Number & Numeral Converter
2 number conversion tools in one: a number base converter (binary, octal, decimal, hexadecimal, and any custom base from 2-36) and a bidirectional Roman numeral converter.
- Pick a mode from the pill bar: Number Base Converter or Roman Numeral Converter.
- For the base converter, select your input base (binary, octal, decimal, hex, or a custom base from 2-36) and type a value — all other bases update live.
- For the Roman numeral converter, pick a direction (Decimal → Roman or Roman → Decimal) and enter a value.
- Invalid input is flagged with a clear error message instead of a blank or NaN result, so you always know what went wrong.
All processing happens entirely in your browser. Your data never leaves your device.
Key Features
- Live simultaneous conversion between binary, octal, decimal, and hexadecimal as you type
- Support for any custom input base from 2 to 36, not just the common four
- Strict per-base digit validation with a clear error message instead of NaN on invalid input
- One-click copy for any of the converted values
- Decimal → Roman conversion for any value from 1 to 3999 using the standard subtractive-notation algorithm
- Roman → Decimal conversion that validates canonical form and rejects malformed numerals like "IIII" or "VV"
- Handles negative numbers in the base converter
- 100% client-side — instant results, no server round-trip
About Number
Number systems beyond decimal come up constantly in programming and computer science: binary and hexadecimal are fundamental to how computers represent data, octal still appears in Unix file permissions, and custom bases show up in encoding schemes and puzzles. Roman numerals, meanwhile, persist in clock faces, book chapters, movie sequels, and formal document numbering — and converting between Roman numerals and decimal by hand is tedious and error-prone to get exactly right (the subtractive notation rules trip up manual conversions constantly).
The Number & Numeral Converter bundles both needs into one tool. The Number Base Converter takes a value in any base from 2 to 36 and instantly shows its equivalent in binary, octal, decimal, and hexadecimal simultaneously, with strict validation against the legal digit set for the chosen base. The Roman Numeral Converter works in both directions: decimal-to-Roman uses the standard greedy subtractive-notation algorithm for any value from 1 to 3999, while Roman-to-decimal validates that the input is a well-formed, canonical Roman numeral (rejecting invalid forms like "IIII" or "VV") before parsing it with the standard left-to-right algorithm.
Both converters run entirely in your browser with straightforward, well-tested algorithms — no server round-trip needed for what is fundamentally simple arithmetic.
Frequently Asked Questions
Any base from 2 to 36 as input, with simultaneous output in binary (base 2), octal (base 8), decimal (base 10), and hexadecimal (base 16), plus your chosen custom base if it's not one of those four.
Each base has a specific set of legal digits — for example, binary only allows 0 and 1, and octal only allows 0-7. If your input contains a digit that's invalid for the selected base (like a "9" in an octal number), the tool shows a clear error explaining exactly which digits are legal instead of silently returning NaN.
Decimal-to-Roman supports values from 1 to 3999. This is the standard range for classical Roman numerals, since there's no widely agreed single-letter symbol for 5000 or beyond in the additive/subtractive system used here.
Standard Roman numerals use subtractive notation, so 4 is written "IV", not "IIII". The converter validates that input follows the canonical rules (no more than three repeated I/X/C/M, no repeated V/L/D, and only the standard subtractive pairs like IV, IX, XL, XC, CD, CM) and rejects anything that doesn't, with an explanation of what's wrong.
Yes. Prefix your input with a minus sign (e.g. "-1010" in binary) and the converter will correctly carry the sign through to all four output bases.