How thick does a cylinder wall have to be? That depends on the standard it was built to. The calculator has the formulas for new European cylinders (ISO-9809 and ISO-7866), older EEC-era cylinders, US cylinders (DOT) and British cylinders (BS-5045). Pick one to see it in detail; all four are worked out for the same cylinder and shown side by side, safety factors and minimum-wall rules included.
The second tab runs the other way: from a measured wall and diameter it estimates the stress at working and test pressure and a rough burst pressure. That is for understanding a cylinder, not a licence to overfill it. To be very clear, this is not an endorsement of overfilling!