How Roman numerals work
Roman numerals use seven letters and two ideas: add and subtract. Letters are written from biggest to smallest and added, so MDCLXVI is 1,666. To avoid four of the same letter in a row, a smaller letter placed in front of a bigger one means subtract, so 4 is IV instead of IIII and 9 is IX instead of VIIII.
The converter follows the standard rules strictly. When decoding, it rebuilds the numeral from the number it read and compares the two, so it can tell you when something is not a correctly written numeral.
Worked examples
2025 is 2000 (MM) + 20 (XX) + 5 (V), so MMXXV. 1994 is M + CM + XC + IV, so MCMXCIV. 3999 is MMMCMXCIX, the largest standard numeral. Going the other way, XLII is XL (40) plus II (2), which gives 42.
Where you still see them
Roman numerals are used for book chapters and prefaces, movie sequels, monarchs and popes (Elizabeth II), the Super Bowl, building cornerstones and clock faces. Copyright dates on films are often written in them as well, so decoding MCMLXXVII means 1977.
For dates and ages, try the age calculator or the date difference calculator.
Questions people ask
How do you write a number in Roman numerals?
Break the number into thousands, hundreds, tens and ones and write each part with the letters M, D, C, L, X, V and I. For 1994 that is 1000 (M) + 900 (CM) + 90 (XC) + 4 (IV), which gives MCMXCIV. The calculator shows these pieces under the answer.
What do the Roman numeral letters mean?
I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1,000. Letters are normally written from largest to smallest and added together, so XVI is 10 + 5 + 1 = 16.
When is a Roman numeral subtracted?
When a smaller letter comes before a larger one, it is subtracted: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. Only I, X and C are used this way, and each only before the next two larger letters, so IC for 99 is not standard and 99 is XCIX.
Why does the calculator stop at 3,999?
Standard Roman numerals have no letter for 5,000 or more. Writing 4,000 would need MMMM, which the standard rules do not allow. Historically, a bar over a numeral multiplied it by 1,000, but that is not part of the simple form used here.
Why is IIII rejected when it is on some clocks?
Some clock faces use IIII instead of IV for 4, a tradition rather than a rule. The calculator accepts only the standard form, so IIII is flagged as invalid, along with other non-standard forms such as VX or IC.