Short answer
Weight per foot, written plf or lb/ft, is how many pounds one linear foot of a section weighs. In an AISC wide-flange designation like W12x26, the first number is the nominal depth in inches (12) and the second is the weight per foot (26 lb/ft). So a 20 ft W12x26 weighs about 520 lb.
Reading W12x26
W means wide flange. 12 is the nominal depth in inches, which is close to but not exactly the actual depth: a W12x26 is 12.22 in deep. 26 is the weight in pounds per foot of length. Its flange is 6.49 in wide and its cross-section area is 7.65 square inches.
That area is where the weight comes from: steel weighs about 3.4028 lb per foot for each square inch of cross-section, and 7.65 x 3.4028 is 26.03 lb/ft. The same check works on any section if you know the area. Full dimensions for this size are on the W12x26 page.
Does the second number always mean weight?
For AISC W, S, M, HP, C and MC shapes, yes: the trailing number is lb/ft. For other families it is not:
- HSS (HSS6x6x1/4): the numbers are outside dimensions and nominal wall thickness. The weight (19.02 lb/ft for that size) comes from the table, and it is based on the design wall thickness, not the nominal.
- Angles (L4x4x1/4): leg, leg, thickness. L4x4x1/4 is 6.6 lb/ft.
- Metric sections: UK designations like 203x133x25 UB end in kg/m, and European ones like IPE 300 carry depth only, so you look up the mass.
When in doubt, look up the weight rather than assuming.
How is plf used in an estimate?
Everything priced by weight starts with plf. Total weight is plf x length x quantity. Material cost is weight x price per pound. Shop hours are often estimated per ton. Freight is by the truckload, which depends on weight. A mistake in plf carries through all of them.
Quick examples from the tables: W10x15 is 15 lb/ft, W16x26 is 26 lb/ft, W24x55 is 55 lb/ft, and W14x90 is 90 lb/ft. Two different shapes can have the same plf (W12x26 and W16x26 both weigh 26 lb/ft) while being very different sizes, which is why you never identify a member by weight alone.
What about metric weight per metre?
Metric tables give mass in kg per metre. To compare with imperial, multiply kg/m by 0.67197 to get lb/ft. A W12x26 is 38.7 kg/m. The conversion is covered in How do you convert kg/m to lb/ft?. SteelFlo shows metric jobs in metric and imperial jobs in imperial.
Quick weight table
| Size | lb/ft | kg/m | Weight of a 20 ft piece |
|---|---|---|---|
| W8x10 | 10 | 14.9 | 200 lb |
| W10x15 | 15 | 22.3 | 300 lb |
| W12x26 | 26 | 38.7 | 520 lb |
| W16x26 | 26 | 38.7 | 520 lb |
| W18x35 | 35 | 52.1 | 700 lb |
| W24x55 | 55 | 81.8 | 1,100 lb |
| W14x90 | 90 | 133.9 | 1,800 lb |
Round numbers help you sanity-check handling too: a 20 ft W18x35 at 700 lb already needs a forklift or a crane, not a crew lifting by hand.
How do you check a callout against the tables?
Look up the designation and compare. If the drawing says W12x28 and the table has no W12x28, the callout is wrong or it is another family. If the weight per foot in the designation does not match the table, trust the table and raise an RFI. Transposed digits (W12x26 written as W12x62, for instance) are a common source of bad callouts.
This is also where unit confusion bites. A designation ending in 25 might be 25 lb/ft (an AISC W) or 25 kg/m (a UK UB). The standard of the drawing set tells you which, so identify the standard first.
Why not just use average plf?
Because it hides the mix. A frame of lightweight beams and a few very heavy transfer girders can share the same average as a frame of medium members, and they behave differently in the shop. Line-by-line weight is what you buy, cut, fabricate and ship. Use averages only as a sanity check on the finished total.
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