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.

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

Mainspring Length
Barrel Measurements:
Inside Diameter of the Barrel:
Diameter of the Barrel Arbor:
Mainspring Thickness:
Needed Mainspring Length:
LMS = 
pi x ( DB2 - DA2 )
2 x 4 x TMS
  • 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.

Watchonista logo watch carefully logo A Timely Perspective logo Mr. WatchMaster logo