The few numbers behind every steel weight
Carbon steel weighs about 490 pounds per cubic foot. Divide by 1,728 cubic inches in a cubic foot and you get 0.2836 lb per cubic inch. Almost every steel weight formula comes from that one number.
Two shortcuts follow from it and are worth memorizing:
- Weight per foot = 3.4 × cross-sectional area. A one-foot length of a section with an area of A square inches is 12A cubic inches, and 12 × 0.2836 = 3.403. So lb/ft ≈ 3.4 × A (in²).
- Plate weighs 40.8 lb per square foot per inch of thickness. One square foot, one inch thick, is 144 cubic inches, and 144 × 0.2836 = 40.8.
For metric work, the density is 7,850 kg per cubic meter, which gives the same kind of shortcuts (covered below). Tables and software use these same relationships, so understanding them lets you check any number you are given.
W shapes and other rolled shapes: the weight is in the name
For most hot-rolled shapes in the AISC designation system, the second number is the weight per foot. A W12X35 is a wide flange with a nominal depth of 12 inches weighing 35 lb/ft. The same goes for S, M and HP shapes, channels (C12X20.7 is 20.7 lb/ft), MC channels, and tees cut from them (a WT6X17.5 is half a W12X35, at 17.5 lb/ft).
The first number is the nominal depth, not the actual depth. A W12X35 is about 12.5 inches deep. That matters for clearances and connections, not for weight.
- Length 24'-6" = 24.5 ft
- 24.5 ft × 35 lb/ft = 857.5 lb
- Twelve of them: 12 × 857.5 = 10,290 lb = 5.15 tons (2,000 lb tons)
Plate
Plate weight = area in square feet × thickness in inches × 40.8. Work in inches and divide by 144 to get square feet.
| Thickness | Weight per square foot |
|---|---|
| 1/4" | 10.2 lb |
| 3/8" | 15.3 lb |
| 1/2" | 20.4 lb |
| 5/8" | 25.5 lb |
| 3/4" | 30.6 lb |
| 1" | 40.8 lb |
- PL 3/4" × 14" × 14"
- 14 × 14 = 196 in² ÷ 144 = 1.361 ft²
- 1.361 × 0.75 × 40.8 = 41.7 lb each
- PL 3/4" × 48" × 96" = 4 ft × 8 ft = 32 ft²
- 32 × 0.75 × 40.8 = 979 lb
Bolt holes are normally ignored at bid time; they remove very little weight. For shaped plates such as gussets, use the bounding rectangle you will cut from, since that is the plate you buy.
Checkered floor plate weighs a little more than plain plate of the same nominal thickness because of the raised pattern, and grating and metal deck are sold by manufacturer weight tables. Use the supplier's published weights for those rather than a formula.
Flat bar, square bar and round bar
Bars are simple solid shapes, so the 3.4 rule applies directly:
- Flat bar: lb/ft = 3.4 × width × thickness. FB 4 × 1/4: 4 × 0.25 = 1.0 in² → 3.40 lb/ft. FB 6 × 3/8: 2.25 in² → 7.65 lb/ft.
- Square bar: lb/ft = 3.4 × side². A 1/2" square bar: 0.25 in² → 0.85 lb/ft.
- Round bar: lb/ft = 3.4 × π/4 × d² = 2.67 × d². A 3/4" round: 2.67 × 0.5625 = 1.50 lb/ft. A 1" round: 2.67 lb/ft.
Angles
Angle designations give the legs and thickness but not the weight: L4X4X3/8 has two 4-inch legs, 3/8 inch thick. Treat the angle as two rectangles that overlap at the heel:
- L4X4X3/8: 0.375 × (4 + 4 − 0.375) = 2.86 in² → 3.4 × 2.86 = 9.7 lb/ft (AISC table: 9.8)
- L6X4X1/2: 0.5 × (6 + 4 − 0.5) = 4.75 in² → 3.4 × 4.75 = 16.2 lb/ft (AISC table: 16.2)
The formula ignores the small fillet at the heel and the rounded toes, so it lands within about one percent of the table. For pricing, use the published table weight; use the formula to sanity-check an unfamiliar size.
