Overview
A two-way Roman numeral converter for the classical range 1 to 3999. Type a number to see its Roman form, or type Roman numerals to get the number back. Useful for chapter numbers, monarchs and popes, film years, clock faces, outline numbering, and the occasional tattoo sanity-check. Both directions compute instantly in the browser.
How It Works
The top field takes a number between 1 and 3999 and shows the Roman numeral directly beneath it; entering 2026 gives MMXXVI. The bottom field works in reverse: type any string of Roman numerals - lowercase is accepted - and the page shows the number, or the word Invalid for strings that cannot be parsed. Non-Roman characters typed into the Roman field are ignored rather than rejected, so pasted text with stray punctuation still converts. Out-of-range numbers in either direction are reported as such instead of producing silent nonsense.
Step-by-Step Usage Guide
- Type a number from 1 to 3999 in the first field.
- Read the Roman numeral in the result card below it.
- Type Roman numerals in the second field to convert the other way.
- Check the result cards for the out-of-range or invalid warnings where relevant.
Technical Specifications & Standards
The to-Roman direction uses the standard greedy algorithm over the thirteen subtractive pairs and singles, from MM (1000) down to I (1): at each step it appends the largest symbol that fits and subtracts its value. Greedy matching against this exact table is what guarantees canonical subtractive form - 4 becomes IV, never IIII - and it also explains the 3999 ceiling: with no symbol above M, and a rule against four consecutive Ms, 4000 is unrepresentable in classical notation. The from-Roman parser maps I, V, X, L, C, D and M to their values, then walks the string adding each symbol's value, and subtracting instead when a smaller symbol precedes a larger one, which is how IX and CM are evaluated. Because the parser accepts any arrangement that yields a value in range, non-canonical strings like VIIII still parse to 9 - it is a tolerant reader, not a strict validator. The range checks in both directions mean the tool never returns a number it cannot round-trip.
Targeted Use Cases
- Dating documents, title pages and copyright lines in the classical style.
- Deciphering film credits, monument inscriptions and book prefaces.
- Numbering outline sections, appendices or monarchs and popes.
- Verifying a numeral design before committing it to a tattoo or engraving.
Notes & Gotchas
- Stay within 1 to 3999 - the classical alphabet simply cannot represent 4000 or 0.
- Expect tolerant parsing: VIIII converts to 9 even though IX is the canonical form.
- Use the round trip to double-check: number to numeral to number should return the start.
- Lowercase input is fine; the parser normalises to uppercase.
Frequently Asked Questions
Why is the range limited to 3999?
Classical Roman notation has no symbol above M (1000) and writes numbers by repetition and subtraction, so 4000 would need MMMM, which is outside the standard form. Overlines for thousands exist in later notation but are not classical.
How does the converter read subtractive forms like IX?
It walks the string once, adding each symbol's value but subtracting it when a smaller symbol precedes a larger one, so IX reads as 10 minus 1.
What happens if I type invalid characters?
Characters other than I, V, X, L, C, D and M are filtered out before parsing, and a string with no valid value returns Invalid rather than a wrong number.
Does it accept lowercase Roman numerals?
Yes. Input is uppercased before parsing, so mmxxvi and MMXXVI both give 2026.