Headsails are often referred to by their size as a percentage of the foretriangle; that is, the triangle formed by the
mast (I), the forestay, and the
deck (J).
Thus, a 120 percent jib would have an area 20 percent greater than that of the foretriangle.
Foretriangle Area = (I x J) ÷ 2
The I dimension is defined as the vertical height in feet of the foretriangle (from the
deck, not the
cabin top), and J is the horizontal distance in feet from the
mast to the root of the forestay.