The Koch snowflake (also known as the Koch star and Koch island) is a mathematical curve and one of the earliest fractal curves to have been described. It is based on the Koch curve, which appeared in a 1904 paper titled "On a continuous curve without tangents, constructible from elementary geometry" (original French title: Sur une courbe continue sans tangente, obtenue par une construction géométrique élémentaire) by the Swedish mathematician Helge von Koch.
The Koch snowflake can be constructed by starting with an equilateral triangle, then recursively altering each line segment as follows:
- divide the line segment into three segments of equal length.
- draw an equilateral triangle that has the middle segment from step 1 as its base and points outward.
- remove the line segment that is the base of the triangle from step 2.
After one iteration of this process, the resulting shape is the outline of a hexagram.
The Koch snowflake is the limit approached as the above steps are followed over and over again. The Koch curve originally described by Koch is constructed with only one of the three sides of the original triangle. In other words, three Koch curves make a Koch snowflake.
The Koch curve has an infinite length because each iteration creates four times as many line segments as in the previous iteration, with the length of each one being one-third the length of the segments in the previous stage. Hence the total length of the curve increases by one third with each iteration and the length of the curve after n iterations will be (4/3)n times the original triangle perimeter, which is unbounded as n tends to infinity.
Perimeter of the Koch snowflake 
After each iteration, the number of sides of the Koch snowflake increase by a factor of 4, so the number of sides after n iterations is given by:
If the original equilateral triangle has sides of length s, the length of each side of the snowflake after n iterations is:
Therefore the perimeter of the snowflake after n iterations is:
Area of the Koch snowflake 
In each iteration a new triangle is added on each side in the previous iteration, so the number of new triangles added in iteration n is:
The area of each new triangle added in an iteration is one ninth of the area of each triangle added in the previous iteration, so the area of each triangle added in iteration n is:
where a0 is the area of the original triangle. The total new area added in iteration n is therefore:
The total area of the snowflake after n iterations is:
Collapsing the geometric sum gives:
Area and perimeter after infinite iterations 
As the number of iterations tends to infinity, the limit of the perimeter is:
since . The limit of the area is:
So the area of the Koch snowflake is 8/5 of the area of the original triangle. Expressed in terms of the side length s of the original triangle this is . Therefore the infinite perimeter of the Koch triangle encloses a finite area.
Tessellation of the plane 
It is possible to tessellate the plane by copies of Koch snowflakes in two different sizes. However, such a tessellation is not possible using only snowflakes of the same size as each other. Since each Koch snowflake in the tessellation can be subdivided into seven smaller snowflakes of two different sizes, it is also possible to find tessellations that use more than two sizes at once.
Thue-Morse Sequence and Turtle graphics 
- If t(n) = 0, move ahead by one unit,
- If t(n) = 1, rotate counterclockwise by an angle of π/3,
the resulting curve converges to the Koch snowflake.
Representation as Lindenmayer system 
- Alphabet : F
- Constants : +, −
- Axiom : F++F++F
- Production rules:
- F → F−F++F−F
Here, F means "draw forward", + means "turn right 60°", and − means "turn left 60°".
Variants of the Koch curve 
Following von Koch's concept, several variants of the Koch curve were designed, considering right angles (quadratic), other angles (Césaro) or circles and their extensions to higher dimensions (Sphereflake):
|1D, 85° angle||The Cesaro fractal is a variant of the Koch curve with an angle between 60° and 90° (here 85°).|
|1D, 90° angle|
|1D, 90° angle|
|1D, ln 3/ln (√5)|
|1D, ln 3.33/ln (√5)||Another variation. Its fractal dimension equals ln 3.33/ln (√5)=1.49.|
|2D, 90° angle||Extension of the quadratic type 1 curve. The illustration at left shows the fractal after the second iteration|
|2D, 90° angle||Extension of the quadratic type 2 curve. The illustration at left shows the fractal after the first iteration.|
|3D, spheres||Eric Haines has developed the sphereflake fractal, which is a three-dimensional version of the Koch snowflake, using spheres.|
Squares can be used to generate similar fractal curves. Starting with a unit square and adding to each side at each iteration a square with dimension one third of the squares in the previous iteration, it can be shown that both the length of the perimeter and the total area are determined by geometric progressions. The progression for the area converges to 2 while the progression for the perimeter diverges to infinity, so as in the case of the Koch Snowflake, we have a finite area bounded by an infinite fractal curve. The resulting area fills a square with the same center as the original, but twice the area, and rotated by π/4 radians, the perimeter touching but never overlapping itself.
The total area covered at the nth iteration is :
While the total length of the perimeter is : which approaches infinity as n increases
See also 
- List of fractals by Hausdorff dimension
- Gabriel's Horn (infinite surface area but encloses a finite volume)
- Weierstrass function
- Addison, Paul S. Fractals and Chaos - An Illustrated Course. Institute of Physics (IoP) Publishing (1997) ISBN 0-7503-0400-6 - Page 19
- Koch Snowflake
- Burns, Aidan (1994), "78.13 Fractal tilings", Mathematical Gazette 78 (482): 193–196, JSTOR 3618577.
- Demonstrated by James McDonald in a public lecture at KAUST University on January 27, 2013.  retrieved 29 January 2013.
- Edward Kasner & James Newman, Mathematics and the Imagination Dover Press reprint of Simon & Schuster (1940) ISBN 0-486-41703-4, pp 344–51.
|Wikimedia Commons has media related to: Koch curve|
|Wikimedia Commons has media related to: Koch snowflake|
- von Koch Curve
- The Koch snowflake in Mathworld
- Application of the Koch curve to an antenna
- "A mathematical analysis of the Koch curve and quadratic Koch curve" (pdf). Retrieved 22 November 2011.