Material Weight Calculator

Weight is volume times density, and neither half is usually the problem. The problem is that the shape is not what somebody thinks it is, or the density belongs to a different alloy, or the size on the drawing is nominal rather than measured.

Density, and the alloys that trip people up

Densities here are in pounds per cubic inch, because that is the unit that lets the shape arithmetic come out without conversions:

• Mild carbon steel 0.2836 lb/in³, which is 490 lb/ft³ or 7.85 g/cm³.
• Stainless 304 0.289, marginally heavier than carbon steel.
• Aluminium 6061 0.0975, about a third of steel.
• Brass 0.307 and copper 0.323, both heavier than steel.
• Lead 0.4097, and titanium 0.1628, roughly 60 percent of steel with comparable strength, which is the whole reason it costs what it does.

Within a family the variation is small: most carbon steels sit within a percent or two of 0.2836, and the aluminium alloys within about the same of 0.0975. If you are told to use a precise alloy density for a structural calculation, use the mill certificate rather than a table.

Getting the cross-section right

Every shape here reduces to a cross-sectional area multiplied by a length.

Flat bar is width times thickness. Round bar is π r². Both are straightforward.

Tube and pipe are the ones that go wrong. The area is the outer shape minus the inner void, and the inner dimension is the outer minus twice the wall thickness, because the wall appears on both sides. Subtracting the wall once is a common slip and it roughly doubles the answer.

Pipe adds a second trap: nominal pipe size is not a measurement of anything. A 2 inch schedule 40 pipe has an outside diameter of 2.375 inches and a wall of 0.154. Neither number is 2. Use the actual dimensions from a pipe schedule table rather than the name.

Weight per foot, which is what suppliers quote

Steel stockholders sell and quote by weight per foot, so the figure above is often more useful than the total. It also gives a quick check on whether an order is plausible: a 2 by 2 by ¼ inch square steel tube runs about 5.4 pounds per foot, so a 20 foot length is about 108 pounds, which is a two-person lift rather than a one-person one.

Wood, and the moisture problem

Timber densities are quoted here for air-dried stock at roughly 12 percent moisture content, which is the normal condition for lumber that has been stored inside.

Green timber, freshly sawn, can weigh half as much again. A green oak beam that the table says is 200 pounds may be 300 in reality. That matters for handling, for the load on temporary supports, and for shipping, and it is the reason a delivery of "dry" lumber that arrives soaked feels so much heavier than expected.

Wood density also varies within a species by a noticeable margin depending on where it grew and how fast. Treat wood figures as an estimate with perhaps 10 percent either way, unlike metals, which are consistent.

What weight is used for

Three questions, usually:

Can it be lifted or carried? Here the total matters and so does the shape, since an awkward 100 pound piece is worse than a compact one.

What will it cost? Metals are priced per pound or per hundredweight, so weight is the invoice. The price field above turns the weight into money directly.

What will it load? Shelf brackets, vehicle racks, trailers and temporary supports all have limits, and the dead weight of the material itself is often a large fraction of the total before anything is put on it.

Shipping weight and dimensional weight

Carriers charge on the greater of actual weight and dimensional weight, which is the volume of the package divided by a divisor set by the carrier. Long thin items such as bar stock and tube frequently ship on dimensional weight rather than on what they actually weigh, so a light aluminium extrusion can cost the same to send as a steel one of the same size.