Roman Numerals Converter

What are Roman numerals?

Roman numerals are a way of writing numbers with letters. The system grew up in ancient Rome and is still in use today on clock faces, building cornerstones, book chapters and prefaces, the names of kings and popes, and the titles of film sequels. It has no place value and no symbol for zero, so a number is built by combining a handful of letters.

How the symbols combine

Three habits cover almost every numeral you will meet:

• Add when the symbols run from large to small. VIII is 5 + 1 + 1 + 1 = 8, and CLX is 100 + 50 + 10 = 160.
• Subtract when a smaller symbol stands directly before a larger one. IV is 5 − 1 = 4 and XC is 100 − 10 = 90.
• Repeat a symbol at most three times in a row. III is 3, but 4 is written IV rather than IIII.

Only I, X and C are used as the subtracted symbol, and each has fixed partners. I may precede V or X, X may precede L or C, and C may precede D or M. That gives the six subtractive pairs IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900).

Converting a number to a Roman numeral

Work from the largest place value down. For each of thousands, hundreds, tens and units, write the numeral for that digit and join them in order.

Example: 1994. 1,000 is M. The remaining 994 has 900 (CM), 90 (XC) and 4 (IV). Joined together: MCMXCIV.

Example: 2024. 2,000 is MM, 20 is XX and 4 is IV, so 2024 is MMXXIV.

Example: 49. 40 is XL and 9 is IX, so 49 is XLIX. Writing IL is a common shortcut, but I can only subtract from V and X.

The converter accepts a string of digits and ignores leading zeros, so 0042 gives XLII. Zero has no Roman numeral, so 0 leaves the answer blank.

Converting a Roman numeral to a number

Read the letters from left to right. Add each value, except when a letter is smaller than the one that follows it, in which case subtract it instead.

Example: MCMXCIV. M = +1000, C before M = −100, M = +1000, X before C = −10, C = +100, I before V = −1, V = +5. The total is 1000 − 100 + 1000 − 10 + 100 − 1 + 5 = 1994.

The converter reads any string made of I, V, X, L, C, D and M, upper or lower case, using exactly that rule. It does not check that the numeral is written in standard form, so IIII is read as 4, IC as 99 and VX as 5. If you need to confirm a numeral is correct, convert the number it gives back into a Roman numeral and compare.

Roman numerals conversion table

Large numbers and zero

With repeats limited to three, the biggest standard numeral is 3999, written MMMCMXCIX. Traditional writing extends the range with a bar over a letter to multiply it by 1,000, which plain text cannot show. This converter keeps things simple and just repeats M, so 4000 comes out as MMMM and 4999 as MMMMCMXCIX. Numbers up to 10,000,000 are accepted.

There is no Roman numeral for zero. Medieval writers used the Latin word nulla when they needed to say "none", but it was never part of the numeral system.