To calculate the weight of structural steel, multiply the volume of steel by its density of 7,850 kg/m³. For rolled shapes like beams, channels and tubes, the shortcut is to take the mass per metre (kg/m) from the section table and multiply by the length. A W310x39 beam weighs about 38.8 kg/m, so 6.1 m of it weighs about 237 kg. Plates, bars and connection material are calculated from their dimensions and added on top.
This guide gives the formulas for each type of member, a worked example for a beam with two posts, and the practical points that make a calculated weight differ from a scale ticket.
The one number you need: 7,850 kg/m³
Carbon structural steel, whether it is a W-beam, HSS, channel, angle or plate, is taken as having a density of 7,850 kg/m³. In other units that is:
| Unit | Value |
|---|---|
| Density | 7,850 kg/m³ |
| Unit weight | about 77 kN/m³ |
| Imperial | about 490 lb/ft³ |
| Per mm² of section, per metre of length | 0.00785 kg |
| Per m² of plate, per mm of thickness | 7.85 kg |
The last two rows are the ones that make day-to-day calculations quick. Any member of constant cross-section weighs:
Mass (kg/m) = area (mm²) x 0.00785
And any flat plate weighs:
Mass (kg) = length (m) x width (m) x thickness (mm) x 7.85
Method 1: rolled shapes from the section table
For beams, columns, channels, angles and HSS, you don't need to calculate the area. The mass per metre is published, and in Canada it is written into the metric name:
- W310x39 is a wide-flange beam about 310 mm deep at about 39 kg/m (38.8 in the table).
- C200x17 is a channel at about 17 kg/m.
- HSS 102x102x6.4 is a square tube; its mass (18.2 kg/m) comes from the table, not the name.
Imperial names work the same way in pounds per foot: a W12x26 weighs 26 lb/ft.
Weight of a piece = length (m) x kg/m
Example: 20 ft of W12x26 = 20 x 26 = 520 lb. In metric, 6.1 m x 38.8 kg/m = 237 kg. Same beam, same answer (520 lb is about 236 kg).
For the published values, use our size and weight charts:
- Beams: steel beam weight per metre and per foot
- Tubes: HSS weight per metre, which also gives the tube formulas
- Channels: C-channel weight per metre
Method 2: formulas for plate, bar and simple shapes
When there is no table, or the piece is cut from plate or bar, calculate the area from the dimensions. All dimensions in millimetres, results in kg per metre of length unless noted.
| Shape | Formula | Example |
|---|---|---|
| Plate (per piece) | L (m) x W (m) x t (mm) x 7.85 | 0.25 x 0.25 x 19 x 7.85 = 9.3 kg |
| Flat bar | width x thickness x 0.00785 | 100 x 10 bar: 7.85 kg/m |
| Square bar | side² x 0.00785 | 25 mm bar: 4.9 kg/m |
| Round bar | diameter² x 0.006165 | 25 mm bar: 3.85 kg/m |
| Equal or unequal angle (approx.) | t x (a + b minus t) x 0.00785 | 102 x 102 x 9.5 angle: about 14.5 kg/m |
| Square or rectangular HSS | 0.0157 x t x (B + H minus 3.3t) | 102 x 102 x 6.35: 18.2 kg/m |
| Round HSS or pipe | 0.02466 x t x (D minus t) | 168.3 x 6.35: 25.4 kg/m |
A quick sanity check on the round bar formula: a 10M reinforcing bar has a nominal diameter of 11.3 mm, and 11.3² x 0.006165 = 0.79 kg/m, which matches the published 0.785 kg/m for 10M.
The angle formula ignores the small fillet at the root and the rounded toes, so it lands slightly under the table value. Use the published mass for anything you will invoice or lift. For more on plate, including kg per square metre by thickness, see our steel plate weight calculator guide.
