How to measure a frying pan?
So what is, say, a 9" pan?
Do you measure the flat surface on the inside of the pan?
The width of the outside bottom?
The width from lip to lip?
Almost always that the size of a frying pan is defined by the longest dimension -- that is the wide from the top (lip to lip).
It isn't so much how I measure the pan. You can definite it however you like as long as you are clear. However, when a company or a store defines a pan, they are talking about the side to side dimension.
Think about it like our TV.
You can describe your TV however you like to your friends, but when you go to a store or read a product brochure from the manufacturer, it is defined as the following:
To be more precise, my understanding is that the size of the pan is generally measured from the inside top lip to the inside top lip (i.e. the size of the pan is the diameter of the circle that the lid must fit into.) The longest dimension Chem mentions would include an amount equal to twice the thickness of the walls of the pan, and I don't believe that additional diameter is usually included in a pan's measured size.
Depends, some manufactures measure the (inside?) bottom, some measure the top, lip to lip. Some are just close to.
I have pans marked SK-8 (2) that use a 10" lid, a pan marked SK-10 that uses a 12" lid, the inside measurement at the bottom of the pan is close to the number, measure on the outside bottom, the pan is ~1" larger than the number. Those are Lodge Cast Iron pans.
I have pans that were advertised as 12", marked SK-12, inside bottom measurement is 9.5", outside measurement is 12.5", I have to use a flat 14" lid for that pan. These are aluminum pans from a restaurant supply store.
My pans marked SK-6 use an 8" lid. These are "tapas" pans from Costco, I use for reheating and fritattas.
I usually don't worry about the "advertised size", I am concerned about lids.
It does pay to know all the dimensions of a pan. Because rim-to-rim diameter is the standard way to refer to them, two skillets of the same nominal dimension can differ significantly in volume and function. (Compare a 9-inch cast iron skillet and its straight, close-to-vertical sides with a 9-inch crepe pan whose shallow 'walls' form a near-horizontal seamless curve with the base.)
Regardless of the manufacturer's part number, sellers (both professional and ebay / flea market) communicate the size of a pan by the rim-to-rim dimension. As Alan notes, it's about lids.
:: Depends, some manufactures measure the (inside?) bottom ::
Could you provide an example of a manufacturer using the base measurement to specify a pan? I've not seen that.
My lodge cast iron pans are marked as follows: SK-8 uses a 10" lid, if I measure the inside bottom, it is ~8" if I measure the outside bottom it is ~8.5", SK-10 uses a 12" lid, inside bottom measurement is ~10", the outside bottom measurement is ~10.5". I have a couple of aluminum pans marked SK-12 that require a 14" lid. The aluminum pans are imported, purchased at a restaurant supply store.
Knowing the pan base size is very helpful for matching pan to burner, particularly for the less conductive materials like cast iron and stainless steel.
On induction cooktops, the coil size limits the pan base sizes that can be used. When considering an induction unit last year, I found myself measuring the inside and outside base dimensions of all my ferrous pans, and it was something of a revelation. Most were smaller than I'd have estimated.
With cast iron it is easiest to think of pre-marketing era and marketing era. The SK is the identifier for skillet and the number represents the approximate interior dimension. Once they figured out that using a lip to lip measurement sounded more impressive they went with that and fell in line with what is now the conventional way of stating pan size. Covers are now marked in both measurements i.e. 10 1/4, 8.