Enter the measurements shown in metric or inches (but not mixed!)
e.g.    D = 50      d = 15         t = 0.45

Barrel diameter (D)        
Arbor diameter (d)          
Spring thickness (t)  


Meadows & Passmore's Clock Mainspring Length Calculator ©

This calculates the length using the formula described in our catalogue.

Length = ((((PI x (D/2)2)-(PI x (d/2)2)))/2) /t

At least 80% of the suggested length should be used.