Roman Numeral Converter

Roman numeral conversion uses an additive and subtractive system based on seven fixed letter values. Understanding the rules allows you to convert any integer from 1 to 3,999 into Roman numerals and back.

Seven base symbols:

Addition rule: When a symbol of equal or lesser value follows a larger symbol, add the values.

• XI = 10 + 1 = 11
• VII = 5 + 1 + 1 = 7

Subtraction rule: When a symbol of lesser value precedes a larger symbol, subtract it.

• IV = 5 − 1 = 4
• IX = 10 − 1 = 9
• XL = 50 − 10 = 40
• XC = 90, CD = 400, CM = 900

Valid subtractive pairs (only these six are used):

• I before V or X (4, 9)
• X before L or C (40, 90)
• C before D or M (400, 900)

Conversion algorithm (Arabic → Roman): Work from largest to smallest value. For each step, subtract the largest applicable symbol value, write that symbol, and repeat.

Repetition limits:

• I, X, C and M may repeat up to three times in a row (III = 3, MMM = 3,000). A fourth is written subtractively instead: IIII is non-standard, IV is correct.
• V, L and D never repeat at all. VV would be 10, and 10 already has its own symbol, X.
• These two rules together are why the system stops at 3,999. Reaching 4,000 would need MMMM, and there is no symbol above M in the standard set.

This converter enforces all of that. Type IIII, IC or VX and it will tell you which rule the numeral breaks and print the correct form, rather than quietly returning a number. Loose converters accept IC as 99, but no Roman ever wrote it that way: only I before V or X, X before L or C, and C before D or M are valid subtractions.

Worked example, 2,024:

• 2,024 ÷ 1,000 = 2 → MM (remainder 24)
• 24 ÷ 10 = 2 → XX (remainder 4)
• 4 = IV → IV
• 2024 = MMXXIV

Worked example, 1,987:

• 1,000 → M (remainder 987)
• 900 = CM → CM (remainder 87)
• 50 → L (remainder 37)
• 10 × 3 → XXX (remainder 7)
• 5 → V (remainder 2)
• 1 × 2 → II
• 1987 = MCMLXXXVII

No zero, and no place value. That is the real difference between this system and the one you grew up with. The Romans had no symbol for nothing, and the position of a letter carries no weight of its own the way the 2 in 200 does. It is why arithmetic in Roman numerals is miserable and why the system survives only for labelling things, never for calculating with them.

You meet them more often than you would guess: clock faces, book chapter and preface pages, film copyright lines in the credits, Super Bowl numbering, and the names of monarchs and popes.

One exception lives on clocks. Four is very often written IIII rather than IV on a dial, purely for visual balance against the VIII opposite. This page recognises that form and tells you what it means rather than calling it an error, because it is not one. It is just not the standard spelling, and every other non-standard form here does get rejected with the rule it breaks.