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Calculate the Length 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; you will need at least a fragment of the old mainspring to specify the appropriate thickness.

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


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

  • LMS is the mainspring length
  • TMS is the mainspring thickness
  • 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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