Roman numeral converter
Convert between Arabic numbers and Roman numerals (1–3999).
Runs in this tab. The file stays here.
Strict subtractive notation only — IIII and VX are rejected. Range is 1–3999 (no vinculum extensions).
This runs in your browser. Nothing is uploaded.
About Roman numeral converter
Convert Arabic numbers ↔ Roman numerals for 1–3999 with strict validation — offline.
Roman numeral converters that accept IIII or VX teach bad habits. This page uses standard subtractive notation and rejects malformed strings by requiring a perfect round-trip to the canonical form.
Range is 1–3999 — the usual limit without vinculum/overline extensions. Out-of-range and invalid inputs are distinguished in the UI.
Useful for outlines, clock faces, and copyright lines when you want the arithmetic to be correct, not decorative.
How to use Roman numeral converter
- 1Type an Arabic integer or a Roman string.
- 2Read the live conversion in the other field.
- 3Fix any inline validation error if the Roman form is non-canonical.
What it won't do
- Integers 1–3999 only.
- No archaic or clock-face special cases beyond the standard map.
Common questions
Why was IIII rejected?
Canonical four is IV. Strict mode refuses additive forms that are not standard subtractive notation.
Can I convert 4000 or larger?
Not on this page. Above 3999 needs non-standard overline notation we deliberately omit.
Is this case-sensitive?
Input is normalized to uppercase before parsing.
Related tools
Why this one doesn't upload your file
There is no server to upload to. This page is a static file, and the work happens in your browser using the same graphics and WebAssembly code that renders every other site you visit. Your file is read from disk into memory, processed, and handed back as a download. Once the page has loaded you can disconnect from the network entirely and it keeps working. The longer explanation