Right Size Your FSI Fins

Sizing of FSI Fins Compared to Typical Fins

How to right size your FSI fins? Our fins have a lot of power—hold and drive. Because they have a lot of power, your right-sized FSI fin or set of fins will have a smaller area than what you’re used to. And because they’re smaller, you’ll have less drag. Less drag translates to quicker acceleration and better wave catching, easier paddling, and longer sessions before you get tired.

Basically we take a typical fin, chop off about 30 percent of its area, then replace about 5 percent in a more modern, more efficient configuration—just likes what’s done in modern airplane and boat-keel design.  This removal and repositioning of surface area, reducing overall skin-friction drag, is one of the key concepts behind our fins.  Our fins are unlike dolphin fins that have dominated the market for the last 50 years, and for a reason. Winglets are solid science and they work.  Winglets and other design features make these fins about 20 to 30 percent more efficient than larger, older styles, so you need to downsize by about 20 to 30 percent as compared to the fin or set of fins you typically use. Read more about lift, drag, hold and drive.

FSI Fins—It’s All About the Area

Because FSI fins are symmetric, you can either get a set of all the same fins, or you can mix smaller and larger fins. We recommend going as small as you can, then adding a bit more area if you need it for your system, body mass, or wave conditions. Enter your fin size in our Fin-Area Calculator, then reference the FSI Fin Combinations table to select a fin combination.  Click the links to see the related products pages.  Any questions, contact us.

In the table below, “SB” refers to our Shortboard Fins, “WG2” refers to our new longboard/SUP fin that replaced the Wavegrinder Classic.


FSI Fin Combinations

(in square inches)

10.14 1 SB small
14.56 1 SB large
20.28 2 SB smalls
29.16 2 SB larges
30.42 3 SB smalls
30.6 1 WG2
34.86 2 SB smalls, 1 large
39.3 1 SB small, 2 larges
40.56 4 SB smalls
43.74 3 SB larges
49.44 2 SB larges, 2 SB smalls
50.88 1 WG2 + 2 SB smalls
58.32 4 SB larges
59.8 1 WG2 + 2 SB larges


Single-fin-fin-sizing example:

Enter the number “42″ in the calculator, which is the size of many typical longboard fins. The calculator yields a range of 29.40 and a high of 33.60 sq. inches. Check the table and you can see that you could get a WG2, or two SB larges, or three SM smalls—and all would be within the range. You might be able to go even smaller by using fewer fins in a multiple-fin set.

2 + 1 fin-sizing example:

If you ride a 2 + 1 setup, probably you have around 40 sq. inches in the center fin, and probably about 14.5 sq. inches in each of the sidebites. Enter “69,” and the calculator yields a range of 48.3 to 55.2 sq. inches. Check the table. You could go with a 2 + 1 setup of a WG2 and 2 SB smalls. And you might be able to go even smaller. I’m about 180 pounds, 5’11″, and generally ride a 9’6″ performance longboard. I popped out the sidebites and ride it as a single fin, shedding about 50 percent of my total fin area—it works great! Many have done the same thing and have reported excellent results. Part of the reason for this is shedding the toed-in fins.  Depending on the board-bottom shape, toe-in can create a lot of drag by virtue of the inward angle of attack.

Thruster-fin-sizing example:

Thruster medium sets often have three fins, each of which is about 14.6 sq. inches, times three = 43.8 square inches. Enter than number in the calculator, and it yields a 20 – 30 percent reduction range of 30.66 – 35.04 sq. inches. So try three smalls. If you feel that it’s too loose, or you’re spinning out, add a bit of area, try 2 smalls and a large, or 2 larges and 1 small.


Quad sets often have a couple big fins up front, like 15.5 fronts with 11.5 rears, a total of 54 sq. inches. Reducing that by 20 – 30 percent yields a range of 37.80 – 43.20 square inches. Try 4 smalls, or 2 larges and 2 smalls.

FSI Fins—FSI Fin Areas Sets

Fin type FSI size Area, each Area, set of 2 Area, set of 3 Area, set of 4
Shortboard fin SM Small 10.14 sq. in. 20.28 sq. in. 30.42 sq. in. 40.56 sq. in.
Shortboard fin LG Large 14.58 sq. in. 29.16 sq. in. 43.74 sq. in. 58.32 sq. in.
Thruster 2 SMs, 1 LG 2 SM, 1 LG     34.86 sq. in.  
Thruster 1 SM, 2 LG 1 SM, 2 LG     39.3 sq. in.  
Quad 2 LG, 2 SM 2 SM, 2 LG       49.44 sq. in.
Longboard-SUP fin (discontinued) Wavegrinder Classic 30.54 sq. in. 61.08 sq. in.    
Longboard-SUP fin WG2 30.60 sq. in. 61.20 sq. in.    
2 + 1 WG2 + 2 SB smalls     30.6 + (2 x 10.14) = 50.88  
2 + 1 WG2 + 2 SB larges     30.6 + (2 x 14.6) = 59.8