Mass of Substance = Volume of Substance x Density of Substance
Density of naphtha, wood = 959.51 kg/m3
Density of naphtha, wood in kilogram/gallon(UK) = 0.027262648373179856
Mass of a Substance = 0.027262648373179856 x 15
15 gallon(UK) of naphtha, wood weighs 0.4089397255976978 kilogram