The mainspring in its barrel is the energy accumulator (motor organ) of a mechanical watch (in an electronic watch, the battery is the energy accumulator).
The approximate thickness of a wristwatch mainspring is 1/100th of the internal diameter of the barrel.
There are three parts to the Barrel: the Barrel lid (obvious), barrel drum (the main body of the barrel that bears the barrel teeth and receives the spring) and the barrel arbour (the central post around which the spring winds and on which the barrel pivots).
Have you ever wondered what the main difference between the lubrication of the barrel wall in an automatic watch and a mechanical is? I’ll tell you: you do NOT lubricate the barrel wall in a manual wind watch; you only grease the internal bottom of the barrel drum and the inside of the lid and the spring itself. In an automatic watch, you would put a small amount of graphite grease (such as MR1) in the recesses on the walls to allow the slipping bridle to slip without excessive friction.
The barrel arbour normally takes up one third of the internal diameter of the barrel, as does the fully wound mainspring, which leaves one third free at all times.
With that in mind, let’s try and calculate the thickness of a mainspring given an arbour core RADIUS of 1.65mm.
Assuming that the Barrel Arbour Diameter (3.3) is one third of the Interior Barrel diameter (9.9), then the theoretical thickness of the mainspring (1/100th of the interior Barrel diameter) should be 0.099, or 0.1 when rounding up.