The AWG is a logarithmic scale, so given an know resistance or diameter for a certain gauge, you can quickly figure out the resistance and diameter of another gauge number, by addition and subtraction.
AWG 15 is 10 mohm/m.
Adding 3 doubles the resistance, and subtracting 3 halves it.
Adding 10 multiplies the resistance by 10, and subtracting 10 it divides by 10.
AWG 18 has a solid diameter of about 1mm
Adding 6 halves the diameter, Subtracting 6 doubles the diameter
Adding 20 divides the diameter by 10, and subtracting 20 multiplies the diameter by 10.
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Ptss, I understand you posted this in the context of the speaker cable resistance. Please take into consideration that most of the speaker impedance is resistive. 8 ohm speaker will have, most likely, resistance of 6 ohm. Your 10 mohms, in comparison, means nothing.