How do you calculate steel tonnage?

The formula

For each line of the takeoff:

Weight (lb) = weight per foot (lb/ft) x length (ft) x quantity

Then total the pounds and divide by 2,000. The weight per foot comes from the shape tables. It is the trailing number in the designation for most AISC shapes (W12x26 is 26 lb/ft), and for HSS, pipe, angles and channels you look it up. Every size in the shape database lists it.

Metric works the same way: kg/m x metres x quantity, divided by 1,000 for tonnes. To move between the two, use the factor in How do you convert kg/m to lb/ft?.

A worked example

24,977 lb divided by 2,000 is about 12.5 US tons. All the weights per foot above are from the tables for those sizes; see W12x26, W14x90 and HSS6x6x1/4.

How do you weigh plates?

Plate has no weight per foot in the table because it depends on width. Steel weighs about 490 lb per cubic foot, which works out to roughly 40.8 lb per square foot for each inch of thickness. A 1/2 in plate is about 20.4 lb per square foot. A plate 12 in x 12 in x 1/2 in is one square foot, so it weighs about 20.4 lb.

For a plate that is not a clean square, multiply width x length in square feet, then multiply by 40.8 and by the thickness in inches. Bar and flat stock follow the same logic.

Do you add waste and connections?

Yes, and you should state where. The takeoff total is the weight of the members as drawn. Material you buy is more, because stock lengths leave drops (commonly a few percent on wide flanges, more on small angles and tube). Fabricated weight is also more, because connection plates, clips, stiffeners and bolts are mostly not on the contract drawings.

Many shops carry connections as a percentage of the member weight or as a per-piece allowance, and carry waste as a separate line in the material buy. Keep these separate from the counted tonnage so the base number stays checkable. See Steel waste factors for typical ranges by member type.

Common tonnage mistakes

• Mixing short tons and metric tonnes in one spreadsheet. A tonne is about 10 percent heavier than a short ton.
• Using nominal instead of actual lengths when the drawing gives a dimension to a gridline and the member runs past it.
• Counting a member on both the plan and its enlarged detail.
• Weighing HSS from the nominal wall instead of the design wall. The tables use the design wall, so use the table value.
• Forgetting that a "typical" note multiplies the pieces.

SteelFlo applies the weight per foot from the section tables to every box it counts and totals the weight for you, so the arithmetic is not where errors come from.

A metric example

Take 20 IPE 300 beams, each 6 m long. Mass is 42.2 kg/m, so one beam is 6 x 42.2 = 253.2 kg and 20 beams are 5,064 kg, or 5.06 tonnes. In US terms that is about 11,164 lb, or 5.58 short tons. The numbers differ by 10 percent between tonnes and short tons, so label the unit every time.

If a drawing set mixes units (a European set with a few imperial details, for example), convert as you go and keep a units column in the spreadsheet.

Sanity checks on your total

Compare tonnage per square foot of floor area against jobs you have done of the same type. A low-rise warehouse and a multi-story office produce very different numbers, so compare like with like. Check the heaviest lines: if one W14x90 line carries a third of the total, confirm the count. Also compare your total with the member count. A very low average weight per piece suggests missed heavy members, and a very high one suggests a duplicated line.

If you are using the weight per foot from a table, a quick cross-check is to compute area x 3.4028. A W14x90 at 26.5 in² gives 90.2 lb/ft. If the two disagree, you have the wrong row.

How should you report tonnage?

Report it by category (beams, columns, bracing, HSS, angles, plates, misc) as well as in total. The category split tells the shop where the labor is, and it lets you apply different rates. State whether the number is net (as drawn) or gross (with waste), whether it includes connections, and which unit you used. A bid that says only "142 tons" invites argument. One that says "142.0 net short tons of framing members, connections excluded" does not.