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Roman Numeral to Number Converter

Choose a direction and enter a Roman numeral or a number to convert.

How it is calculated

In Roman to number mode, the letters (case does not matter) are read left to right using I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000. If a symbol is smaller than the one after it, its value is subtracted; otherwise it is added. In number to Roman mode, the largest value from the list M, CM, D, CD, C, XC, L, XL, X, IX, V, IV, I is taken repeatedly until nothing remains.

Number to Roman works for whole numbers from 1 to 3999; anything else gives no result, and a decimal such as 12.9 is cut to 12. Roman to number accepts only the seven letters, but it does not check that the numeral is properly formed, so IIII reads as 4 and IC as 99, even though the standard forms are IV and XCIX.

Worked example

Convert MCMXCIV. Read it in parts: M = 1000, CM = 1000 − 100 → 900, XC = 100 − 10 → 90, IV = 5 − 1 → 4. Adding gives 1000 + 900 + 90 + 4 = 1,994. In the other mode, entering 1994 gives MCMXCIV back, so the two directions can be used to check each other.

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Common mistakes

  • Typing a number above 3999 in number to Roman mode: standard numerals stop at MMMCMXCIX (3999), so nothing is shown.
  • Trusting a non-standard numeral because it returned a value: IIII and IC are read as 4 and 99, though the standard forms are IV and XCIX.
  • Leaving the mode on Roman to number while typing digits (or the reverse): digits are not Roman letters, so no result appears.
  • Entering 0 or a negative number: Roman numerals have no zero and no negatives, so the range starts at 1.

About this calculator

Roman numerals build numbers from seven symbols — I=1, V=5, X=10, L=50, C=100, D=500, M=1000 — normally added together left to right, but with a subtractive rule where a smaller symbol placed before a larger one is subtracted instead (IV = 4, IX = 9, XL = 40, CM = 900). Converting the other direction, from an integer to Roman numerals, works by greedily subtracting the largest possible symbol value (including the subtractive pairs) until nothing remains — standard Roman numerals cover integers from 1 to 3999.

Students learning the numeral system, programmers implementing or testing the conversion algorithm, and anyone decoding a copyright year in film credits, a clock face, or a book chapter number use this converter to move between the two number systems instantly.

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Comments (1)

  • Editorial team

    A mismatch on the round trip can be perfectly legitimate. Clock faces traditionally show four as IIII, and this tool reads that as 4, but typing 4 in the other mode gives IV. Several old inscriptions and some film credits use such variants, so a different spelling coming back does not mean you misread the original. Have you met a numeral that this converter reads differently from what you expected?

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