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
There are 128 characters in the original ASCII set. Seven bits, of which 33 are control codes rather than printable characters. Unicode's first 128 code points are deliberately identical, which is why plain ASCII is valid UTF-8.
- Media & popular culture
There are 128 note numbers in the MIDI specification (worldwide, MIDI 1.0). Seven bits, so 0 to 127 — the same reason a colour channel stops at 255, one bit smaller. Middle C is note 60, and the range runs about five octaves wider than a piano.
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
- Scientific notation
- 1.280 × 10^2
Roman numerals
C X X V I I I
Binary
1 0 0 0 0 0 0 0
1 × 128 + 0 × 64 + 0 × 32 + 0 × 16 + 0 × 8 + 0 × 4 + 0 × 2 + 0 × 1 = 128
Octal
2 0 0
2 × 64 + 0 × 8 + 0 × 1 = 128
Hexadecimal
8 0
8 × 16 + 0 × 1 = 128
Read as powers of two: 128 = 128.
In historical numeral systems
Egyptian
1 coil of rope, 2 hobbles, 8 strokes
1 × 100 + 2 × 10 + 8 × 1 = 128. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 vertical wedges (sixties place), 8 vertical wedges (units place)
2 × 60 + 8 = 128. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
rho, then kappa, then eta, marked as a numeral
rho-kappa-eta + keraia
rho + kappa + eta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
qof, then kaf, then het
qof-kaf-het
qof (100) + kaf (20) + het (8) = 128, written in descending order.
Maya
1 bar + 1 dot (upper, twenties place) over 1 bar + 3 dots (lower, units place)
6 × 20 + 8 = 128. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
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
- is a factor of256
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
There are 128 characters in the original ASCII set. Seven bits, of which 33 are control codes rather than printable characters. Unicode's first 128 code points are deliberately identical, which is why plain ASCII is valid UTF-8.
DocumentedTechnology & computingGeography: worldwideThere are 128 bits in an IPv6 address (worldwide, IETF). Four times the 32 bits of IPv4, which raises the address count by a factor of about 79 thousand million million million million. The size was chosen so that address exhaustion could not recur rather than because that many addresses are needed.
DocumentedTechnology & computingGeography: worldwide, IETFISO 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
Media & popular culture
There are 128 note numbers in the MIDI specification (worldwide, MIDI 1.0). Seven bits, so 0 to 127 — the same reason a colour channel stops at 255, one bit smaller. Middle C is note 60, and the range runs about five octaves wider than a piano.
DocumentedMedia & popular cultureGeography: worldwide, MIDI 1.0
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: 7 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026