Under FAA regulations, to qualify as an ultralight — and thus be exempt from registration and licensing requirements — the aircraft must weigh no more than 254 pounds, have a fuel capacity of five gallons, have a maximum speed of 55 knots and carry a single seat.

Under FAA Part 103 regulations in the United States, there isn't actually a "weight limit" for the pilot themselves, but there is a very strict weight limit for the aircraft. A powered ultralight must have a maximum "empty weight" of no more than 254 pounds (115 kg). If the aircraft has floats or a ballistic parachute, it is allowed a small amount of additional weight. Because the planes are so light and have low-power engines (usually limited to 5 gallons of fuel), the "useful load" (the pilot + fuel + gear) is naturally restricted. Most ultralight designs are built for a pilot weight between 150 and 250 pounds. If a pilot is significantly heavier, the aircraft may struggle to climb, have a dangerously high stall speed, or exceed its structural stress limits. While you don't need a medical certificate or a license to fly an ultralight, it is vital for the pilot to be within the specific weight range listed by the aircraft manufacturer to ensure that the center of gravity remains within safe limits and that the plane can actually get off the ground.