One half

For the computer virus, see OneHalf.

"Half" redirects here; for other uses that do not relate to "one half" as a number (½), see Half (disambiguation).

One half is the multiplicative inverse of 2. It irreducible fraction with a numerator of 1 and a denominator of 2. It often appears in mathematical equations, recipes and measurements.

← −0.5 0.5 1.5 →
−1 0 1 2 3 4 5 6 7 8 9 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	one half
Ordinal	1⁄2th (halfth)
Binary	0.12
Ternary	0.11111111113
Senary	0.36
Octal	0.48
Duodecimal	0.612
Hexadecimal	0.816
Greek	∠
Roman numerals	S
Egyptian hieroglyph	𓐛
Hebrew	חֵצִ
Malayalam	൴
Chinese	半
Tibetan	༪

As a word

One half is one of the few fractions which are commonly expressed in natural languages by suppletion rather than regular derivation. In English, for example, compare the compound "one half" with other regular formations like "one-sixth".

A half can also be said to be one part of something divided into two equal parts. It is acceptable to write one half as a hyphenated word, one-half.

Mathematics

One half is the rational number that lies between 0 and 1 on the number line. Multiplication by one half is equivalent to division by two, or "halving"; conversely, division by one half is equivalent to multiplication by two, or "doubling".

A square of side length one, here dissected into rectangles whose areas are successive powers of one half.

A number raised to the power of one half is equal to its square root.

The area of a triangle is one half its base and altitude (or height).

Fundamental region of the modular j-invariant in the upper half-plane (shaded gray), with modular discriminant $|\tau |\geq 1$ and $-{\tfrac {1}{2}}<{\mathfrak {R}}(\tau )\leq {\tfrac {1}{2}}$, where $-{\tfrac {1}{2}}<{\mathfrak {R}}(\tau )<0\Rightarrow |\tau |>1.$

The gamma function evaluated at one half is the square root of pi.

It has two different decimal representations in base ten, the familiar $0.5$ and the recurring $0.4{\overline {9}}$, with a similar pair of expansions in any even base; while in odd bases, one half has no terminating representation.

The Bernoulli number $B_{1}$ has the value $\pm {\tfrac {1}{2}}$ (its sign depending on competing conventions).

The Riemann hypothesis is the conjecture that every nontrivial complex root of the Riemann zeta function has a real part equal to ${\tfrac {1}{2}}$.

Computer characters

½
vulgar fraction one half
In Unicode	U+00BD ½ VULGAR FRACTION ONE HALF
Related
See also	U+00BC ¼ VULGAR FRACTION ONE QUARTER U+00BE ¾ VULGAR FRACTION THREE QUARTERS

The "one-half" symbol has its own code point as a precomposed character in the Number Forms block of Unicode, rendering as ½.

The reduced size of this symbol may make it illegible to readers with relatively mild visual impairment; consequently the decomposed forms 1⁄2 or ⁠1/2⁠ may be more appropriate.

See also

Postal stamp, Ireland, 1940: one halfpenny postage due.

• Division by two