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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.

Estimated Mainspring
Number of Wheels
in Drive Train:
5 WheelsCenter Wheel
or Center Pinion
4 Wheels2nd Wheel
Main/Barrel Wheel
Expected Duration
on a Full Wind
Barrel Measurements
30 Hours
8 Days
30 Days
Inside Diameter of the Barrel
Diameter of the Barrel Arbor
Number of Desired Turns of Barrel:
Needed Mainspring Thickness:
Needed Mainspring Length:

The mainspring width should be about .1" (2.5mm) less than the barrel depth


nB = nC x P2 x WC
WB x W2
nB = nC x WC
TMS = (2 x (DB + DA))½ - (DB + DA) x pi
NB x (3/2)
LMSn = 1 x ( ( pi x DB2 ) - ( pi x DA2 ) )
2 x TMSn 4 4

  • 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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