Calculate an Estimated Mainspring

If the mainspring is missing or if the existing mainspring is a suspect replacement, use this calculator to determine the correct sizing.

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

Estimated Mainspring
Teeth in Movement:
 # of Wheels in Drive Train:
Wheel Teeth
Pinion Leaves
5 Wheels
Center Wheel or Center Pinion:
4 Wheels
2nd Wheel:
Main/Barrel Wheel:
Number of Needed Turns:
 Expected Duration on a Full Wind
30 Hours
Number of Turns of Center Wheel:
8 Days
Number of Desired Turns of Barrel:
30 Days
(Override if desired)
Barrel Measurements:
Inside Diameter of the Barrel:
Diameter of the Barrel Arbor:
Needed Mainspring Thickness:
Needed Mainspring Length:
NB = 
NC x 
P2 x WC
WB x W2
NB = 
NC x 
TMS = 
( 2 x ( DB2 + DA2 ))½ - ( DB + DA )
NB x 2
Corrected Oct. 2020 with a very able assist
from Paul Bauerle.
LMS = 
pi x ( DB2 - DA2 )
2 x 4 x TMS
  • NB is the number of turns of the barrel
  • NC is the needed number of turns of the center wheel, i.e., number of hours: 30 or (8 days x 24) or (31 days x 24)
  • WB is the number of teeth on the barrel wheel
  • W2 is the number of teeth on the second wheel
  • WC is the number of teeth on the center wheel, or center pinion for a 4 wheel train
  • P2 is the number of leaves on the second wheel pinion
  • DB is the inside diameter of the barrel
  • DA is the outside diameter of the barrel arbor
  • TMS is the mainspring thickness
  • LMS is the mainspring length

5 Wheel Drive Train

4 Wheel Drive Train

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

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