Roman Numeral Converter: Numbers to Roman and Back

How the converter works

Number to numeral uses a greedy method with 13 building blocks: M (1,000), CM (900), D (500), CD (400), C (100), XC (90), L (50), XL (40), X (10), IX (9), V (5), IV (4), and I (1). Starting from the largest, it writes each block as many times as it fits and subtracts, then moves to the next. Including the six subtractive pairs in the list is what guarantees the standard form.

Numeral to number runs the same list in reverse, reading blocks from the left and adding their values. Then it checks its work: it converts the total back to a numeral and compares. If the two differ, the input was not standard form. That is how it catches IIII, VV, and XXXX, and suggests IV, X, and XL instead.

Worked examples

1994. The largest block that fits is M: 1994 − 1000 = 994. Next is CM (900): 994 − 900 = 94. D, CD, and C are too big, but XC (90) fits: 94 − 90 = 4. L, XL, X, IX, and V are too big; IV fits: 4 − 4 = 0. Result: MCMXCIV.

Decoding MMXXVI: M + M = 2,000, X + X = 20, V = 5, I = 1. Total 2,026. Decoding XLIX: XL is 40 and IX is 9, so 49. A common wrong answer for 49 is IL, which breaks the rule that I can only precede V and X.

The largest standard numeral is MMMCMXCIX for 3,999. The longest numeral below 4,000 is MMMDCCCLXXXVIII (3,888) at 15 characters.

The rules of standard form

Symbols are written largest to smallest, left to right, and added. I, X, C, and M can repeat up to three times in a row. V, L, and D never repeat, because VV is just X.

Subtraction only happens in six pairs. I goes before V and X. X goes before L and C. C goes before D and M. V, L, and D are never subtracted. So 45 is XLV, not VL, and 990 is CMXC, not XM.

These rules were standardized well after Rome. Romans themselves were inconsistent: IIII and VIIII appear on ancient inscriptions, and the Colosseum's entrance arches include XLIIII for 44. The strict form taught in US schools is a later convention, but it is the one expected on anything official today.

Where you will see Roman numerals

Super Bowls: the NFL has numbered Super Bowls in Roman numerals since Super Bowl V, except Super Bowl 50 in 2016, which used Arabic digits because L looked weak on branding. Super Bowl LX was played in February 2026, and LXI follows in 2027.

Movie and TV copyright years, often in end credits, such as MMXXVI for 2026. Monarchs and popes: Charles III, Leo XIV. Book front matter, where preface pages are numbered i, ii, iii in lowercase. Outlines, where main sections use I, II, III. Clock faces, watch dials, and building cornerstones. Tattoos of birth and wedding dates, where a mistake is permanent, so double-check with the breakdown this converter shows.

Common mistakes

Writing dates digit by digit. 2026 is MMXXVI, not II-0-XX-VI or anything assembled one digit at a time. There is no zero, so digits cannot be converted independently.

Using IC for 99 or IM for 999. These are tempting shortcuts but not standard. 99 is XCIX and 999 is CMXCIX.

Formatting full dates. For 9/25/2026 in US order, write IX.XXV.MMXXVI, with a separator between month, day, and year. Separators vary: dots, dashes, and middle dots are all common. Pick one and use it consistently.

Mixing case inconsistently. Uppercase is standard for years, monarchs, and outlines. Lowercase (i, ii, iii, iv) is the convention for book front matter and some list styles, including the lower-roman option in CSS list-style-type. The converter accepts either case as input and always outputs uppercase, so lowercase the result yourself if your style guide calls for it.