You can put this solution on YOUR website! Mass in English units is given in slugs, in metric it's kilograms.
Pounds are units of force or weight (mass x acceleration).
The fake unit is lb-m (for pounds mass).
One gallon of water contains 3785 ml, so 1 gallon has a mass of in units of g.
There are 454 grams in 1 lb-m.
So then lb-m
So 1 gallon of water has a mass of approximately 8.34 lb-m.