Under Part 107, the maximum takeoff weight of a small UAS (including payload) must be less than:

Why →Under 14 CFR § 107.3, a small unmanned aircraft is defined as one weighing less than 55 pounds on takeoff, including everything on board or attached. Aircraft at or above 55 pounds fall outside Part 107 entirely.

The trap →The threshold is "less than 55 pounds," not "55 pounds or less." An aircraft weighing exactly 55.0 pounds is not a small UAS, and 25 kilograms converts to just over the line at 55.1 pounds.