Ask any site engineer how to calculate steel bar weight and you will hear "D square by 162". It is one of the most useful shortcuts in the steel trade. This guide explains the round bar weight formula, where it comes from, how to use it for TMT bars and how to adapt it for square, hexagon, flat and stainless bars.
The D²/162 formula
For a steel round bar:
Weight (kg per metre) = D² / 162
where D is the diameter in millimetres.
Example: a 20 mm bar weighs 20 × 20 / 162 = 400 / 162 = 2.47 kg per metre.
Where 162 comes from
Weight per metre = cross-section area × length × density.
- Area of a circle = π/4 × D² (in mm²)
- Steel density = 7,850 kg/m³
Weight per metre (kg) = (π/4 × D²) × 1,000 mm × 7.85 × 10⁻⁶ kg/mm³ = 0.006165 × D²
And 1 / 0.006165 ≈ 162.2. Hence D²/162 — a neat shortcut that is accurate to about 0.1%.
TMT and round bar weight chart
| Diameter (mm) | kg per metre | kg per 12 m bar |
|---|---|---|
| 6 | 0.222 | 2.66 |
| 8 | 0.395 | 4.74 |
| 10 | 0.617 | 7.40 |
| 12 | 0.888 | 10.66 |
| 16 | 1.580 | 18.96 |
| 20 | 2.470 | 29.64 |
| 25 | 3.850 | 46.20 |
| 28 | 4.830 | 57.96 |
| 32 | 6.310 | 75.72 |
| 40 | 9.860 | 118.32 |
TMT rebars are usually supplied in 12 m lengths, so builders often count bars per tonne. For example, about 135 bars of 10 mm make one tonne (1000 / 7.40).
TMT bars have ribs, but standards such as IS 1786 define nominal mass per metre using the same nominal diameter, with permitted tolerances. So D²/162 works well for rebar estimates.
Square bar weight
Area = a² (side in mm):
Weight (kg/m) = a² × 0.00785
Example: 25 mm square bar = 625 × 0.00785 = 4.91 kg/m.
Hexagon bar weight
For a hex bar measured across flats (a):
Weight (kg/m) = 0.0068 × a²
Example: 24 mm hex = 576 × 0.0068 = 3.92 kg/m.
Flat bar weight
Weight (kg/m) = width (mm) × thickness (mm) × 0.00785
Example: 50 × 10 mm flat = 500 × 0.00785 = 3.93 kg/m.
Stainless steel and other metals
The 162 factor assumes carbon steel at 7.85 g/cm³. For other metals, scale by density:
| Material | Density (g/cm³) | Round bar factor |
|---|---|---|
| Carbon steel | 7.85 | D² / 162 |
| Stainless 304 | 7.93 | D² × 0.00623 |
| Stainless 316 | 7.98 | D² × 0.00627 |
| Aluminium | 2.70 | D² × 0.00212 |
| Brass | 8.50 | D² × 0.00668 |
| Copper | 8.96 | D² × 0.00704 |
Example: a 50 mm SS 316 round bar = 2,500 × 0.00627 = 15.7 kg/m. Using the carbon steel formula would give 15.4 kg/m — almost 2% low.
Prefer a quick tool? Use our free bar weight calculator, which covers round, square, hex and flat bars in multiple materials.
Hollow bars and pipes
For hollow bars and pipes, calculate the outer circle minus the inner circle, or use the pipe formula: (OD – WT) × WT × 0.02466 kg/m (for steel, dimensions in mm). See steel pipe weight calculation and our pipe weight calculator.
Estimating steel for a construction site
Builders and contractors often need to convert a bar bending schedule into tonnes for ordering. The method is simple:
- List each bar diameter and the total running length needed, including laps and bends.
- Multiply the running length in metres by the kg per metre from the chart above.
- Add the totals for each diameter to get the overall weight.
- Add a small allowance for wastage and cutting, often around 2–3% depending on site practice.
For example, 1,200 m of 12 mm bar needs 1,200 × 0.888 = 1,066 kg, and 800 m of 16 mm bar needs 800 × 1.58 = 1,264 kg. Together that is about 2.33 tonnes before wastage. Traders who help customers with these quick calculations become their first call when it is time to order.
Using bar weights in quotations
- Confirm whether the buyer wants price per kg, per metre or per piece.
- State bar length (random or fixed, such as 3 m or 6 m).
- For bright bars, mention tolerance (h9, h11) and finish (peeled, ground, cold drawn).
- For forging bars and blanks, mention cutting allowance.
When enquiries list many sizes and grades, QuoteMitra calculates weights and line totals automatically, so your quote goes out faster and error-free.
Frequently asked questions
What is the weight of a 12 mm TMT bar per metre?
About 0.888 kg per metre, or roughly 10.66 kg for a 12 m bar.
Why do we divide by 162?
Because π/4 × steel density (7.85 g/cm³) works out to 1/162.2 when diameter is in mm and length in metres.
Is D²/162 accurate for stainless steel?
It under-estimates slightly. Use D² × 0.00623 for 304 and D² × 0.00627 for 316.
How many 8 mm bars make one tonne?
At 4.74 kg per 12 m bar, about 211 bars make one tonne.
Key takeaways
- Round bar: D²/162 kg/m (steel).
- Square: a² × 0.00785; hex: 0.0068 × a²; flat: w × t × 0.00785.
- Adjust for stainless and non-ferrous densities.
- Always state price basis and length in quotes.
Small formulas, big impact: accurate weights mean accurate prices — and fewer arguments at delivery.
