Roman numeral converter for whole numbers

How to convert a number or a Roman numeral

Choose a direction first. The Direction menu offers Arabic to Roman and Roman to Arabic, and the field below changes with it. Under Arabic to Roman, the field is labeled Number, its placeholder shows 1994, and it accepts whole-number digits from 1 through 3999. Under Roman to Arabic, the label switches to Roman numeral, the placeholder shows MCMXCIV, and it accepts standard Roman letters in upper, lower or mixed case.

The Roman letter case (result and breakdown) menu below the field sets the Roman result and breakdown letters, in both directions; an Arabic result always prints digits. Select Convert, or press Enter in the field, to run the conversion. The Result panel shows the converted value with a breakdown line beneath it.

Select Copy result to copy the value, or Download .txt to save it as roman-numeral.txt or arabic-number.txt, depending on the direction chosen. Reset clears the field, the result and the breakdown, and sets Direction and letter case back to Arabic to Roman and UPPERCASE.

Roman numeral symbols and subtractive notation

Seven letters carry a fixed value: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. Standard notation also uses six subtractive pairs for the digits 4 and 9 in each place: IV for 4, IX for 9, XL for 40, XC for 90, CD for 400 and CM for 900. The converter builds a Roman result by taking the largest available symbol or pair again and again until nothing is left, so 1994 becomes M, then CM, then XC, then IV, for 1000 plus 900 plus 90 plus 4.

The breakdown line under the result spells this out in full: first the symbols and pairs, then their values in the same order, then the total, so you can check the arithmetic. Reading a Roman numeral back works the same way in reverse, matching the largest symbol or pair at each position and adding the values, again with a breakdown beneath the number.

Limits and what the converter rejects

Standard notation without overbars covers whole numbers from 1 through 3999, the range this converter supports. Zero, negative numbers, fractions and numbers of 4000 or more are rejected, not rounded or approximated. On the Roman side, only the ASCII letters I, V, X, L, C, D and M are accepted; digits, spaces inside the numeral, punctuation, overbars and look-alike glyphs from other alphabets are all rejected. Spaces before or after the value, and leading zeros in a number, are ignored.

Spelling must be canonical: I, X, C and M may repeat at most three times, V, L and D appear at most once, and each subtractive pair may appear only once, so IIII and IC are rejected in favor of IV and XCIX (see Questions). Typing a Roman numeral into the Number field, or digits into the Roman numeral field, names the mismatch instead of guessing. The converter has no date, calendar or birthday mode: it converts a number or a Roman numeral, nothing else.