How Roman numerals work
Roman numerals use seven letters, each with a fixed value:
| Letter | I | V | X | L | C | D | M |
|---|---|---|---|---|---|---|---|
| Value | 1 | 5 | 10 | 50 | 100 | 500 | 1,000 |
You build a number by writing letters from the largest value to the smallest and adding them up. So VIII is 5 + 1 + 1 + 1 = 8, and MDC is 1,000 + 500 + 100 = 1,600.
The twist is subtraction. When a smaller letter comes right before a larger one, you subtract it. That is how 4 and 9 are written in every place:
Converting a number to Roman numerals
Split the number into thousands, hundreds, tens and ones, then write each place on its own and join them. The converter does the same thing with a “greedy” method: it keeps taking the largest value that still fits.
Example: 1994. 1,000 is M. 900 is CM. 90 is XC. 4 is IV. Together that is MCMXCIV.
Example: 2026. 2,000 is MM, 20 is XX and 6 is VI, so the year is MMXXVI.
Example: 49. It is tempting to write IL (50 − 1), but that breaks the rules. Split it into 40 + 9 instead: XL + IX = XLIX.
The rules that make a numeral valid
Many strings of Roman letters can be read as a number, but only one form is standard. The converter checks every rule and tells you which one a numeral breaks:
- I, X, C and M repeat at most three times in a row. 4 is IV, not IIII. 40 is XL, not XXXX.
- V, L and D never repeat. VV would be 10, which is simply X.
- Only I, X and C are subtracted, and only from the next two letters up: I from V and X, X from L and C, C from D and M. So 99 is XCIX, never IC.
- Only one letter is subtracted at a time. IIX is not 8; 8 is VIII.
- Places go from largest to smallest. IXI or XCX are not valid, even though you could guess a value.
Clock faces often show IIII instead of IV. That is a long design tradition, not the standard form, so the converter flags it and suggests IV.
Numbers above 3,999: the vinculum
With the seven letters and the three-repeat rule, the largest number is MMMCMXCIX (3,999). For bigger numbers, Roman and medieval writers drew a bar over letters, called a vinculum. A bar multiplies the letters under it by 1,000.
So 4,700 is IV with a bar (4,000) followed by DCC (700). The largest number this converter writes is 3,999,999: MMMCMXCIX with a bar, then CMXCIX.
A bar is hard to type, so the converter accepts three ways to enter it:
| You type | Meaning | Value |
|---|---|---|
| (IV)DCC | letters inside brackets carry the bar | 4,700 |
| _I_VDCC | an underscore before each barred letter | 4,700 |
| I̅V̅DCC | letters followed by a combining overline | 4,700 |
The result shows a real overline, and you can copy it either with Unicode overlines or in the bracket form, which works everywhere. The barred part must come first, and the part after it must be below 1,000. Numbers from 1,000 to 3,999 are always written with M, so the bar starts at 4,000.
Be aware that the bar was never fully standardised. Some ancient inscriptions used it for other things, and some writers used frames or extra symbols for large numbers. For anything official, stick to 1 to 3,999.
Where you still see Roman numerals
- Years on films, buildings and monuments, such as MCMXCIV for 1994.
- Outlines and lists, where I, II, III mark sections and i, ii, iii mark sub-points.
- Monarchs, popes and sequels, like Elizabeth II or Rocky IV.
- Clock faces and watches, often with the IIII variation.
- Book front matter, where page numbers like xiv come before page 1.
There is no Roman numeral for zero, and the system has no negative numbers or decimals, so the converter only accepts whole numbers from 1 upward. If you are converting units rather than numerals, try the length converter. If you need to count the years since a date written in Roman numerals, convert it here and then use the age calculator or the date calculator.