The sum of all the internal angles of any octagon is 1080°. As with all polygons, the external angles total 360°.

If squares are constructed all internally or all externally on the sides of an octagon, then the midpoints of the segments connecting the centers of opposite squares form a quadrilateral that is both equidiagonal and orthodiagonal (that is, whose diagonals are equal in length and at right angles to each other).

The midpoint octagon of a reference octagon has its eight vertices at the midpoints of the sides of the reference octagon. If squares are constructed all internally or all externally on the sides of the midpoint octagon, then the midpoints of the segments connecting the centers of opposite squares themselves form the vertices of a square.

A regular octagon is a closed figure with sides of the same length and internal angles of the same size. It has eight lines of reflective symmetry and rotational symmetry of order 8. A regular octagon is represented by the Schläfli symbol {8}. The internal angle at each vertex of a regular octagon is 135° ( 3 π 4 radians). The central angle is 45° ( π 4 radians).

The area of a regular octagon of side length *a* is given by

A = 2 cot π 8 a 2 = 2 ( 1 + 2 ) a 2 ≃ 4.828 a 2 . In terms of the circumradius *R*, the area is

A = 4 sin π 4 R 2 = 2 2 R 2 ≃ 2.828 R 2 . In terms of the apothem *r* (see also inscribed figure), the area is

A = 8 tan π 8 r 2 = 8 ( 2 − 1 ) r 2 ≃ 3.314 r 2 . These last two coefficients bracket the value of pi, the area of the unit circle.

The area can also be expressed as

A = S 2 − a 2 , where *S* is the span of the octagon, or the second-shortest diagonal; and *a* is the length of one of the sides, or bases. This is easily proven if one takes an octagon, draws a square around the outside (making sure that four of the eight sides overlap with the four sides of the square) and then takes the corner triangles (these are 45–45–90 triangles) and places them with right angles pointed inward, forming a square. The edges of this square are each the length of the base.

Given the length of a side *a*, the span *S* is

S = a 2 + a + a 2 = ( 1 + 2 ) a ≈ 2.414 a .

The area is then as above:

A = ( ( 1 + 2 ) a ) 2 − a 2 = 2 ( 1 + 2 ) a 2 ≈ 4.828 a 2 . Expressed in terms of the span, the area is

A = 2 ( 2 − 1 ) S 2 ≈ 0.828 S 2 . Another simple formula for the area is

A = 2 a S . More often the span *S* is known, and the length of the sides, *a*, is to be determined, as when cutting a square piece of material into a regular octagon. From the above,

a ≈ S / 2.414. The two end lengths *e* on each side (the leg lengths of the triangles (green in the image) truncated from the square), as well as being e = a / 2 , may be calculated as

e = ( S − a ) / 2. The circumradius of the regular octagon in terms of the side length *a* is

R = ( 4 + 2 2 2 ) a , and the inradius is

r = ( 1 + 2 2 ) a .

A regular octagon at a given circumcircle may be constructed as follows:

- Draw a circle and a diameter AOE, where O is the center and A, E are points on the circumcircle.
- Draw another diameter GOC, perpendicular to AOE.
- (Note in passing that A,C,E,G are vertices of a square).
- Draw the bisectors of the right angles GOA and EOG, making two more diameters HOD and FOB.
- A,B,C,D,E,F,G,H are the vertices of the octagon.

A regular octagon can be constructed using a straightedge and a compass, as 8 = 2^{3}, a power of two:

Each side of a regular octagon subtends half a right angle at the centre of the circle which connects its vertices. Its area can thus be computed as the sum of 8 isosceles triangles, leading to the result:

Area = 2 a 2 ( 2 + 1 ) for an octagon of side *a*.

The coordinates for the vertices of a regular octagon centered at the origin and with side length 2 are:

(±1, ±(1+√2))(±(1+√2), ±1).Coxeter states that every parallel-sided 2*m*-gon can be divided into m(m-1)/2 rhombs. For the octagon, *m*=4, and it can be divided into 6 rhombs, with one example shown below. This decomposition can be seen as 6 of 24 faces in a Petrie polygon projection plane of the tesseract.

A **skew octagon** is a skew polygon with 8 vertices and edges but not existing on the same plane. The interior of such an octagon is not generally defined. A *skew zig-zag octagon* has vertices alternating between two parallel planes.

A **regular skew octagon** is vertex-transitive with equal edge lengths. In 3-dimensions it will be a zig-zag skew octagon and can be seen in the vertices and side edges of a square antiprism with the same D_{4d}, [2^{+},8] symmetry, order 16.

The regular skew octagon is the Petrie polygon for these higher-dimensional regular and uniform polytopes, shown in these skew orthogonal projections of in A_{7}, B_{4}, and D_{5} Coxeter planes.

The *regular octagon* has Dih_{8} symmetry, order 16. There are 3 dihedral subgroups: Dih_{4}, Dih_{2}, and Dih_{1}, and 4 cyclic subgroups: Z_{8}, Z_{4}, Z_{2}, and Z_{1}, the last implying no symmetry.

On the regular octagon, there are 11 distinct symmetries. John Conway labels full symmetry as **r16**. The dihedral symmetries are divided depending on whether they pass through vertices (**d** for diagonal) or edges (**p** for perpendiculars) Cyclic symmetries in the middle column are labeled as **g** for their central gyration orders. Full symmetry of the regular form is **r16** and no symmetry is labeled **a1**.

The most common high symmetry octagons are **p8**, a isogonal octagon constructed by four mirrors can alternate long and short edges, and **d8**, an isotoxal octagon constructed with equal edge lengths, but vertices alternating two different internal angles. These two forms are duals of each other and have half the symmetry order of the regular octagon.

Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the **g8** subgroup has no degrees of freedom but can seen as directed edges.

The octagonal shape is used as a design element in architecture. The Dome of the Rock has a characteristic octagonal plan. The Tower of the Winds in Athens is another example of an octagonal structure. The octagonal plan has also been in church architecture such as St. George's Cathedral, Addis Ababa, Basilica of San Vitale (in Ravenna, Italia), Castel del Monte (Apulia, Italia), Florence Baptistery, Zum Friedefürsten church (Germany) and a number of octagonal churches in Norway. The central space in the Aachen Cathedral, the Carolingian Palatine Chapel, has a regular octagonal floorplan. Uses of octagons in churches also include lesser design elements, such as the octagonal apse of Nidaros Cathedral.

Architects such as John Andrews have used octagonal floor layouts in buildings for functionally separating office areas from building services, notably the Intelsat Headquarters in Washington D.C., Callam Offices in Canberra, and Octagon Offices in Parramatta, Australia.

The *octagon*, as a truncated square, is first in a sequence of truncated hypercubes:

As an expanded square, it is also first in a sequence of expanded hypercubes: