Just a note: some people use "girth" which is the measurement around the shaft. This is also known as the "circumference", but girth sounds sexiee to say.
The "diameter" is the cross section, or measurement from one side directly to the opposite side.
Since most dildos are sold as a diameter measurement, the easiest was to estimate the girth would be to multiply by 3-1/8.
Simarly, if any of you are reading about the girth and wondering how wide it is, just divide by 3-1/8.
Yes, technically it is 3.141579, but we don't need to carve it so close, do we?