Roman Numerals 1–3999: Rules and Examples
The tool maps I=1, V=5, X=10, L=50, C=100, D=500 and M=1000. It forms a number from descending place values; I
Decimal to Roman Numerals
Roman Numerals to Decimal
Method and units
The tool maps I=1, V=5, X=10, L=50, C=100, D=500 and M=1000. It forms a number from descending place values; IV, IX, XL, XC, CD and CM use subtraction.
Worked example
42 decomposes into 40 + 2, so 40 is XL and 2 is II: 42 = XLII. Entering XLII returns 42.
Assumptions and limits
Decimal input must contain digits only and represent an integer from 1 through 3999. Roman input must match the calculator’s standard subtractive pattern; lowercase letters are converted to uppercase. Zero, negatives, overlines and noncanonical spellings are outside this tool.
Specific questions
Why does the calculator reject IIII for four?
Its standard pattern uses IV for four, although IIII appears in some historical and clock contexts.
Sources and related calculators
Last reviewed: 2026-09-28.