A properly sized mainspring will fill half of the available area in the barrel. It is easy to calculate the dimensions of the barrel; you will need at least a fragment of the old mainspring to specify the appropriate thickness.

Change the items in the  yellow  boxes and see the Results below.

Calculator

Mainspring Length

Barrel Measurements:

Inside Diameter of the Barrel:

Diameter of the Barrel Arbor:

Mainspring Thickness:

Results:

Needed Mainspring Length:

Formula:

L_MS = 

x ( D_B² - D_A² )

2 x 4 x T_MS

where,

• L_MS is the mainspring length
• T_MS is the mainspring thickness
• D_B is the inside diameter of the barrel
• D_A is the outside diameter of the barrel arbor

This calculator, provided by Horology - The Index, is one in a series of repair calculations.