Worked example: a basement beam with two posts
Say the drawings call for one W310x39 beam, 6.1 m long, on two HSS 127x127x6.4 posts 2.6 m tall, each with a base plate and a cap plate. (The sizes here are for illustrating the arithmetic only. Your sizes come from your drawings.)
| Item | Calculation | Weight |
|---|---|---|
| Beam W310x39, 6.1 m | 6.1 x 38.8 | 236.7 kg |
| Posts HSS 127x127x6.4, 2 at 2.6 m | 5.2 x 23.2 | 120.6 kg |
| Base plates 250 x 250 x 19 mm, 2 | 2 x 0.25 x 0.25 x 19 x 7.85 | 18.6 kg |
| Cap plates 150 x 300 x 12 mm, 2 | 2 x 0.15 x 0.30 x 12 x 7.85 | 8.5 kg |
| Total | about 384 kg |
Two things fall out of this straight away. The beam alone is close to 240 kg, so getting it into a basement is a planned lift, not a carry. And the plates are a small share of the total here, but on a building with many connections, plates, clips and bolts add up.
Gross weight, net weight and allowances
Estimators and fabricators talk about steel weight in a few different ways:
- Gross (or calculated) weight: member lengths times kg/m, plus plates. This is what a take-off produces.
- Net weight: gross weight minus holes, copes and cut-outs. The difference is small for most members and is often ignored.
- Connection allowance: at early stages, before connections are designed, estimators add an allowance for plates, clips and bolts on top of the main member weight. How much depends on the type of building and connections, so it comes from the estimator's own experience with similar work rather than a fixed rule.
- Scale weight: what the truck actually weighs. Rolled sections and plate are produced to a mass and thickness tolerance, so the scale rarely matches the calculation exactly.
Coatings add weight too. Primer adds very little; galvanizing adds more. If weight is critical (crane picks, floor loading during construction), ask the coater for a figure.
If you are building up a full quantity from drawings, our guide on how to do a structural steel takeoff covers the process member by member.
Why the weight matters
Steel weight is used for more than curiosity:
- Estimating and pricing. Structural steel is commonly estimated by weight, with fabrication, finishing, delivery and erection built up from there.
- Lifting and rigging. The heaviest single piece decides the lifting method, whether that is people, a material lift, a forklift or a crane.
- Delivery. Total weight and the longest piece decide the truck and the number of loads.
- Structural design. Steel self-weight is part of the dead load. The engineer includes it, which is why a beam's own weight shows up in the design calculations.
- Temporary loads. A bundle of steel set down on a suspended slab or a deck during construction is a concentrated load. Check before landing it.
Unit conversions you will use
- 1 kg/m = 0.672 lb/ft
- 1 lb/ft = 1.488 kg/m
- 1 tonne (metric) = 1,000 kg = about 2,205 lb
- 1 short ton = 2,000 lb = about 907 kg
- 1 m = 3.281 ft
Watch the word "ton". Canadian steel quotes and take-offs are normally in metric tonnes, while US sources may mean short tons. The difference is about 10%.
Get a take-off from your drawings
If you need a total steel weight for a project, the most reliable route is a take-off from the drawings. We prepare detailed take-offs and quotes free of charge from structural or architectural drawings, then produce shop drawings in-house, fabricate with CWB certified welding and deliver across the GTA and Ontario.
Send your drawings for a quote, or see our steel products for beams, posts, plates and channels.
Frequently asked questions
What is the formula for steel weight?
Weight equals volume times density. Structural steel is taken as 7,850 kg/m³, so a member's mass in kg/m is its cross-sectional area in mm² times 0.00785. Multiply by the length in metres for the weight of the piece.
What is the unit weight of structural steel?
Structural steel has a density of about 7,850 kg/m³, which is a unit weight of about 77 kN/m³ or 490 lb/ft³.
How do you estimate the weight of a steel beam?
Read the mass per metre from the designation or a section table and multiply by the length. A W310x39 weighs about 38.8 kg/m, so a 6.1 m beam is about 237 kg before any plates or connections are added.
How much does a steel plate weigh per square metre?
About 7.85 kg per square metre for every millimetre of thickness. A 10 mm plate is about 78.5 kg/m² and a 20 mm plate about 157 kg/m².
