Originally Posted by Vettronics
It measures across at about 10-11mm but I am told it is called 25mm cable.
I assume that figure of 10-11mm diameter includes the insulation (and perhaps the armouring if it's armoured cable).
25mm cable has a diameter of about 5.6mm.
Area = Pi x radius squared
CSA = 3.142 x 2.8 x 2.8 = 24.63 sq mm