Enter the number of rings, the outside spiral diameter, and inside spiral diameter into the calculator to determine the spiral length.

Spiral Length Formula

The following equation is used to calculate the Spiral Length.

SL = 3.14159*N*(OD+ID)/2

  • Where SL is the spiral length
  • N is the number of rings
  • OD is the outside diameter
  • ID is the inside diameter

The units for the OD and ID should remain the same. The result will be the spiral length in the same units.

What is Spiral Length?


Spiral length is the circumference, or length, of the spiral. The spiral length can vary, depending on whether it is measured from the inside or outside the ring.

For example, a 90° bend in a spiral with an outside diameter of 1 inch and an inside diameter of 7/8 inches has a spiral length of 1.48 inches (1.414 times its outer diameter). A bend with a radius of 1/4 inch has a spiral length of 1.02 inches (1.26 times its outside diameter).

Spiral length is used to determine the capacity of a tube that contains multiple spirals. When spirals are nested together, their combined area increases by adding together the areas of all rings that fit within that tube. To measure the area in this case, use the following formula: Area = Pi x Spiral Length x Radius

Spiral length is the distance, measured along the helical axis, from the center of the helix to its apex. It is a key dimension in the design of spiral bevel gears.

The length of a spiral gear is sometimes expressed as twice the spiral pitch, which amounts to approximately one-fourth of the total gear length. The spiral length may also be referred to as pitch diameter or simply pitch in applications where it is equal to half the transverse pitch