper overline.
There is no zero
The system has no symbol for zero and no way to write a negative number or a fraction of one, because it was built for counting and tallying rather than calculation. That absence is precisely why Roman numerals were displaced by Hindu-Arabic digits for arithmetic, surviving today mostly on clock faces, in copyright years, for monarchs and popes, and in the numbering of Super Bowls.