If you want to determine them mathamatically do the following for 12/1 as example. Add the dia of 12ga (.729") to the dia of 11ga (.751") then divide by 2 & drop any figures beyond 3 decimal places, do not round, just drop. IE .729" + .751" = 1.48/2 = .740" for 12/1. 10ga & larger have an extra division ie 10/1 & 10/2. For these you take 1/3 & 2/3 of the difference between the gauge & next larger one & again drop to 3 decimal places. Remember also even though a gauge is not in use these are all based on the next larger whole number gauge ie from 10 to 9, not 8 which is the next larger gauge in common usage. The formula for the Diameter(d) of any gauge with D as the diameter of 1 gauge is the cube root of the diameter of 1gauge cubed divided by the gauge number; thus d= 3√ Dcube/Ga. To exactly fit the British charts which extent all the way from onne down to 172.28 (.300") requires D to be carried to 6 decimal places. The best figure I have come up with which does fit all the way is D= 1.669285. I this case though the answer is "Rounded" to 3 decimal places. The in-betweens thus are truly averages & not strictly a mathamatical formula.
Remember all the inbetweens are larger than the gauge number & include 2 steps for 5Ga down through 10 ga & 1 step for gauges 11 down through 17ga. Gauges 4 & above & 18 & down are not divided.
