Seems to be an amazing resource, but so sad that it's tailored for an American audience with an English-only catalogue and those unreadable units (wtf is a lbs/ft3, seriously ?).
Edit: Ah ! They have meters and kg/m3 on each detail page, that's better !
The standard unit for purchasing lumber in the United States is a board-foot which is 1"x12"x12" or 1/12th of a cubic foot. Cubic feet is used because "board-foot" is too industry specific but it's still easy enough to convert when you need to calculate the weight of your woodworking/construction.
Edit: Ah ! They have meters and kg/m3 on each detail page, that's better !