I'm not sure there is a simple answer to that question. 18 gauge wire is 1.02 mm in diameter. If you twist two strands of wire and look at it from the end it will look like two wires next to each other and as you move back the twist two wires on top of each other. The bottom line is that 2 x wire width = 1.02 mm, so your wire should be about .51 mm, which is 24 gauge.
Now, having said that, a lot depends on how much you twist. The wires can be loose, or they can be twisted so much that it looks more like a coil from the end. If you do a tight twist and don't deform the metal much, 24 gauge should get you close.
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Jim