Large whales are frequently entangled in fishing gear and sometimes swim while carrying gear for days to years. Entangled whales are subject to additional drag forces requiring increased thrust power and energy expenditure over time. To classify entanglement cases and aid potential disentanglement efforts, it is useful to know how long an entangled whale might survive, given the unique configurations of the gear they are towing. This study establishes an approach to predict drag forces on fishing gear that entangles whales, and applies this method to ten North Atlantic right whale cases to estimate the resulting increase in energy expenditure and the critical entanglement duration that could lead to death. Estimated gear drag ranged 11–275 N. Most entanglements were resolved before critical entanglement durations (mean ± SD 216 ± 260 days) were reached. These estimates can assist real-time development of disentanglement action plans and U.S. Federal Serious Injury assessments required for protected species.