The seven symbols and how they combine
Roman numerals use seven letters: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1,000. Numbers are written largest to smallest and added up, so XVII is 10 + 5 + 1 + 1 = 17 and MDCLXVI is 1,666, a number that happens to use every symbol once in descending order. A symbol may repeat at most three times in a row, which is why 30 is XXX but 40 is not XXXX.
Instead, six subtractive pairs cover the gaps: IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. The rule is that only I, X and C subtract, and only from the next two larger symbols, so 99 is XCIX (90 + 9) and never IC. V, L and D never repeat and never subtract. There is no zero and no way to write a fraction or a negative number, and because M is the largest symbol the practical range without overbars is 1 to 3,999, which is what this converter accepts.
Worked examples
Take 2,026. Break it into place values: 2,000 is MM, 0 tens of hundreds contributes nothing, 20 is XX, and 6 is VI, giving MMXXVI. Take 1,994: 1,000 is M, 900 is CM, 90 is XC and 4 is IV, so MCMXCIV. Reading in reverse, scan left to right and subtract whenever a symbol is smaller than the one after it: in MCMXCIV that is M (+1000), C before M (-100), M (+1000), X before C (-10), C (+100), I before V (-1), V (+5), which totals 1,994.
Common sightings: Super Bowl LVIII is number 58; a copyright line reading MMXXV is 2025; the Rocky sequels run II through VI; a cornerstone marked MDCCLXXVI is 1776. On a watch dial the hours run I through XII, though most makers print IIII instead of IV at four o clock so that the left and right halves of the dial look balanced.
Where you still meet them, and what to watch for
Roman numerals survive in outline headings, book prefaces numbered i, ii, iii, monarch and pope names, Super Bowls and Olympiads, chord symbols in music theory, and dates on monuments and film credits. Chemists and some legal documents use them for series numbering. They are terrible for arithmetic, which is exactly why medieval European accounting switched to Arabic digits once they arrived from India through the Arab world.
Two cautions. First, many inscriptions and clocks are not in standard form: IIII for 4 is traditional, and older texts sometimes use IIXX for 18. This converter insists on the standard form when you type a numeral, and if your input adds up to a valid number it tells you the standard spelling instead of silently accepting it. Second, values above 3,999 historically used a vinculum, a bar over a letter meaning multiply by 1,000, so a barred V is 5,000. Since the bar cannot be typed reliably, the tool stops at MMMCMXCIX, which is 3,999.