Roman Numeral Converter

Two converters sit above this. The first switches between a number and a Roman numeral in either direction, using the toggle at the top, and shows the working underneath. The second handles dates and any other string of values, converting each part and leaving your separators alone, so 2026/08/16 comes back as MMXXVI/VIII/XVI.

The seven numerals

Roman numerals are a system for writing numbers that came out of the Roman Empire and stayed in common use across Europe long after it fell, until Arabic numerals displaced them from about the fourteenth century. Modern Roman numerals use seven symbols:

Roman numeral	I	V	X	L	C	D	M
Arabic number	1	5	10	50	100	500	1,000

Each symbol stands for a power of ten or five times a power of ten, and there is no symbol for zero. That absence is not an oversight. The system counts things rather than holding a place, so it has no need for a digit meaning nothing, and it also has no negative numbers and no decimal point.

How to read and write them

There is no single authority on Roman numerals and usage has varied across two thousand years, but the modern conventions are settled enough to state as rules.

1. Equal or descending symbols add. Read left to right and add as you go: III is 3, VI is 6, MCL is 1,150.
2. A smaller symbol before a larger one subtracts. IV is 5 − 1, or 4. The rest of the numeral carries on adding normally, so XIV is 10 + 5 − 1 = 14, CD is 500 − 100 = 400, and CDXIV is 500 − 100 + 10 + 5 − 1 = 414.
3. V, L and D are never repeated and never subtracted. VX does not exist. It is not 15, which is XV, and it is not 5. LC and DM are not valid either, for the same reason.
4. No symbol repeats more than three times. III, XXX and MMM are fine; IIII, XXXX and MMMM are not. Combined with rule 3, that is why 4 is IV rather than IIII and 9 is IX rather than VIIII.
5. Only certain pairs subtract. I goes before V and X only, X goes before L and C only, and C goes before D and M only. So 99 is XCIX, not IC: you cannot jump from 1 to 100 in a single subtraction.

Those last two rules together are what make the system unambiguous. Without them 4 could be written IIII, IV or VI read backwards, and reading would depend on guessing the writer's intent.

Numbers above 3,999

M is the largest symbol and nothing repeats more than three times, so plain numerals stop at MMMCMXCIX, which is 3,999. Two systems extend past that.

The vinculum

A vinculum is a line drawn above a numeral, and it multiplies that numeral by 1,000. This converter uses it, and since the input box cannot draw an overline you type an underscore before the letter instead: _V is 5,000, _X is 10,000.

Because the vinculum multiplies by a thousand and the largest ordinary numeral is 3,999, the largest value this converter can express is 3,999,999. Above 3,999 the answer is written as an overlined thousands group followed by the remainder, so 5,999 is VCMXCIX, which you can type back in as _VCMXCIX.

The apostrophus

The older method uses the symbol Ↄ, a reversed C, paired with C. Think of CↃ as a bracket around a number: each extra pair multiplies by ten, and each Ↄ added on the right of IↃ does the same.

This is where two familiar symbols came from. D is a worn-down IↃ, and M is a worn-down CIↃ, which is why both mean what they mean. The largest value the apostrophus system reaches is CCCIↃↃↃ, or 100,000, and it gets unwieldy quickly, which is why the vinculum is the more practical of the two today.

Converting a number into Roman numerals

Split the number by place value, convert each place on its own, then write the pieces from largest to smallest.

The place-value split is what keeps you out of trouble. Working digit by digit means each part only ever needs the three symbols around its own scale, so you never have to think about whether a subtraction is legal across two different orders of magnitude.

Converting Roman numerals into a number

Break the numeral into runs of the same symbol, convert each run, and add. Watch for subtractive pairs, where a smaller symbol sits in front of a larger one, and treat each pair as a single value.

With practice the pairs stop needing separate attention and you read the whole numeral in one pass, which is roughly how the steps panel above sets it out.

Dates in Roman numerals

The straightforward approach converts each part of the date and keeps the order you already use. In YYYY/MM/DD, 4 March 2025 is MMXXV/III/IV. In DD/MM/YYYY the same date is IV/III/MMXXV. The date converter above keeps whatever separators you type, so the order is entirely yours.

Mixed forms are common too, with the month in Roman numerals and the day and year left as digits, as in III/4/2025 or 4/III/2025. That style is mostly European, and the year-first version of it is rare.

One thing to watch on jewellery and tattoos: a date written as a single unbroken string is ambiguous. MMXXVIIIVI could be read several ways depending on where you break it. Keep the separators.

Days of the week

Roman numerals are sometimes used for weekdays, counting Monday as the first day and Sunday as the seventh, which follows the ISO convention rather than the American one that starts on Sunday.

Day	Roman numeral
Monday	I
Tuesday	II
Wednesday	III
Thursday	IV
Friday	V
Saturday	VI
Sunday	VII

Where Roman numerals are still used

They survive wherever a number is a label rather than a quantity. Clock and watch faces, chapter and volume numbers, act and scene divisions in plays, and the front matter of books, which is numbered i, ii, iii so that the main text can start at page 1.

They also mark editions of recurring events and generations of people: Olympiads, Super Bowls, monarchs and popes, World Wars, and the suffix on a name such as Henry VIII or a third-generation namesake. Film and television copyright dates are often set in Roman numerals as well, a habit from an era when a hard-to-read year made a rerun feel less dated.

What they are never used for is arithmetic. Adding LXIII to XLVII in the numerals themselves is miserable, and long multiplication is close to impossible, which is precisely why Arabic numerals won.

Why clocks often show IIII

Look at a traditional clock face and the 4 is usually IIII, not IV, sitting opposite a VIII. The rules above say IV, so this looks like a mistake that everybody agreed to make.

There is no settled explanation, only good candidates. IIII balances VIII visually across the dial, and the two together give the face a symmetry that IV does not. It also suits manufacturing: with IIII, a set of numerals for one dial needs exactly twenty I, four V and four X, which casts neatly from a single mould of IIIIVX repeated four times. A third story holds that IV was avoided as the opening letters of IVPPITER, the Latin form of Jupiter. Any of them may be true; none is documented well enough to settle it.

Reading your result

If you typed a numeral that reads clearly but is not standard, such as IIII or VIIII, the converter still gives you the value and tells you the conventional spelling. That is deliberate. Older inscriptions and clock faces genuinely use those forms, and refusing to read them would be less useful than reading them and saying so.

Zero, negatives and fractions all return a message rather than an answer, because the system has no way to write them. The Romans did handle fractions, but in a separate duodecimal notation built on twelfths, with S for one half and dots for twelfths, and it does not combine with the numerals here.

For converting between number bases such as binary and hexadecimal rather than between numeral systems, use the Hex Calculator, and for the date arithmetic behind a date you are converting, the Date Calculator works out the days between two of them.