To calculate steel weight, multiply the volume of the piece by the density of steel. Using a working density of 7850 kg/m³, a steel plate of 2 m × 1 m × 10 mm weighs 2 × 1 × 0.010 × 7850 = 157 kg. The same method works for flat bar, round bar, square bar and pipe once you use the right volume. The result is a theoretical weight; real material can differ slightly, as explained below.
What is the basic rule for calculating steel weight?
Weight = volume × density
Volume is length × width × thickness for a flat piece, or the cross-section area × length for a bar or pipe. Density is the mass of steel per cubic metre. This guide uses 7850 kg/m³ as a working value. It falls inside the range of roughly 7,750 to 8,050 kg/m³ that Wikipedia gives for steel, which varies with the alloying elements.
The most common mistake is units. Density is per cubic metre, so convert every length to metres before you multiply: 10 mm = 0.010 m, and 50 mm = 0.050 m.
Steel weight formulas by shape
Each formula below is derived from weight = volume × 7850 kg/m³. Sizes are in millimetres unless stated.
| Shape | Weight formula | Result in |
|---|---|---|
| Plate, sheet or flat bar | length (m) × width (m) × thickness (m) × 7850 | kg |
| Plate or sheet, per area | thickness (mm) × 7.85 | kg per m² |
| Round bar | diameter² × 0.006165 | kg per metre |
| Square bar | side² × 0.00785 | kg per metre |
| Round pipe (hollow) | (outside diameter − wall thickness) × wall thickness × 0.024662 | kg per metre |
The constants come from the density: 0.006165 is π ÷ 4 × 7850 ÷ 1,000,000, and 0.024662 is π × 7850 ÷ 1,000,000. To get the weight of a full length, multiply the kg per metre by the length in metres.
Worked examples
1. A steel plate, 2.5 m × 1.25 m × 6 mm 6 mm = 0.006 m. Weight = 2.5 × 1.25 × 0.006 × 7850 = 147.19 kg (147.1875 before rounding).
2. A round bar, 16 mm diameter, 6 m long Per metre = 16 × 16 × 0.006165 = 1.578 kg. For 6 m: 1.578 × 6 = 9.47 kg.
3. A flat bar, 50 mm × 6 mm, 3 m long Weight = 0.050 × 0.006 × 3 × 7850 = 7.07 kg (7.065 before rounding).
4. A square bar, 20 mm side Per metre = 20 × 20 × 0.00785 = 3.14 kg.
5. A round pipe, 60 mm outside diameter, 3 mm wall, 5 m long Per metre = (60 − 3) × 3 × 0.024662 = 4.22 kg. For 5 m: about 21.09 kg. These sizes are only an example, not a standard pipe size.
If you need pounds, multiply kilograms by 2.20462.
Quick reference tables (7850 kg/m³)
Round bar weight per metre:
| Diameter (mm) | Weight (kg per metre) |
|---|---|
| 10 | 0.617 |
| 12 | 0.888 |
| 16 | 1.578 |
| 20 | 2.466 |
| 25 | 3.853 |
| 32 | 6.313 |
Plate or sheet weight per square metre:
| Thickness (mm) | Weight (kg per m²) |
|---|---|
| 1 | 7.85 |
| 2 | 15.7 |
| 3 | 23.55 |
| 5 | 39.25 |
| 6 | 47.1 |
| 10 | 78.5 |
| 12 | 94.2 |
| 20 | 157 |
Why can the real weight differ from your calculation?
A calculation gives a theoretical weight for a perfect shape at one assumed density. Steel's density changes with its alloy content, so the answer moves with the number you choose. For a 10 mm plate, one square metre weighs:
| Density used | Weight per m² of 10 mm plate |
|---|---|
| 7750 kg/m³ | 77.5 kg |
| 7850 kg/m³ | 78.5 kg |
| 8050 kg/m³ | 80.5 kg |
The actual dimensions of the piece, its grade and any coating also play a part. For anything where weight matters, such as transport, lifting, pricing or structural work, use the figures given by your supplier or the relevant standard, and treat the calculated number as an estimate for planning.
Common mistakes and a checklist
- Leaving millimetres in the formula. If density is in kg/m³, convert lengths to metres first.
- Using the outside diameter as if the pipe were solid. A pipe needs the wall thickness in the formula.
- Forgetting the length step for bars. The 0.006165 constant gives kg per metre, so multiply by the length in metres afterwards.
- Forgetting to multiply by the number of pieces.
- Treating the result as exact. It is a theoretical figure.
Before you rely on a number:
- All dimensions are in the same unit.
- You used the right formula for the shape.
- You multiplied by length and quantity.
- You noted the density you assumed.
- You compared with your supplier's figure for important jobs.
Frequently asked questions
What is the formula for steel weight? Weight = volume × density. With density 7850 kg/m³ and sizes in metres, the answer is in kilograms.
How much does a 10 mm steel plate weigh per square metre? 78.5 kg at 7850 kg/m³ (10 × 7.85).
How do I calculate the weight of a round bar? Square the diameter in millimetres and multiply by 0.006165 to get kg per metre. A 16 mm bar gives 1.578 kg per metre.
What density of steel should I use? This guide uses 7850 kg/m³ as a working value, within the roughly 7,750 to 8,050 kg/m³ range given for steel. Use your supplier's figure when it matters.
Is a calculated weight the same as the weighed weight? Not always. A calculation is theoretical, so a weighed or supplier-stated weight can differ.
Skip the manual maths
If you calculate bar weights often, the free Round Bar Weight Calculator on BytePriva can do the round bar calculation for you. Keep in mind that any calculated weight is theoretical, so confirm important figures with your supplier.
Disclaimer
This guide gives theoretical weights for general information, using a working density of 7850 kg/m³. Actual weights vary with grade, dimensions, tolerances and coatings. Do not use calculated values alone for structural design, lifting, transport or pricing; follow your supplier's data and the applicable standards. Written by the BytePriva Team. Last reviewed: 23 October 2026.