Mass And Volume

A group of my friends recently went camping in the desert. Over dinner one night, we were discussing the amount of water they were taking (5 x 55 Gal drums) and one of them was trying to work out how much that would weigh (mass restrictions on the vehicle being the limiting factor for how much they could take).

He never did manage to work out what it weighed in pounds, took me all of 5 seconds to convert it to liters and give him the approximate mass in kg (275 US gallons x 3.8 = 1,045 liters = 1,045 kg).

Remind me why we use the old system still?