Sail Area / Displacement Ratio (SA/D) and Bruce Number (BN)
(Please note this page has been derived from the SA/D calculator at

This calculation indicates the power of the sailplan relative to the displacement of the boat, regardless of length.

SA/D range of values: 16 to 18
18 to 22
22 to 26
26 to 30+
Heavy offshore cruisers
Medium cruisers
Inshore cruisers, racing boats
Extreme racing boats

SA/D Calculator The Formulae

Sail Area in sq. ft.
SA/D = -------------------------------------
(Displacement / 64)^0.6666

(Sail Area in sq. ft.) ^ 0.5
BN = -------------------------------------

* Displacement in pounds
Sail area in sq. ft.
Displacement in lbs.
SA/D =
Bruce no. =

Caution & Warning....

Oh the ways we can deceive. As with the D/L ratio, the displacement figure used must reflect the real-world weight of the boat in the water. Where you're most likely to be led astray is with the sail area figure. Total area has traditionally been considered to be the sum of the foretriangle, plus the triangular area of the main, roach not included. Because of the highly roached mains found on many of the newer boats, it's becoming common to quote the full area of the main. This is fine, you just need to be sure you're figuring it the same way for all the boats you want to compare.