Translate integers to Roman symbols I,V,X,L,C,D,M and back with validation.
Overview
Roman numerals use letters for values: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Subtractive pairs like IV (4) and IX (9) avoid four repeated I characters in standard modern notation.
Conversion typically greedily subtracts symbol values from largest to smallest, with special cases for IV, IX, XL, XC, CD, CM. Reverse parsing scans left to right, adding or subtracting symbol values based on whether the next symbol is larger.
Modern usage appears on clock faces, book chapters, outlines, and movie credits. Historical manuscripts varied; this tool follows common modern school rules, not every ancient variant.
There is no Roman zero. Large numbers historically used vinculum overbars; many calculators cap at 3999 (MMMCMXCIX) when using strict classical rules without overbar extension.
Steps
Decimal to Roman or Roman to decimal.
Type Arabic number or Roman string.
Reject illegal sequences like IL or VV.
Handle IV, IX, XL, XC, CD, CM pairs.
Display converted form.
Confirm value within supported maximum.
Math
Greedy decomposition with subtractive pairs {IV,IX,XL,XC,CD,CM}
Subtract symbol value when a smaller symbol precedes a larger one; otherwise add symbol values left to right.
Worked cases
1000+900+90+4 => M+CM+XC+IV
MCMXCIV.
50−10 + 1+1 = 42
42.
Non-standard; use IV for 4
Validation error in strict mode.
Why use this
Questions