Nalgene polypropylene carboys are safe to lift by the handles when full, as they are capable of supporting at least double their intended weight on a single handle.
To test the strength of the carboy handles, carboys are suspended through a single handle on the carboy body. 10 L, 20 L, and 50 L carboys are tested by filling the container with steel ballast pellets to a weight in excess of double the weight of the carboy filled to nominal capacity with water.