A generally accepted rule of thumb is to use a fin that is the same size (in inches) as the length of the board (in feet), so for a 9-foot board you would use a single 9-foot fin. Like surfboards, the fins are sized according to the weight of the surfer. The typical size starts in XS and goes up to L or XL. By knowing where your weight is in the manufacturer's size chart, you eliminate up to 75% of the remaining options.
If you're close to the cut line to get a fin size, aggressive surfers who drive hard when cornering may increase their size, while surfers who don't push as hard on curves will probably like the smaller of the two sizes. Also, when you're on the cut line, keep in mind that narrower tail boards like smaller fins, since they tend to surf at higher speeds, while wide-tailed boards like larger fins to compensate for the added tail area. When choosing the fins for your surfboard, you should start with your weight. Choosing the right fin size is the first essential step in choosing the right fins.
Once you've decided on the size, examine the configuration of the fins or the configuration of the fins on the board. Your board will be equipped with between 1 and 5 boxes of fins. The soft, flexible fins are ideal for making quick and sharp turns and provide freesurfing enthusiasts with a lively feel on their board, but they are more difficult to control compared to rigid fins.
Surfers who work hard when cornering or who ride faster, more powerful waves will appreciate the added stiffness that carbon or solid fiberglass adds to their fins, while surfers who only flow in their curves may find that these structures are too stiff.
The center and outer flaps have different shapes, so make sure you use the right flap for the right box.The blade refers to the shape of the outer and inner sides of the fin, the thinnest one near the tip and the thickest one near the base. However, glazed flaps are difficult to repair and do not offer the versatility of removable flaps. For surfers who want a board that is easy to control, a higher fin will be more tolerant and will handle turns in a relaxing way. For example, a flap that is straight up and down and inside the wingbox has no inclination (90°) and will likely make you drive faster, while if it is above 90° it will increase responsiveness.
The shorter fins don't hold as well to the water, but rather allow the board to be more “buttery” when turning, which is ideal for swivel maneuvers in water, for example. For these fins, think of chubby, enclosed beach escapes, where you only have limited space to do your work. Runners who are not in the 9'0 range will want a minimum fin of 8.5 inches and up to 10 inches, depending on the cyclist. Side fins are often referred to as “pointed inward”, with the front of the fin angled toward the center of the board.
While everyone has their own specific preferences, there are some fairly universal standards when it comes to the size of longboard fins. So what type of fin is best for you? The best way to know which fin is best for you and for the waves you surf is to try different sets. Trip Forman is the host of the popular and informative series of REAL Watersports surfboard video reviews, including “How to choose the right size surfboard”, shown below.