Math

ROMAN NUMERALS

Convert between Roman numerals and decimal numbers. Supports 1–3999 with conversion breakdown.

Decimal → Roman

—

⇄

Roman → Decimal

—

Current Year in Roman Numerals

REFERENCE TABLE

Decimal	Roman	Name
1	I	Unus
4	IV	Quattuor
5	V	Quinque
9	IX	Novem
10	X	Decem
40	XL	Quadraginta
50	L	Quinquaginta
90	XC	Nonaginta
100	C	Centum
400	CD	Quadringenti
500	D	Quingenti
900	CM	Nongenti
1000	M	Mille

Developer Reference

Core Algorithm & Standalone Script

Standalone, zero-dependency JavaScript implementation powering this tool. Free to inspect, copy, and build upon.

const MAP = [[1000,'M'],[900,'CM'],[500,'D'],[400,'CD'],[100,'C'],[90,'XC'],[50,'L'],[40,'XL'],[10,'X'],[9,'IX'],[5,'V'],[4,'IV'],[1,'I']];

  function toRoman(n) {
    let result = '', parts = [];
    for (const [val, sym] of MAP) {
      while (n >= val) { result += sym; parts.push(`${sym}=${val}`); n -= val; }
    }
    return { result, parts };
  }

  function fromRomanNum(s) {
    const vals = { I:1,V:5,X:10,L:50,C:100,D:500,M:1000 };
    s = s.toUpperCase();
    if (!/^[IVXLCDM]+$/.test(s)) return null;
    let total = 0;
    for (let i = 0; i < s.length; i++) {
      const cur = vals[s[i]], nxt = vals[s[i+1]] || 0;
      if (cur < nxt) total -= cur; else total += cur;
    }
    return total;
  }

  function fromDec() {
    const n = parseInt(document.getElementById('decInput').value);
    const errEl = document.getElementById('decErr');
    const bd = document.getElementById('breakdown');
    errEl.textContent = '';
    if (isNaN(n)) { document.getElementById('decOutput').textContent = '—'; bd.style.display = 'none'; return; }
    if (n < 1 || n > 3999) { errEl.textContent = 'Range: 1–3999'; document.getElementById('decOutput').textContent = '—'; return; }
    const { result, parts } = toRoman(n);
    document.getElementById('decOutput').textContent = result;
    bd.style.display = '';
    bd.innerHTML = `<strong>${n}</strong> = <span class="eq">${result}</span> = ${parts.join(' + ')}`;
  }

  function fromRom() {
    const s = document.getElementById('romInput').value.trim();
    const errEl = document.getElementById('romErr');
    errEl.textContent = '';
    if (!s) { document.getElementById('romOutput').textContent = '—'; return; }
    const n = fromRomanNum(s);
    if (n === null || n < 1) { errEl.textContent = 'Invalid Roman numeral'; document.getElementById('romOutput').textContent = '—'; return; }
    document.getElementById('romOutput').textContent = n;
  }

  // Current year
  const yr = new Date().getFullYear();
  document.getElementById('currentYear').textContent = toRoman(yr).result + ' (' + yr + ')';