The aspect ratio of a geometry shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter sidethe ratio of width to height, when the rectangle is oriented as a "landscape".
The aspect ratio is most often expressed as two integer numbers separated by a colon (x:y), less commonly as a simple or decimal fraction. The values x and y do not represent actual widths and heights but, rather, the proportion between width and height. As an example, 8:5, 16:10, 1.6:1, and 1.6 are all ways of representing the same aspect ratio.
In objects of more than two dimensions, such as , the aspect ratio can still be defined as the ratio of the longest side to the shortest side.
Applications and uses
The term is most commonly used with reference to:

Graphic / image

Image aspect ratio

Display aspect ratio

Paper size

Standard photographic print sizes

Motion picture film formats

Standard ad size

Pixel aspect ratio

Photolithography: the aspect ratio of an etched, or deposited structure is the ratio of the height of its vertical side wall to its width.

HARMST High Aspect Ratios allow the construction of tall microstructures without slant

Tire code

Tire sizing

Turbocharger impeller sizing

Wing aspect ratio of an aircraft or bird

Astigmatism of an optical lens

Nanorod dimensions

Shape factor (image analysis and microscopy)
Aspect ratios of simple shapes
Rectangles
For a rectangle, the aspect ratio denotes the ratio of the width to the height of the rectangle. A
square has the smallest possible aspect ratio of 1:1.
Examples:

4:3 = 1.: Some (not all) 20th century computer monitors (VGA, XGA, etc.), standarddefinition television

$\backslash sqrt\{2\}:1\; =\; 1.414...$: international paper sizes (ISO 216)

3:2 = 1.5: 35mm still camera film, iPhone (until iPhone 5) displays

= 1.6: commonly used widescreen (WXGA)

Φ:1 = 1.618...: golden ratio, close to 16:10

5:3 = 1.: super 16 mm, a standard film gauge in many European countries

16:9 = 1.: widescreen TV

2:1 = 2: dominoes

64:27 = 2.: ultrawidescreen,

32:9 = 3.: super ultrawidescreen
Ellipses
For an ellipse, the aspect ratio denotes the ratio of the
major axis to the
minor axis. An ellipse with an aspect ratio of 1:1 is a circle.
Aspect ratios of general shapes
In
geometry, there are several alternative definitions to aspect ratios of general
in a ddimensional space:

The diameterwidth aspect ratio (DWAR) of a compact set is the ratio of its diameter to its width. A circle has the minimal DWAR which is 1. A square has a DWAR of $\backslash sqrt\{2\}$.

The cubevolume aspect ratio (CVAR) of a compact set is the dth root of the ratio of the dvolume of the smallest enclosing axesparallel dcube, to the set's own dvolume. A square has the minimal CVAR which is 1. A circle has a CVAR of $\backslash sqrt\{2\}$. An axisparallel rectangle of width W and height H, where W>H, has a CVAR of $\backslash sqrt\{W^2/WH\}\; =\; \backslash sqrt\{W/H\}$.
If the dimension d is fixed, then all reasonable definitions of aspect ratio are equivalent to within constant factors.
Notations
Aspect ratios are mathematically expressed as
x:
y (pronounced "xtoy").
Cinematographic aspect ratios are usually denoted as a (rounded) decimal multiple of width vs unit height, while photographic and videographic aspect ratios are usually defined and denoted by whole number ratios of width to height. In there is a subtle distinction between the display aspect ratio (the image as displayed) and the storage aspect ratio (the ratio of pixel dimensions); see Distinctions.
See also