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In general, people buy certain SUP lengths dependent on how much weight the board will carry. The lighter you are, the safer a shorter board will be for you. If you will be sharing your SUP with anyone on the water, like your kids or other family members, go with a longer board to make sure it can hold the extra weight. If you’re very light, or 50–125 pounds (23–57 kg), and will be riding alone, you can also opt for the shortest all-around boards, which are 9–10.5 feet (2.7–3.2 m). The medium weight class will need a medium length board in most cases. Many all-around boards come in this size. This size is often the easiest to find for multipurpose use. These longest boards are often wider as well, making them safer to balance more weight. If your weight or your combined family weight is 200–275 pounds (91–125 kg), plan on going with the largest available board. The surfing SUP sizing often goes by volume recommendations instead of length, since they are all shorter boards. This system has you multiply your body weight by a certain decimal depending on your experience level, to get a volume recommendation number.  For example, beginners would multiply their body weight in pounds by 1 to 1.4. Intermediates would multiply their body weight by 0.8 to 1. Advanced surfers would multiply theirs by 0.6 to 0.8. The resulting number gives you a board volume recommendation in liters.
Get a 10.5–11.5 feet (3.2–3.5 m) board if you weigh under 175 pounds (79 kg). Use a 11.5–12.5 feet (3.5–3.8 m) board if you weigh 150–225 pounds (68–102 kg). Choose a board that’s longer than 12.5 feet (3.8 m) if you weigh 200–275 pounds (91–125 kg). Follow a volume recommendation if you plan to surf.