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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.

Mainspring Thickness
Barrel Measurements
Inside Diameter of the Barrel
Diameter of the Barrel Arbor
Mainspring Length
Needed Mainspring Thickness:


TMS = 1 x ( ( pi x DB2 ) - ( pi x DA2 ) )
2 x LMS 4 4

  • 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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