Roman Numeral Converter

This Roman numeral converter changes whole numbers into Roman numerals and reads Roman numerals back as numbers. Roman numerals still turn up on clock faces, book chapters, film credits and monuments. This converter changes whole numbers into Roman numerals and back, for anything from 1 to 3,999, and shows exactly which symbols make up the answer.

The building blocks

I=1  V=5  X=10  L=50  C=100  D=500  M=1000
4=IV  9=IX  40=XL  90=XC  400=CD  900=CM
1994 = M + CM + XC + IV = MCMXCIV

A smaller symbol placed before a larger one subtracts; otherwise symbols add from left to right.

Canonical form only

The converter uses the greedy subtractive rules, so it never produces IIII for 4 or VIIII for 9. When decoding, it round-trips the result — converting the number back to Roman numerals — and rejects anything that does not match, which catches malformed inputs.

Worked example

Converting 2026, the greedy algorithm takes the largest symbol that fits at each step:

2026 − 1000 = 1026   M     → M
1026 − 1000 =   26   M     → MM
  26 −   10 =   16   X     → MMX
  16 −   10 =    6   X     → MMXX
   6 −    5 =    1   V     → MMXXV
   1 −    1 =    0   I     → MMXXVI

The largest value in range is 3999 = MMMCMXCIX, where the greedy step picks CM for 900 and IX for 9 rather than DCCCC and VIIII. Going the other way, MMXXVI decodes back to 2026, and so does mmxxvi — input is upper-cased before it is read.

Why the range stops at 3,999

Standard Roman numerals have exactly one symbol for a thousand, M, and no agreed symbol for anything larger. Writing 4,000 as MMMM breaks the subtractive convention the rest of the system depends on, so the converter declines rather than inventing an answer.

Historically a horizontal line over a numeral — a vinculum — multiplied it by a thousand, which gave a way to reach into the millions. There is no single plain-text way to write that line and sources render it differently, so it is deliberately not supported here.

There is also no zero. Roman numerals are additive and subtractive but not positional, so a placeholder digit was never needed and the count starts at 1 rather than 0.

What gets rejected, and why IIII is one of them

Decoding does not simply add up the symbols. The total is converted back to Roman numerals and compared with what you typed, so only canonical forms are accepted:

• IIII totals 4, but 4 converts to IV, so the round-trip fails and it is rejected.
• VX and IC fail the same way — subtractive pairs are valid in only six combinations: IV, IX, XL, XC, CD and CM.
• Any character outside M, D, C, L, X, V and I is refused before the arithmetic starts.

This is a deliberate choice, and it is the one place the converter disagrees with the real world. Plenty of clock faces and watch dials show 4 as IIII for visual balance. That is a stylistic tradition rather than a canonical numeral, so it will not decode here — read it as 4. Number Base Converter handles the same kind of positional-symbol question for binary, octal and hexadecimal.

Private and instant

Conversions happen in your browser with no upload and no sign-up.