Though the facilities used in discus competition appear to be simple, just a painted circle or metal ring on the ground, they have rigid specifications. The measurements of the discus circle -- diameter, radius, thickness of the ring and the sector lines, must be accurate for competition. It is not just a circle in which an athlete throws the discus. It is essential that these specifications be exact, so that a competitor can compete on identical facilities, no matter the venue.

### Diameter

The diameter of a discus circle is 8 feet 2 1/2 inches or 2.5 meters. This is the area in which the discus thrower must perform the throw of the discus. Though a circle that large, at first glance, appears plenty large enough to accommodate a discus thrower, it can be a challenge to remain in the circle during the throw. In general, the thrower begins at the back of the circle and spins to the front of the circle, using every inch as he finishes the throw at the front of the circle.

### Radius of Circle

The radius of the discus circle is 4 feet 1 1/4 inches or 1.25 meters. The radius of the circle is significant for a couple of reasons. Most discus circles have a small dot painted in the centre of the discus circle, marking the point of the radius. Many discus throwers used that dot as a mark in their throwing protocol. Additionally, that dot is the point at which the end of the tape is held to measure each successful throw.

### Thickness of Circle

The legal height and thickness of a discus circle are 3/4-inch and 1/4-inch respectively. (Reference One) However, in some recreational and high school competitions, it is acceptable to replace the legal metal rimmed circle with a painted ring, provided the legal inside diameter is maintained.

### Throwing Sector

The legal throwing sector for high school discus throws is 34.92 degrees. (Reference Two) Though the sector lines do not extend into the circle, the measurement of them initiates from the dot, on the start of the radius. The lines that mark the throwing sector connect to the circle and extend outward from the outer edge, providing a boundary for legal landing.