Because fixed basin wrenches are designed specifically for removing tap backnuts, their sizes correspond with the standard sizes of plumbing nuts and fittings, typically BSP 1/2 inch and 3/4 inch which give an AF (across flats) measurement of 27mm and 32mm.
Adjustable basin wrenches
Adjustable basin wrenches can fit over a wider range of nuts but they are still limited to certain sizes.
For example, this model has jaws that can fit onto nuts 6mm-25mm (1/4 – 1 inch).
Adjustable basin wrenches are available with larger jaws or you can buy one that has interchangeable jaws for a wider range of uses.
Smaller and larger interchangeable jaws can be bought separately.
Jaws typically come in sizes that can be used on nuts and fittings measuring between 15mm-50mm (5/8 – 2 inch).
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Box spanner basin wrenches
Box spanner basin wrenches come in a wide variety of sizes depending on your needs.
However, the most common sizes match the typical sizes of nuts used in plumbing fixtures and pipe fittings: 8mm, 10mm, 15mm, 22mm, 28mm, 32mm, and 38mm etc.
Box spanner basin wrenches are measured by the internal width of their hexagonal sockets across the flats.