The internal area after the border is 20 × 20.
400 bits = 50 codewords
ECC200: 30 data codewords and 20 error-correction codewords.
This is the grid size, not the label size in millimetres.When a code is described as 22×22, that refers to the number of modules in the matrix, not the physical size of the label in millimetres.
Such a symbol has 484 modules in total. After removing the outer finder pattern, the inner mapping region is 20×20.
That corresponds to 400 bits, or 50 codewords of 8 bits each. For ECC200 22×22, the typical split is 30 data codewords and 20 error-correction codewords.
Codewords are not the same thing as visible characters. The same human-readable information can be encoded in different ways and occupy different amounts of space.
For packaged water, 22×22 is very common in practice, which is why it often appears in our examples.
Knowing the matrix size helps build the correct grid and apply the right recovery logic.