jun 08, 2017 there are 128 oz in a gallon and 29.5735 milliliters in an oz so there are 3785.41 milliliters in a gallon or 3.78541 liters in a gallon.1 gallon equals 3785.41 milliliters,how many milliliters in 1 gallon uk,how do you convert 1 gallon uk into milliliters? to transform 1 gallon uk into milliliters, you just need to multiply the quantity in gallons uk by the conversion factor, 0.000001. so, 1 gallon uk in milliliters 1 times 0.000001 4546.09 milliliters, exactly. see details on the formula below on this page.