Roman Numeral Converter

Roman numerals still show up on building cornerstones, film copyright lines, clock faces, monarch names, and book chapters. The Roman Numeral Converter translates in both directions: type 2026 to get MMXXVI, or paste MCMLXXXIV to get 1984. It validates the input rather than blindly summing symbols, so malformed numerals such as IC or VV are flagged instead of quietly returning a wrong number.

Two Conversions in One Tool

Choose a direction with the toggle. To Roman takes a whole number from 1 to 3,999 and produces standard notation. To Arabic takes a Roman numeral string and returns its decimal value. Input is accepted as you type, the result appears instantly, and a copy button puts the output on your clipboard. The parser is case-insensitive and ignores surrounding whitespace, which is handy when pasting text out of a document.

How the Conversion Works

Both directions rely on a fixed table of thirteen value-and-symbol pairs, ordered from largest to smallest:

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

Converting a number to Roman is greedy: walk the table from the top, append each symbol and subtract its value while the remainder is large enough. Because the table includes the six subtractive pairs (CM, CD, XC, XL, IX, IV), the output is automatically in the canonical shortest form.

Converting back to a number reads the string left to right, checking two-character pairs first and then single characters. When a pair such as IV is seen, its value is added as a unit. This is what allows a leading smaller symbol to reduce a larger one without a separate subtraction pass.

The Subtraction Rules in Full

Subtraction is not free-form. Only three symbols may act as a subtractor: I, X, and C. Each has exactly one permitted partner, and only the forms below are canonical:

Pair	Value	Meaning
IV	4	5 minus 1
IX	9	10 minus 1
XL	40	50 minus 10
XC	90	100 minus 10
CD	400	500 minus 100
CM	900	1000 minus 100

V, L, and D never subtract, and no symbol subtracts from one more than one step away. That single restriction rules out a family of tempting shortcuts: IL for 49, IC for 99, XD for 490, and XM for 990 are all invalid, even though they look economical. The correct forms are XLIX, XCIX, CDXC, and CMXC. The converter’s round-trip check catches every one of these, because the parsed value converts back to the canonical spelling rather than the shortcut.

How to Read an Unfamiliar Numeral

Reading a long numeral by eye is easier with a left-to-right pass. Compare each symbol with the next: if the following symbol is larger, subtract the current one; otherwise add it. MCMXLV becomes 1000, then (100 before 1000, so minus 100), then 1000, then (10 before 50, so minus 10), then 50, for 1945. Keeping the six legitimate pairs in mind means the rule rarely has to be applied explicitly, since most numerals are built from those pairs. When a value looks impossible, the tool’s error is usually right and the Roman spelling is the thing to check.

Worked Examples

Decimal	Roman	How it breaks down
4	IV	5 minus 1
9	IX	10 minus 1
40	XL	50 minus 10
444	CDXLIV	400 + 40 + 4
1812	MDCCCXII	1000 + 500 + 300 + 10 + 2
1984	MCMLXXXIV	1000 + 900 + 80 + 4
2026	MMXXVI	1000 + 1000 + 20 + 6
3999	MMMCMXCIX	3000 + 900 + 90 + 9

What Counts as Invalid

The converter enforces standard notation with two checks. First, it rejects any run of four or more identical symbols, so IIII and XXXX fail. Second, it parses the input, converts the numeric result back to Roman, and requires an exact match.

Input	Result	Reason
IIII	Invalid	Four repetitions; should be IV
VV	Invalid	Parses to 10, which round-trips to X
IC	Invalid	Parses to 101, which round-trips to CI
MIM	Invalid	Parses to 2001, which round-trips to MMI
XLII	42	Valid
mcmlxxxiv	1984	Valid, case-insensitive
0	Invalid	No zero in Roman numerals
4000	Invalid	Above the 3,999 ceiling

Why the Limit Is 3,999

The seven basic symbols stop at M, which is 1,000. Expressing 4,000 would require MMMM, and 5,000 has no single symbol at all. To go higher, historical writers used a bar over a symbol to multiply it by 1,000 or a variant called apostrophus, but those conventions vary and are not universally readable. Capping at 3,999 keeps every output unambiguous and matches how years and chapter numbers are normally written today.

Where Roman Numerals Still Appear

• Copyright years at the end of films and television shows, such as MMXXVI.
• Foundation stones, monuments, and building dedications.
• Clock faces, where IIII is sometimes preferred to IV for visual symmetry.
• Numbering for monarchs (Charles III), Super Bowls (Super Bowl LVIII), chapters, and appendices.
• Tattoos, engravings, and design work that needs a classical look.

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