Square and rectangular HSS (tube)
HSS are designated by outside dimensions and wall thickness: HSS8X4X3/8 is 8 inches by 4 inches with a 3/8-inch nominal wall. The quick estimate treats the tube as four flat walls:
That reads slightly heavy because real tubes have rounded corners. AISC calculates HSS properties with an outside corner radius of 2t, which gives a closer formula:
- HSS6X6X1/4, quick: 2 × 0.25 × (6 + 6 − 0.5) = 5.75 in² → 19.6 lb/ft
- HSS6X6X1/4, closer: 0.25 × (24 − 1.64) = 5.59 in² → 19.0 lb/ft (AISC table: 19.02)
- HSS8X4X3/8, quick: 0.75 × (8 + 4 − 0.75) = 8.44 in² → 28.7 lb/ft
- HSS8X4X3/8, closer: 0.375 × (24 − 2.47) = 8.07 in² → 27.5 lb/ft (AISC table: 27.48)
A note on wall thickness. For HSS made to ASTM A500 (electric-resistance welded), the AISC Specification uses a design wall thickness of 0.93 times the nominal wall when calculating section properties for design, because mills are allowed to roll the wall thinner than nominal. HSS made to ASTM A1085 use the nominal wall. The AISC tables list the design wall next to the nominal one, but the tabulated weight per foot is based on the nominal wall. For estimating weight, use the nominal thickness. Using 0.93t understates the weight by about 7 percent.
Pipe and round HSS
For any round tube, the area is π × t × (D − t), where D is the outside diameter. Multiplied by 3.4, that gives the standard pipe weight formula:
Pipe is named by nominal size, not outside diameter, so look up the OD. Pipe 4 Std (Schedule 40) is 4.500" OD with a 0.237" wall; Pipe 1-1/2 Std is 1.900" OD with a 0.145" wall.
- Pipe 4 Std: 10.69 × 0.237 × (4.500 − 0.237) = 10.8 lb/ft
- Pipe 1-1/2 Std: 10.69 × 0.145 × (1.900 − 0.145) = 2.72 lb/ft
- 100 ft of double-pipe guardrail: 2 rails × 100 ft × 2.72 = 544 lb
- Posts at 5'-0" on center: 21 posts × 3.5 ft × 2.72 = 200 lb
- Rails and posts ≈ 744 lb, before base plates and fittings
Round HSS follow the same formula, again using the nominal wall for weight.
Metric equivalents
In metric units the density is 7,850 kg/m³, which gives two matching shortcuts:
- Plate: 7.85 kg per square meter per millimeter of thickness. A 10 mm plate, 1,000 × 2,000 mm: 2 m² × 10 × 7.85 = 157 kg.
- Linear weight: kg/m = area in mm² × 0.00785. A 100 × 10 mm flat: 1,000 mm² × 0.00785 = 7.85 kg/m.
To convert, 1 lb/ft = 1.488 kg/m and 1 kg/m = 0.672 lb/ft. Metric W shapes are designated by depth in millimeters and mass in kg/m, so a W12X35 (35 × 1.488 = 52.1 kg/m) is a W310X52. One metric ton is 1,000 kg, or about 2,205 lb.
From piece weights to what you actually buy
The weight you price is rarely the net weight of the pieces. Steel is bought in stock lengths (W shapes commonly in 20, 40 and 50 ft lengths; tube and pipe in their own lengths), and the drops left over after cutting are part of the cost unless you can use them on the job. A cut list that nests the pieces into stock lengths gives you the purchased weight and the waste.
Connection material, stiffeners and plates add weight beyond the main members. If the connections are detailed, take them off. If not, use an allowance based on your own past jobs, since it varies with the framing and with how your shop details. And confirm how your galvanizer prices, since galvanizing is commonly billed by weight.
Let the software carry the arithmetic
Kredna includes about 1,600 standard AISC, UK and EU shapes with weights from the AISC Shapes Database (US shapes), locked so nobody overwrites a table weight by accident. Take off a member and its weight is calculated from the length; plate is entered by width × length × thickness with the weight shown live; railings and bar joists are assemblies whose parts are weighed one by one. Switch between feet-inches and metric at any time and the totals stay identical.
Pricing then buys the steel in your stock lengths, with drops and stock waste carried into the price, and the cut list and material list are ready for your supplier. See estimating and pricing.