Extension Spring is also referred as tension spring, where the helical wound coils are wrapped together tightly to create tension. The spring has hooks or loops, attached at both ends to hold the other components. When the components moves apart in opposite direction, spring push them back together. Wire length refers to the length of the extension spring at rest, measured between the inner surfaces. Here is the online calculator to find the Wire length of Extension spring.
An extension spring with diameter 15 cm and total coils as 8.
Wire Length = Π x D x (N+2)
= 3.14 x 15 x (8+2)
= 471.2389 cm