Boil-off gas (BOG) from a liquefied natural gas (LNG) storage tank depends on the amount of heat leakage however, its assessment often relies on the static value of the boil-off rate (BOR) suggested by the LNG tank vendors that over/under predicts BOG generation. Thus, the impact of static BOR on BOG predictions is investigated and the results suggest that BOR is a strong function of liquid level in a tank. Total heat leakage in a tank practically remains constant, nonetheless the unequal distribution of heat in vapor and liquid gives variation in BOR. Assigning the total tank heat leak to the liquid is inappropriate since a part of heat increases vapor temperature. At the lower liquid level, BOG is under-predicted and at a higher level, it is over-predicted using static BOR. Simulation results show that BOR varies from 0.012 wt% per day for an 80% tank fill to 0.12 wt% per day at 10% tank fill.