ⅫRoman Numeral Converter

Convert numbers to Roman numerals (1–3999) and Roman numerals back to numbers, with strict validation that rejects non-standard forms like IIIM or VX.

Worked examples

2026 → Roman

GET /api/v1/converters/roman-numeral-converter?value=2026&op=to_roman

Result: MMXXVI

MMXXVI → number

GET /api/v1/converters/roman-numeral-converter?value=mmxxvi&op=from_roman

Result: 2026

1994 → Roman

GET /api/v1/converters/roman-numeral-converter?value=1994&op=to_roman

Result: MCMXCIV

Machine API (x402)

$0.001 / call

This tool is also a JSON API for AI agents. Requests without payment receive 402 Payment Required plus instructions; agents pay USDC on Base via the x402 protocol — no accounts, no API keys.

GET /api/v1/converters/roman-numeral-converter?value=2026&op=to_roman HTTP/1.1
Host: agenttools-hub.vercel.app

→ 402 (payment required, instructions in headers)
→ 200 (after X-PAYMENT header; JSON body below)

{
  "tool": "converters/roman-numeral-converter",
  "input": {"value":"2026","op":"to_roman"},
  "result": { "value": 2026, "answer": "MMXXVI" }
}

Agent docs: /llms.txt · OpenAPI spec · integration guide

About this tool

Two-way Roman numeral conversion for years, chapter numbers, outlines and clock faces. Numbers map to the standard subtractive notation, and Roman input is strictly validated against that same standard form.

greedy decomposition: M(1000), CM(900), D(500), CD(400), C(100), XC(90), L(50), XL(40), X(10), IX(9), V(5), IV(4), I(1)

Frequently asked questions

What is the largest number in Roman numerals?

With the standard seven symbols and no overline notation, the range is 1–3999 (MMMCMXCIX).

Why was my input rejected?

The converter only accepts standard-form numerals. Forms like IIII (instead of IV), VX or IIIM are historically incorrect and are rejected with an error.