Roman Numeral Converter

Roman numerals are the number system of ancient Rome. They use seven letters, each with a fixed value, and build every other number by combining them. The system remained Europe's standard way of writing numbers until Hindu-Arabic digits spread from the twelfth century onward, and it survives today on clock faces, in book chapters and outlines, in the names of monarchs and popes, in movie copyright dates and in event titles such as the Super Bowl.

How to read and write Roman numerals

Read the symbols from left to right and add their values. When symbols are written from largest to smallest, simply total them: VIII is 5 + 1 + 1 + 1 = 8, and MDCLXVI is 1,000 + 500 + 100 + 50 + 10 + 5 + 1 = 1,666.

A smaller symbol placed before a larger one is subtracted instead. Only six such pairs are used in the standard form: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). So 14 is XIV, 49 is XLIX rather than IL, and 1990 is MCMXC.

A few rules keep each number's spelling unique. The symbols I, X, C and M can repeat up to three times in a row; V, L and D never repeat, because VV would just be X. Only I, X and C are used as subtractive prefixes, and only before the next two larger symbols. Under these rules the largest number you can write with the seven basic letters is MMMCMXCIX, or 3,999.

Older inscriptions do not always follow these rules. Romans often wrote IIII for 4, and many clock faces still do, and variants such as XIIX for 18 appear on ancient monuments. The strict modern form is what this converter produces and accepts.

Large numbers

Numbers of 4,000 or more need an extension, and the most common is the vinculum: a horizontal bar over a numeral multiplies it by 1,000. V with a bar means 5,000, X with a bar means 10,000, and M with a bar means one million. A number is written as its thousands part with a bar followed by the remainder in ordinary numerals, so 25,430 is X̅X̅V̅CDXXX.

Medieval and Renaissance writers also used other conventions. Some framed numerals with vertical bars on each side as well as a bar on top to multiply by 100,000, and the apostrophus system wrote large values with C and a reversed C around an I, such as CIↃ for 1,000 and IↃↃ for 5,000. These forms are rare today. Very large values were usually written out in words or reckoned on an abacus instead.

How to convert between Roman and Arabic numerals

To write a number in Roman numerals, split it into thousands, hundreds, tens and ones, convert each part separately and join them. For 2,749: 2,000 is MM, 700 is DCC, 40 is XL and 9 is IX, giving MMDCCXLIX. Each digit place only ever uses three symbols, its one, five and ten, so 4 is IV, 40 is XL and 400 is CD.

To read a Roman numeral, scan it from left to right. If a symbol is smaller than the one after it, subtract it; otherwise add it. In MCMXCIV: M is 1,000; C before M subtracts 100 and M adds 1,000; X before C subtracts 10 and C adds 100; I before V subtracts 1 and V adds 5. The total is 1,000 + 900 + 90 + 4 = 1,994.

A useful check is to convert your answer back. If MCMXCIV turns back into 1994, both directions are correct.

Dates in Roman numeral form

Dates are written in Roman numerals by converting the day, month and year separately and joining them with dots, dashes, slashes or spaces. The order follows local custom: Americans usually write month, day, year, most of the world writes day, month, year, and the ISO 8601 standard puts the year first. July 4, 1776, for example, becomes VII·IV·MDCCLXXVI in US order.

Roman-numeral dates are popular for tattoos, wedding rings, monuments and the copyright line at the end of films and television programs. Because there is no zero in Roman numerals, a two-digit year such as 2000 written as 00 has no direct equivalent, so the full year is safer.

Frequently asked questions