Measure inside the pan. The size on the box may include the outer rim.
Measure between the inside walls. Leave handles and the outer rim out.
Three measurements, three decisions
The rim is the size across the top, and it's what a lid has to fit. The flat floor inside is how much room your food gets. The base underneath is what touches the burner or the induction zone. Each answers a different question, so don't swap one for another.
For storage, the full length with the handle and the height matter too: a pan can fit a cabinet's width and still stop the door closing because of its handle. Some listings give the box size rather than the pan's, so check which one you're reading.
A real example
Made In's 8-inch frying pan is 8.5 inches across the rim and has a 5-inch flat cooking surface. Its 10-inch pan is 10.5 inches across the rim with a 7.5-inch cooking surface. Same pans, three different numbers.
Floor area grows faster than width. A 7.5-inch floor gives about 44.2 square inches of room and a 5-inch floor about 19.6, less than half. That tells you how much more fits in one layer, though not how many people it feeds.
Measure it yourself
Measure the flat part inside the pan separately from the rim. When you read a maker's figures, note whether they give the cooking surface inside or the flat bottom outside. If a listing gives only the label size, you don't know the others yet.
The pan capacity calculator works from your measurements. For a round pan with sloped sides it treats the pan as a cone with its tip cut off; for a rectangular pan it assumes straight sides. Rounded corners and curved walls make the real figure a little smaller, and the answer is the volume filled to the brim, so leave room for stirring and bubbling.