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Wire diameter: millimetres and AWG
Type either side. Useful when a drawing is in AWG and the order is in millimetres.
Between 40 AWG and 4/0. Outside that range the formula is no longer meaningful for our wire, and a millimetre size is the honest way to order it.
Resistance per metre: NiCr and CuNi
An element is designed from ohms per metre. This gives the figure from grade and diameter, at 20 °C.
Values are nominal at 20 °C, from the resistivity of each grade. For an element that runs hot, use the hot resistance — the figure rises with temperature, and the supplier's own measured resistivity for the lot is the number to design against.
These are planning figures, not acceptance criteria. The certificate for your lot states the measured resistivity, and that is the value that binds.