Safe Working Load (SWL) sometimes stated as the Normal Working Load (NWL) is the force that a piece of lifting equipment, lifting device or accessory can safely use to lift, suspend or lower a mass without fear of breaking. Usually marked on the equipment by the manufacturer it is a calculation of the Minimum Breaking Strength (MBS) and its risk factor, ie 5:1 for lifting equipment although other fractions may be used such as 1/4, 1/6 and 1/10.
Other synonyms include Working Load Limit (WLL), which is the maximum working load designed by the manufacturer. This load represents a force that is much less than that required to make the lifting equipment fail or yield, also known as the Minimum Breaking Load (MBL). SWL or WLL are calculated by dividing MBL by a safety factor (SF). An example of this would be a chain that has a MBL of 2000 lbf (8.89 kN) would have a SWL or WLL of 400 lbf (1.78 kN) if a safety factor of 5 (5:1, 5 to 1, or 1/5) is used.
As such:
WLL = MBL / SF