Calculate the Thickness & Length of an In-Barrel Mainspring
If you are replacing with a mainspring of a different width

If you are replacing a mainspring with a different width, then you will want to determine the thickness so as to have the same strength characteristic. This calculation, performed iteratively, determines the correct mainspring thickness and length even if the old mainspring is incorrect.

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

Change to Mainspring Width
Barrel Measurements:
Inside Diameter of the Barrel:
Diameter of the Barrel Arbor:
Old Mainspring Thickness:
Old Mainspring Length:
Old Mainspring Width:
New Mainspring Width:
Needed Mainspring Thickness:
Needed Mainspring Length:
TMSn = 

LMS = 
pi x ( DB2 - DA2 )
2 x 4 x TMS
  • TMSn is the new mainspring thickness
  • LMSn is the new mainspring length
  • LMSo is the old mainspring length
  • WMSn is the new mainspring width
  • WMSo is the old mainspring width
  • TMSo is the old mainspring thickness
  • LMSn 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.