The best thickness for a lever belt is typically anywhere between 10mm and 13mm depending on the material used. Leather and suede are usually seen in the 10-13mm range, whereas nylon generally goes up to 15mm thick. The thicker the belt, the more support it will provide while lifting heavy weights or performing powerlifting exercises. Thinner belts can still be effective when using lighter weights as they offer plenty of flexibility without sacrificing stability. Ultimately, choosing the correct thickness for your specific needs will depend on what type of activities you plan on doing with your belt and how much support you need from it during those activities.