Roman numeral converter — both directions
This tool converts any whole number from 1 to 3999 into a Roman numeral, and decodes a Roman numeral back into a number. It is handy for writing years on documents, reading clock faces and monument inscriptions, numbering book chapters or appendices, and checking homework. Decoding uses strict validation, so it tells you when a sequence is not a legal numeral.
Greedy encoding, checked decoding
To encode, the tool walks a fixed table from the largest value down — 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) — appending each symbol while it still fits and subtracting its value.
This is why 1994 becomes MCMXCIV, not MIM. To decode, it adds each symbol’s value
but subtracts when a smaller symbol precedes a larger one (IV = 5 − 1). It then
re-encodes the result and rejects the input if the two do not match, catching
malformed strings like IIII or VX.
Quick reference table
| Number | Roman | Notes |
|---|---|---|
| 4 | IV | Subtractive — not IIII |
| 9 | IX | Subtractive — not VIIII |
| 40 | XL | Subtractive — not XXXX |
| 90 | XC | Subtractive — not LXXXX |
| 400 | CD | Subtractive — not CCCC |
| 900 | CM | Subtractive — not DCCCC |
| 1994 | MCMXCIV | Common exam/quiz year |
| 2026 | MMXXVI | Current year |
| 3999 | MMMCMXCIX | Maximum standard value |
The six subtractive pairs
Roman numerals normally add values left to right. The six exceptions where a
smaller symbol precedes a larger one and is subtracted are: IV (4), IX (9),
XL (40), XC (90), CD (400) and CM (900). These are the only valid
subtractive combinations; sequences like IL (49) or IC (99) are not standard,
even though they look logical. The correct forms are XLIX and XCIX.
Common uses
- Years — film credits, copyright notices and monuments routinely use Roman numerals for years. For example, MCMXCIX is 1999 and MMXXVI is 2026.
- Clock faces — traditional analogue clocks often use Roman numerals, though many use IIII for 4 rather than IV (a historical convention specific to clocks that has been in use for centuries).
- Book chapters and appendices — legal and academic documents use Roman numerals for front matter (pages i, ii, iii) and appendix labels (Appendix I, II, III).
- Super Bowl and Olympiad numbering — major recurring events number their editions in Roman numerals.
Beyond 3999: how the Romans wrote big numbers
The classic seven-symbol system runs out at MMMCMXCIX (3999) — but the Romans did write larger numbers. Two historical extensions existed. The vinculum placed a bar over a numeral to multiply it by 1,000, so V̄ meant 5,000 and X̄ meant 10,000. Earlier still, the apostrophus system built large values from bracket-like forms: ↀ (1,000), ↁ (5,000), ↂ (10,000). Unicode preserves these in its Number Forms block (U+2150–U+218F), which also contains precomposed numerals Ⅰ through Ⅻ used on clock faces and in East Asian typography.
This converter deliberately sticks to plain ASCII letters and the 1–3999 range, because that is what “Roman numerals” means in modern use — page numbering, years, outlines and inscriptions. If you need 12,000 in Roman numerals, the honest answer is that there is no single standard way to write it, and any modern usage should probably use ordinary digits.
Why the subtractive rules feel arbitrary (they partly are)
Classical Roman inscriptions were far less consistent than the rules taught today — IIII for 4 appears throughout Roman sources, and additive forms lingered for centuries. The strict subtractive convention (exactly six pairs, no symbol repeated four times) solidified in medieval and early-modern Europe and is now the standard that exams, style guides and this validator follow. The clock-face IIII is the most visible survivor of the older additive style: many traditional dials use IIII for 4 while keeping IX for 9, a typographic convention (often attributed to visual balance with VIII opposite) that long predates the standardised rules.
Validation: why IIII is rejected
This converter checks every decoded numeral by re-encoding the result and comparing
it to the input. If they do not match, the input is flagged as invalid. This catches
non-standard repetitions like IIII (should be IV), VV (should be X), and
impossible sequences like VX (which would mean 5 before 10, a subtractive
combination that is not one of the six permitted pairs).
For the history of the system and its medieval standardisation, Britannica’s Roman numeral entry is a reliable reference.
Both directions run entirely in your browser — nothing is uploaded.