🏛️ Roman Numeral Converter
Convert numbers and dates to Roman numerals and back — and unlike most converters, it rejects invalid numerals like IIII, VX and IC instead of guessing.
What Roman Numeral Converter Does
Roman numerals are built from seven letters — I, V, X, L, C, D and M — and two rules. Symbols are written largest to smallest and added up, except when a smaller symbol sits before a larger one, in which case it is subtracted. MCL is 1,150 by addition; CDXIV is 414 because CD subtracts and IV subtracts.
What most converters skip is that not every string of those letters is a valid numeral. IIII looks like 4 and is not, because no symbol may repeat more than three times. VX looks like 5 and is meaningless. IC looks like 99 and is not permitted, because I may only be subtracted from V or X. Type any of those into most online converters and you will get a confident answer to an invalid question.
This one validates first. If the numeral is malformed it says so and names the rule that was broken, rather than quietly returning a number. That matters more than it sounds, because the usual reason someone is checking a Roman numeral is to make sure it is right before committing it to something permanent.
Which is the other thing worth knowing about this search: a large share of it is dates. Anniversaries, birthdays, tattoos and engraved jewellery. There is no single standard for writing a Roman-numeral date — order, separator and whether to mix Arabic and Roman are all conventions — so it is worth choosing deliberately rather than accepting whatever a converter happens to output.
How to Use Roman Numeral Converter
- Choose Number to Roman or Roman to Number conversion mode
- Enter your value in the input field
- View the accurate converted output instantly
Formula Used by Roman Numeral Converter
The seven symbols
I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1,000
- No zero
- The system has no symbol for nothing — the concept was absent from it entirely
- Order
- Written largest to smallest, left to right, apart from subtractive pairs
Worked example
Converting 768 by place value
- 768 = 700 + 60 + 8
- 700 = DCC
- 60 = LX
- 8 = VIII
Result: DCCLXVIII
Reading a numeral back
Add each value, unless a symbol is smaller than the one after it — then subtract it
- Subtractive pair
- Only IV, IX, XL, XC, CD and CM are permitted
Worked example
MMMCCLXXVIII
- MMM = 3,000
- CC = 200
- LXX = 70
- VIII = 8
Result: 3,278
A date
Convert day, month and year separately, then join them in your chosen order
- Separator
- A dot, slash or space — all conventional, none standard
Worked example
7 September 2026, day-month-year
- 7 = VII
- 9 = IX
- 2026 = MMXXVI
Result: VII · IX · MMXXVI
The rules, and what breaks them
Every one of the invalid forms below is read as a number by most online converters. All are rejected here.
| Rule | Valid | Invalid | Commonly misread as |
|---|---|---|---|
| No symbol repeats more than three times | III, XXX, MMM | IIII, XXXX, MMMM | 4, 40, 4000 |
| V, L and D never repeat | XV, LX, DC | VV, LL, DD | 10, 100, 1000 |
| I subtracts only from V or X | IV, IX | IL, IC, ID, IM | 49, 99, 499, 999 |
| X subtracts only from L or C | XL, XC | XD, XM | 490, 990 |
| C subtracts only from D or M | CD, CM | — | — |
| V, L and D are never subtracted | XV, LX | VX, LC, DM | 5, 50, 500 |
Common values
Including the numbers people most often look up.
| Number | Roman | Number | Roman |
|---|---|---|---|
| 1 | I | 26 | XXVI |
| 4 | IV | 50 | L |
| 5 | V | 100 | C |
| 9 | IX | 400 | CD |
| 10 | X | 500 | D |
| 14 | XIV | 900 | CM |
| 19 | XIX | 1000 | M |
| 20 | XX | 2026 | MMXXVI |
| 24 | XXIV | 3999 | MMMCMXCIX |
Beyond 3,999
The seven letters run out at MMMCMXCIX. Two historical systems extend the range; the vinculum is the one modern converters use.
| System | How it works | Example | Ceiling |
|---|---|---|---|
| Standard letters | I through M, no repeats beyond three | MMMCMXCIX = 3,999 | 3,999 |
| Vinculum | An overline multiplies the symbol by 1,000 | V̄ = 5,000 | 3,999,999 |
| Apostrophus | CIↃ forms, each added Ↄ multiplying by ten | CIↃ = 1,000, CCIↃↃ = 10,000 | CCCIↃↃↃ = 100,000 |
How to Read Your Result
Why IIII appears on clock faces
It breaks the three-repeat rule and it is on a great many clocks anyway. The usual explanations are visual: IIII balances VIII on the opposite side of the dial, and the first four hours then read I, II, III, IIII in a way that looks orderly at a glance. Whatever the reason, it is a deliberate typographic convention rather than correct notation, and a converter that accepts IIII as 4 without comment is not doing you a favor.
Why there is no zero
The system was built for recording and tallying quantities, not for calculation, and a placeholder for nothing was not needed. That absence is also why arithmetic in Roman numerals is so awkward — there are no columns to carry between — and why Arabic numerals displaced them for computation from roughly the 14th century while Roman numerals persisted for numbering, dating and formal display.
Only six subtractive pairs exist
IV, IX, XL, XC, CD and CM. That is the complete list. The pattern is that only the powers of ten — I, X and C — may be subtracted, and only from the next two symbols above them. This is why IC is not 99: I may precede V or X and nothing else. Ninety-nine is XCIX, which is XC followed by IX. It looks longer because it is doing the subtraction twice rather than once.
Dates have no standard, which matters for permanent things
The same date can be written several defensible ways. Order varies by region — day-month-year or month-day-year — and the separator may be a dot, a slash, a space or nothing at all. Some conventions mix systems, putting the month in Roman numerals and leaving day and year in Arabic. None of this is wrong, but the versions look quite different from one another, so anyone converting a date for a tattoo or an engraving should pick a convention on purpose and check the result letter by letter.
Where you still meet them
Clock faces, book front matter and chapter numbers, film copyright lines, monarch and pope regnal numbers, Super Bowls and Olympiads. The common thread is that all of these are labels rather than quantities — nobody needs to add two Super Bowls together. Roman numerals survive precisely in the roles where their arithmetic weakness does not matter and their formality does.
Limitations & Accuracy Notes
- The converter covers 1 to 3,999, the full range of the seven standard letters. Vinculum and apostrophus notation are explained above but not computed here.
- There is no representation of zero or of negative numbers, and fractions were handled by a separate duodecimal system of dots and symbols that has no modern converter equivalent.
- Validation enforces the standard modern rules. Historical inscriptions frequently break them — IIII, XXXX and other non-canonical forms appear on genuine Roman monuments — so a numeral rejected here may still be authentic to its period.
- Date conventions are not standardized. The output follows the order and separator you select, which is a choice rather than a correct answer.
- Years are limited to 3,999 like any other number, which is ample for dates but means the tool cannot express years beyond that without the vinculum.
Frequently Asked Questions
What are the seven Roman numerals?
How does subtractive notation work?
Why is IIII invalid if it seems to mean 4?
How do I write a date in Roman numerals?
What is the largest number you can write?
What is the largest number Roman numerals can represent?
Is there a Roman numeral for zero?
Why is 4 written IV rather than IIII?
Which subtractive combinations are valid?
Where are Roman numerals still used?
Is my data stored?
References & Further Reading
- Britannica — Roman numeral — Encyclopaedic account of the system, its symbols and its historical use
- Calculator.net — Roman Numeral Converter — The most complete competing treatment, including the vinculum and apostrophus systems; its worked examples were used to cross-check the conversions here