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The piece's volume, length × width × thickness, times the material's density. Pieces are taken as solid rectangles: a slab with a live edge or a tapered leg weighs less than its bounding box, so measure its average width and thickness.
Water is part of a board's weight. Solid wood is listed at 12% moisture content, close to kiln-dried lumber kept indoors, which settles at 6 to 9%. Air-dried lumber usually settles at 12 to 15%. Freshly sawn wood holds much more: northern red oak heartwood about 80% and western white pine sapwood about 150% of its dry weight. Where the Wood Handbook lists a species' green moisture content, the readout gives the weight of the same piece freshly sawn.
Between oven-dry and 30% moisture content, wood gains weight and also swells, so its density changes less than its weight. The calculator follows the Wood Handbook's equations for both. Above 30% the wood no longer swells, and the extra water only adds weight.
Density varies within a species: the Wood Handbook puts the variation of oven-dry density at about 10%. Heartwood, sapwood and growth rate all play a part, so two boards of the same size can differ by a fifth. Sheet-goods densities are typical of what stores sell and differ between brands.
Kilograms measure mass; pounds here are pounds of weight at standard gravity, which is what a bathroom or shipping scale reads. One kilogram is 2.205 lb.
The volume line includes board feet, 144 cubic inches each, for comparing with a lumber yard's tally. Yards count rough lumber by its nominal thickness, so a surfaced board's board feet here, from its actual size, will be lower. The board foot calculator totals a list of boards and their cost.
Steel, aluminum, glass and plastics don't take on water, so moisture doesn't apply to them. Every carbon steel weighs the same, about 490 lb per cubic foot; stainless is slightly heavier and aluminum about a third as heavy.