Convert from nanogram to firkin [butter, soap]

Unit Definition (nanogram)
The nanogram is a metric unit of mass equal to 10E-9 gram, or one millionth of a milligram.

Unit Definition (firkin [butter, soap])
The firkin [butter, soap] is a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms).