Daily fraction of cryogenic cargo lost to boil-off, expressed as % of cargo mass per day.
Regulatory detail & full definition
BOR is a guarantee figure in newbuild contracts. For LNG carriers, BOR < 0.15%/day is now standard for Q-Flex and Q-Max; older steam-turbine vessels run 0.20-0.25%/day. The rate varies with sea state, ambient temperature, and cargo level (sloshing). Charterers monitor BOR closely because it directly affects voyage economics.