Bidirectional conversion between Base91 and decimal with detailed steps
Input Value
Result
Conversion Result
Step-by-Step Derivation
Base Conversion Principle
Base91 → Decimal: index in alphabet × 91ⁿ and sum
Decimal → Base91: Repeated division by 91 (reverse remainders)
Alphabet: 91 ASCII characters (excluding some control characters)
Base91 is a binary-to-text encoding scheme that uses 91 ASCII characters. Developed by Joachim Henke, it's known for high efficiency, representing data more compactly than Base64 while remaining printable and safe.
⚠Base91 uses 91 characters: ASCII 33-45, 47-126 (excluding some special characters). Each character represents a value from 0 to 90. This tool demonstrates Base91 as a positional number system.
What Is Base91?
Base91 is a binary-to-text encoding scheme developed by Joachim Henke. It uses 91 ASCII characters, making it very efficient—achieving about 85% of the original binary size (compared to 75% for Base64). It avoids problematic characters while remaining printable and safe.
Highly Efficient
Base91 is about 85% efficient (vs 75% for Base64). It encodes data more compactly than many other encoding schemes.
Place Value Expansion
Base91 to decimal: index × 91^position. For example, A8 = 32×91¹+40×91⁰ = 3152.
Repeated Division by 91
Decimal to Base91: repeatedly divide by 91, collect remainders, use as index into alphabet, read in reverse.
Printable & Safe
Base91 uses only printable ASCII characters, making it safe for transmission through various systems that might otherwise corrupt data.
💡 Teaching Example: Convert Base91 A8 to decimal. Place value expansion: 32×91¹+40×91⁰ = 32×91+40 = 2912+40 = 2952. Conversely, 2952: 2952÷91=32 r40, 32÷91=0 r32 → A8.
Applications
Data EncodingFile SharingEmail AttachmentsBinary TransmissionCompact Data
Frequently Asked Questions
What is Base91 encoding?▼
Base91 is a binary-to-text encoding scheme that uses 91 ASCII characters. It was developed by Joachim Henke and is known for its high efficiency, representing data more compactly than Base64.
How do you convert Base91 to decimal?▼
Place value expansion: find each character's value in the Base91 alphabet, multiply by its power of 91, then sum. This tool demonstrates Base91 as a positional number system.
How do you convert decimal to Base91?▼
Repeated division by 91: repeatedly divide by 91, collecting remainders. Use the remainder as an index into the Base91 alphabet. Read remainders in reverse.
What are the advantages of Base91?▼
Base91 is very efficient, achieving about 85% of the original binary size (compared to 75% for Base64). It avoids problematic characters while using a large character set for compactness.
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