One half is the irreducible fraction resulting from dividing one by two (½), or any number by its double; multiplication by one half is equivalent to division by two. It is the fraction occurring most often in mathematical equations, recipes, measurements, etc.

For instance, the area S of a triangle is computed

S = ½ × base × altitude

One half also figures in the formula for calculating figurate numbers, such as triangular numbers and pentagonal numbers:

½ × n ( ( s - 2 ) n - ( 4 - s ) )

and in the formula for computing magic constants for magic squares

M₂(n) = ½ × ( n ( n² + 1 ) )

One half is also:

• One of the few fractions to get a key of its own on typewriters. It also gets its own point in some early extensions of ASCII at 189; and in Unicode, it gets its own code point at 189 in the C1 Controls and Latin-1 Supplement block, and a cross-reference in the Number Forms block, which contains some other fractions.
• One of the few fractions which is commonly expressed in natural languages by suppletion rather than regular derivation; compare English one half with regular formations like one sixth from six.

See also

• List of numbers