8 yards is a general rule to equal a tank, as PaulHenry says it is not a fixed number.

But "tank" also implies 16 oz fabric here on XMarks. 13 oz = medium weight (uparmored humvee, not tank). Again, since we made up the term "tank" we can define it however we want. But it was intended to mean the heaviest (non-military) weight pure Scottish wool in an "8 yd" length, and hand sewn.

Obviously, its use is very flexible, though.