Why is 4 written IV and not IIII?

Subtractive notation: a smaller numeral before a larger one subtracts (IV = 5-1, CM = 900). Clock faces traditionally break this rule and print IIII, but standard Roman numerals use subtraction.

What is the largest number this handles?

Standard notation tops out around 3999 (MMMCMXCIX) because there is no classical symbol above M. Anything larger needs vinculum or archaic forms that no modern convention agrees on.

Why does my Roman input fail validation?

Likely an illegal pattern like IC for 99 (must be XCIX) or repeated V, L, D. The validator enforces classical rules so you do not engrave a typo in stone.

Is IIII ever correct?

On clocks, watch dials and some monuments, yes — IIII is the traditional engraved form, often said to be chosen for visual symmetry with VIII opposite it. It is simply not the subtractive form this tool validates, so treat it as decoration rather than as input.

How would I write 5,000 or 100,000?

You cannot in plain ASCII. The classical extension places an overline (vinculum) over a numeral to multiply it by 1,000, so V with a bar means 5,000. With no way to render or parse a bar, the tool stops at 3,999.

Roman numeral converter — Guide

Convert Roman numerals to numbers and convert numbers to Roman numerals.

What it is and when to use it

Convert between Roman numerals and Arabic numbers in both directions, with strict validation on the Roman side. Reading years on buildings and film credits (MCMXCIV), parsing chapter or monarch numbering, or generating stylistic numerals for a design — all from one field. The Roman input is checked against the canonical subtractive form, which means it has to be a numeral this tool itself could have produced: MMMCMXCIX passes, while IIII and VX do not, even though both appear on real clocks and monuments.

Converts between Roman numerals and Arabic numbers — useful for ordinals and copyright years.

How to use it

Enter a number or a Roman numeral on either side
The other side updates automatically
Copy and use it

A worked example

Fill in a sample number

Copy this into the tool, or open Roman numeral converter

Things that trip people up

The supported range is 1 to 3999. Beyond that, classical notation needs a vinculum (overline) to multiply by 1,000, which has no ASCII form.
Out-of-range numbers return an empty result rather than an error — a blank output can mean “too big”, not “broken”.
Validation is canonical-form only: IIII is a legitimate historical engraving but not a valid modern numeral.

Questions people ask

Why is 4 written IV and not IIII?: Subtractive notation: a smaller numeral before a larger one subtracts (IV = 5-1, CM = 900). Clock faces traditionally break this rule and print IIII, but standard Roman numerals use subtraction.
What is the largest number this handles?: Standard notation tops out around 3999 (MMMCMXCIX) because there is no classical symbol above M. Anything larger needs vinculum or archaic forms that no modern convention agrees on.
Why does my Roman input fail validation?: Likely an illegal pattern like IC for 99 (must be XCIX) or repeated V, L, D. The validator enforces classical rules so you do not engrave a typo in stone.
Is IIII ever correct?: On clocks, watch dials and some monuments, yes — IIII is the traditional engraved form, often said to be chosen for visual symmetry with VIII opposite it. It is simply not the subtractive form this tool validates, so treat it as decoration rather than as input.
How would I write 5,000 or 100,000?: You cannot in plain ASCII. The classical extension places an overline (vinculum) over a numeral to multiply it by 1,000, so V with a bar means 5,000. With no way to render or parse a bar, the tool stops at 3,999.

Next steps

Open the Roman numeral converter tool

Spotted something wrong on this page? Tell us via Contact Us