Roman Numeral Converter Guide: Convert Between Roman & Arabic Numbers (2026)

Rule 1: Additive (left to right, large to small)

When a larger symbol comes before a smaller one, add them: VII = 5 + 1 + 1 = 7

Rule 2: Subtractive (small before large)

When a smaller symbol comes before a larger one, subtract it: IV = 5 - 1 = 4

The 6 Subtractive Combinations

Rule 3: Repetition (max 3 times)

A symbol can repeat up to 3 times: III = 3, XXX = 30, CCC = 300. But never IIII (use IV instead), never XXXX (use XL).

Rule 4: V, L, D never repeat

V (5), L (50), D (500) are never repeated — VV is wrong, use X instead.

Arabic → Roman

function toRoman(num) {
  const vals = [1000,900,500,400,100,90,50,40,10,9,5,4,1];
  const syms = ["M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"];
  let result = "";
  for (let i = 0; i < vals.length; i++) {
    while (num >= vals[i]) {
      result += syms[i];
      num -= vals[i];
    }
  }
  return result;
}

Roman → Arabic

function fromRoman(str) {
  const map = {I:1,V:5,X:10,L:50,C:100,D:500,M:1000};
  let total = 0;
  for (let i = 0; i < str.length; i++) {
    const curr = map[str[i]], next = map[str[i+1]] || 0;
    total += curr < next ? -curr : curr;
  }
  return total;
}

The key insight: if a smaller value appears before a larger value, subtract it. Otherwise, add it. This handles all subtractive cases (IV, IX, XL, XC, CD, CM) naturally.