A gauge number is not one thickness — it is four. Each sheet material carries its own gauge system: steel uses the Manufacturers’ Standard Gauge (MSG), derived from a weight basis of 41.82 lb per square foot per inch of thickness; galvanized sheet adds a 0.0037 in zinc-coating allowance to every MSG thickness; stainless follows its own gauge; and aluminum keeps the Brown & Sharpe wire-gauge system, the same geometric progression as AWG electrical wire, following t = 0.005 × 92^((36−n)/39) inches. The spread at a single number is real money: 16 gauge spans 0.0508 in (aluminum) to 0.0635 in (galvanized).
In every system the number runs opposite to the thickness — higher gauge, thinner sheet — a relic of gauge numbers counting wire-drawing passes. The tables also have honest holes: there is no galvanized gauge 7, because published galvanized tables start at gauge 8, and a chart should leave that cell blank rather than extrapolate.
The working rule is to convert the gauge to its published decimal equivalent immediately and calculate in real units: the inch values are the standard published decimals (adjudicated across independent supplier and standards charts), millimetres follow at exactly 25.4 mm per inch, and downstream math — flat-pattern bend allowances, plate weights — takes the decimal thickness directly, never the gauge number.