In United States, according to US customary measurement system, 1 US gallon = 3.785 litres, one litre of mortar weight around 2.162kg or 4.75lb, so 1 gallon of mortar weight = 3.785 × 2.162 = approx 8kg or 18lb, regarding this, what is the weight of mortar per gallon, generally on average weight of mortar is about 8kg or 18lb per gallon.