Regular 120-gon
Regular polygon 120.svg
A regular 120-gon
TypeRegular polygon
Edges and vertices120
Schläfli symbol, t, tt, ttt
Coxeter diagramCDel node 1.pngCDel 12.pngCDel 0x.pngCDel node.png
CDel node 1.pngCDel 6.pngCDel 0x.pngCDel node 1.png
Symmetry groupDihedral (D120), order 2×120
Internal angle (degrees)177°
Dual polygonSelf
PropertiesConvex, cyclic, equilateral, isogonal, isotoxal

In geometry, a 120-gon is a polygon with 120 sides. The sum of any 120-gon's interior angles is 21240 degrees.

Alternative names include dodecacontagon and hecatonicosagon.[1]

Regular 120-gon properties[edit]

A regular 120-gon is represented by Schläfli symbol and also can be constructed as a truncated hexacontagon, t, or a twice-truncated triacontagon, tt, or a thrice-truncated pentadecagon, ttt.

One interior angle in a regular 120-gon is 177°, meaning that one exterior angle would be 3°.

The area of a regular 120-gon is (with t = edge length)

and its inradius is

The circumradius of a regular 120-gon is

This means that the trigonometric functions of π/120 can be expressed in radicals.


Since 120 = 23 × 3 × 5, a regular 120-gon is constructible using a compass and straightedge.[2] As a truncated hexacontagon, it can be constructed by an edge-bisection of a regular hexacontagon.


The symmetries of a regular 120-gon. Symmetries are related as index 2 subgroups in each box. The 4 boxes are related as 3 and 5 index subgroups.

The regular 120-gon has Dih120 dihedral symmetry, order 240, represented by 120 lines of reflection. Dih120 has 15 dihedral subgroups: (Dih60, Dih30, Dih15), (Dih40, Dih20, Dih10, Dih5), (Dih24, Dih12, Dih6, Dih3), and (Dih8, Dih4, Dih2, Dih1). And 16 more cyclic symmetries: (Z120, Z60, Z30, Z15), (Z40, Z20, Z10, Z5), (Z24, Z12, Z6, Z3), and (Z8, Z4, Z2,Z1), with Zn representing π/n radian rotational symmetry.

These 32 symmetries are related to 44 distinct symmetries on the 120-gon. John Conway labels these lower symmetries with a letter and order of the symmetry follows the letter.[3] He gives d (diagonal) with mirror lines through vertices, p with mirror lines through edges (perpendicular), i with mirror lines through both vertices and edges, and g for rotational symmetry. a1 labels no symmetry.

These lower symmetries allows degrees of freedom in defining irregular 120-gons. Only the g120 symmetry has no degrees of freedom but can seen as directed edges.


Coxeter states that every zonogon (a 2m-gon whose opposite sides are parallel and of equal length) can be dissected into m(m-1)/2 parallelograms.[4] In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. For the regular 120-gon, m=60, and it can be divided into 1770: 30 squares and 29 sets of 60 rhombs. This decomposition is based on a Petrie polygon projection of a 60-cube.

120-gon rhombic dissection.svg 120-gon rhombic dissection2.svg


A 120-gram is a 120-sided star polygon. There are 15 regular forms given by Schläfli symbols , , , , , , , , , , , , , , and , as well as 44 compound star figures with the same vertex configuration.

Regular star polygons
Picture Regular polygon 120.svg
Star polygon 120-7.svg
Star polygon 120-11.svg
Star polygon 120-13.svg
Star polygon 120-17.svg
Star polygon 120-19.svg
Star polygon 120-23.svg
Star polygon 120-29.svg
Interior angle 177° 159° 147° 141° 129° 123° 111° 93°
Picture Star polygon 120-31.svg
Star polygon 120-37.svg
Star polygon 120-41.svg
Star polygon 120-43.svg
Star polygon 120-47.svg
Star polygon 120-49.svg
Star polygon 120-53.svg
Star polygon 120-59.svg
Interior angle 87° 69° 57° 51° 39° 33° 21°


  1. ^ Norman Johnson, Geometries and Transformations (2018), Chapter 11, section 11.5 Spherical Coxeter groups, 11.5 full polychoric groups
  2. ^ Constructible Polygon
  3. ^ John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, (2008) The Symmetries of Things, ISBN 978-1-56881-220-5 (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275-278)
  4. ^ Coxeter, Mathematical recreations and Essays, Thirteenth edition, p.141