Calculate the Thickness of an In-Barrel Mainspring

A properly sized mainspring will fill half of the available area in the barrel. It is easy to calculate the dimensions of the barrel.

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

Mainspring Thickness
Barrel Measurements:
Inside Diameter of the Barrel:
Diameter of the Barrel Arbor:
Mainspring Length:
Needed Mainspring Thickness:
TMS = 
pi x ( DB2 - DA2 )
2 x 4 x LMS
  • TMS is the mainspring thickness
  • LMS is the mainspring length
  • DB is the inside diameter of the barrel
  • DA is the outside diameter of the barrel arbor

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

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