Roman numeral converter

MMXXVI: MM = 2000, XX = 20 and VI = 6. Years are converted one place at a time, so 1999 is MCMXCIX (1000 + 900 + 90 + 9) rather than MIM, and 2000 is simply MM. The same rule gives MDCCLXXVI for 1776, the year inscribed on the tablet held by the Statue of Liberty.

In standard form the largest number is 3999, written MMMCMXCIX, because there is no letter for 5000 and a letter repeats at most three times. Excel's ROMAN function returns #VALUE! above 3999 for the same reason. Larger numbers used an overline (vinculum) that multiplies by 1000: V̅ is 5000 and I̅V̅ is 4000, which takes this converter up to 3,999,999.

IV is the standard form; IIII is the additive form, and both mean 4. Standard form uses only six subtractive pairs: IV, IX, XL, XC, CD and CM. Many clock faces still show IIII for four o'clock while using IX for nine, a practice that goes back to clocks such as the late 14th-century Wells Cathedral clock. The converter reads IIII as 4 and gives IV as the standard spelling.

Standard form lets a letter be subtracted only from the next two larger letters: I from V and X, X from L and C, and C from D and M. So 99 is 90 + 9 = XC + IX = XCIX, and 49 is XLIX rather than IL. Excel's ROMAN function breaks the rule only on request: its form 4 writes 499 as ID, while the default form gives CDXCIX.

No. The Roman system had no symbol for zero, so the converter starts at 1. Medieval scholars wrote the Latin word nulla ("none") instead: Dionysius Exiguus used it in 525, and around 725 Bede or a colleague abbreviated it to the letter N.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 8 worked examples whose answers come from independent sources; for example, “2421” is checked against Wikipedia — Roman numerals, standard-form examples (2421 = MMCDXXI); Python greedy conversion (forkA/verify_roman.py).

Wikipedia — Roman numerals (standard form); Microsoft Excel ROMAN and ARABIC functions.