you cannot reliablly determine core size of SWA from overall size, if you new the manufacturer and date of manufacture and had access to the manufacturers specs you could but if you have that level of information you probablly know the size anyway . Differences in the packing armour,sheath and for large cables core shape can make considerablly more difference to overall diameter than a couple of steps of core size.
you need to measure the actual copper core area either by comparing against samples of known size or by using a suitable measuring device (verniar caliper or micrometer) to get the diameter and then calculating from there.
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