New Delhi, July 24 -- DUBAI - The bill for moving crude oil through the world's most contested waterways has become, for most insurers, almost impossible to calculate. War-risk premiums on vessels transiting the Strait of Hormuz have reached 7.5 to 10 percent of hull value, according to insurance professionals tracking the market, a figure that dwarfs the 1 to 3 percent ceiling that defined even the most volatile periods in recent memory.

A 270,000-metric-tonne tanker now carries an insurance tab of roughly $21 million for a single voyage. That figure once represented the total annual exposure some mid-tier operators budgeted for an entire fleet. The shift happened in weeks.

Traffic through Hormuz, which averaged 120 to 140 vessel trans...