128
128 is 2⁷, and the number of characters in the original ASCII set.
- Even
- Composite
Where 128 fits
Why 128 is special
128 is why plain English text has always been cheap to store: ASCII assigned every letter, digit, punctuation mark and control code a slot in 7 bits, giving exactly 128 positions. The eighth bit was left spare, which is what later let a single byte carry 256 values and made the extended character sets — and eventually the Unicode transition — possible.
Same number, different meanings
How 128 is understood across different contexts — never implying one meaning because of another.
- Mathematics
128 is why plain English text has always been cheap to store: ASCII assigned every letter, digit, punctuation mark and control code a slot in 7 bits, giving exactly 128 positions. The eighth bit was left spare, which is what later let a single byte carry 256 values and made the extended character sets — and eventually the Unicode transition — possible.
- Everyday life
A cord is 128 cubic feet — a stack four feet high, four wide and eight long. Firewood is one of the few goods still sold by a stacked-volume unit, and because loosely stacked wood contains more air, the definition specifies the stack rather than the wood.
- Technology & computing
ISO 128 specifies general presentation principles for technical drawings, including line types, views and sections, which ISO 129's dimensioning rules are applied within.
128 is part of
Factors & digit properties
- Divisors (8)
- 1, 2, 4, 8, 16, 32, 64, 128
- Prime factorisation
- 2^7
- Digit sum
- 11
- Digital root
- 2
Representations
- Decimal
- 128
- Binary
- 10000000
- Octal
- 200
- Hexadecimal
- 80
- Roman numeral
- CXXVIII
- Scientific notation
- 1.280 × 10^2
Other mathematical patterns
- Deficient number
- Powerful number
Related numbers & connections
256 is exactly twice 128, which is the whole relationship between the ASCII repertoire and the byte: seven bits give 128 characters, the eighth bit doubles the space to 256, and that spare bit is what later encodings and UTF-8 were built on.
View 256 →128 is 2⁷, the seventh doubling — and the size of the ASCII repertoire, which is why seven bits were enough for English text.
A signed byte spans −128 to 127, so the two values bound the same eight bits from opposite ends.
View 127 →Mathematically related
- has factor256
why?
Strength 0.50, confidence "high".
Shares a distinctive class
- is associated withPowers of two
why?
Strength 0.50, confidence "high".
Everyday life
A cord is 128 cubic feet — a stack four feet high, four wide and eight long. Firewood is one of the few goods still sold by a stacked-volume unit, and because loosely stacked wood contains more air, the definition specifies the stack rather than the wood.
DocumentedEveryday lifeGeography: United States and Canada
Technology & computing
ISO 128 specifies general presentation principles for technical drawings, including line types, views and sections, which ISO 129's dimensioning rules are applied within.
DocumentedTechnology & computingGeography: International — ISO member bodiesPeriod: 1982–ASCII needed 128 slots and 7 bits gave exactly that, which left the eighth bit of a byte unused. That spare bit is the reason the same file could later be read as one of dozens of incompatible 256-character encodings, and the reason UTF-8 could be designed so that plain ASCII text remains byte-for-byte valid UTF-8.
DocumentedTechnology & computingASCII defines exactly 128 characters, which fits in seven bits and left the eighth free — and every incompatible use of that spare bit is the origin of the encoding chaos that UTF-8 eventually settled. The first 128 characters of Unicode are still ASCII unchanged.
DocumentedTechnology & computing
Common questions
Is 128 a prime number?
No, 128 is not prime.
What are the factors of 128?
The factors of 128 are 1, 2, 4, 8, 16, 32, 64, 128.
What is 128 in Roman numerals?
128 in Roman numerals is CXXVIII.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 4 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 3 August 2026