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 diameter helps describe the opening and may be relevant to a lid. The interior flat cooking diameter helps you judge room for food on the base. The exterior magnetic or contact-base diameter helps with a cooktop fit check. Do not substitute one for another.
For storage, total handle-to-handle length and total height matter too. A pan can fit the width of a cabinet but still prevent its door closing because of its handle. Product dimensions sometimes describe packaging, so read the dimension label carefully.
A real example
Made In publishes an 8-inch frying pan with an 8.5-inch total diameter and a 5-inch flat cooking diameter. Its 10-inch pan has a 10.5-inch total diameter and a 7.5-inch flat cooking diameter. Those are different measurements of the same objects, not interchangeable size claims.
For a circular base, area is π × radius squared. A 7.5-inch flat base has about 44.2 square inches of area; a 5-inch base has about 19.6. That difference is useful for comparing room, but it does not tell you how many servings will fit.
Measure without overstating precision
Measure the interior flat section separately from the rim. Record whether a source gives an interior cooking surface or an exterior flat bottom. If the listing gives only a nominal size, leave the other dimensions unknown.
The calculator estimates geometry from your inputs. Its tapered round-pan formula approximates a truncated cone, while the rectangular option assumes vertical walls. Rounded corners, curved sides and practical headroom mean the result is theoretical brim-full volume, not a recipe capacity or safety limit.