Pan Mass Index
The mark is on the underside, most pans went unmarked after the 1960s, and at least six foundries cast an eight. Mass does not lie. Weigh the pan, measure the flat of the cooking surface, and the index puts the casting in a manufacturing class — thin-wall, transitional or heavy-wall — so you know which era of advice actually applies to the thing in your hand.
Kitchen scale, no lid, no handle cover, nothing in the pan.
Mass unit
The flat interior floor — not the rim. Measure to where the wall starts to rise.
Length unit
Floor to rim, measured inside. Add it and the tool also solves an implied average iron thickness.