The rebar weight formula is weight (kg/m) = cross-sectional area (mm²) × 0.00785, because steel weighs about 7,850 kg per cubic metre. If you only know the diameter, the shortcut is kg/m ≈ d² ÷ 162, using the bar's nominal diameter in millimetres. For Canadian bar, the area version is the one to trust: 100 mm² for 10M gives 0.785 kg/m and 200 mm² for 15M gives 1.570 kg/m, exactly the CSA G30.18 values.
Below is where each constant comes from, the imperial version, the most common mistake people make with metric bar sizes, and how to use the formula backwards to identify a bar.
The formula, step by step
A metre of rebar is, in effect, a cylinder of steel one metre long. Its weight is volume times density.
- Volume of one metre of bar = cross-sectional area × 1 m.
- Weight = volume × density of steel.
With area in mm², you convert to m² by dividing by 1,000,000. So:
kg/m = A (mm²) ÷ 1,000,000 × 1 m × 7,850 kg/m³ = A × 0.00785
Check it against the standard values:
| Bar size | Nominal area | Area × 0.00785 | CSA G30.18 nominal mass |
|---|---|---|---|
| 10M | 100 mm² | 0.785 kg/m | 0.785 kg/m |
| 15M | 200 mm² | 1.570 kg/m | 1.570 kg/m |
| 20M | 300 mm² | 2.355 kg/m | 2.355 kg/m |
| 25M | 500 mm² | 3.925 kg/m | 3.925 kg/m |
| 30M | 700 mm² | 5.495 kg/m | 5.495 kg/m |
| 35M | 1,000 mm² | 7.850 kg/m | 7.850 kg/m |
| 45M | 1,500 mm² | 11.775 kg/m | 11.775 kg/m |
| 55M | 2,500 mm² | 19.625 kg/m | 19.625 kg/m |
They match exactly because the Canadian sizes were set up around round-number areas. That is also why weight goes up in neat steps: two 10M bars weigh the same as one 15M, and five 10M bars weigh the same as one 25M. For a ready-to-use table by length and per tonne, see our rebar weight calculator.
The d² ÷ 162 shortcut
Most people know a bar by its diameter, not its area. The area of a round bar is π × d² ÷ 4, so substitute that in:
kg/m = (π ÷ 4) × d² × 0.00785 = d² × 0.006165 ≈ d² ÷ 162.2
where d is the diameter in millimetres. Rounded, that is the "d squared over 162" rule you will see in site handbooks.
Worked through for Canadian sizes:
| Bar size | Nominal diameter | d² ÷ 162.2 | Nominal mass |
|---|---|---|---|
| 10M | 11.3 mm | 0.787 kg/m | 0.785 kg/m |
| 15M | 16.0 mm | 1.578 kg/m | 1.570 kg/m |
| 20M | 19.5 mm | 2.344 kg/m | 2.355 kg/m |
| 25M | 25.2 mm | 3.915 kg/m | 3.925 kg/m |
The small differences come from the published diameters being rounded to one decimal place. The area-based values are the official ones, so use the shortcut for quick checks and the area formula (or the table) for anything that goes on paper.
The mistake: putting 10 in for 10M
The number in a Canadian bar size is a rounded label, not the diameter. A 10M bar is 11.3 mm, a 15M bar is 16.0 mm and a 30M bar is 29.9 mm. Our post on how rebar sizes work explains the naming.
Plug the label into the formula and the error is large:
- 10² ÷ 162 = 0.62 kg/m, against the correct 0.785. About 21% light.
- 15² ÷ 162 = 1.39 kg/m, against the correct 1.570. About 12% light.
- 25² ÷ 162 = 3.86 kg/m, against the correct 3.925. Close, by luck.
That is enough to throw off a budget or a crane pick badly. If a spreadsheet or online calculator asks for "diameter" and someone has typed in the bar size, its answers will be low.
What "nominal diameter" means
A deformed bar is not a smooth cylinder. It has ribs rolled onto it, so a caliper reading across the ribs will be bigger than the nominal diameter, and a reading between ribs will be smaller.
The RSIC Manual of Standard Practice defines the nominal diameter as the diameter of a plain round bar with the same weight per metre as the deformed bar. In other words, the nominal diameter is chosen so that the formula works. It is a weight-equivalent diameter, which is why you should use it (11.3 mm, 16.0 mm and so on) rather than anything you measure over the ribs.
The imperial version
For US work or imperial lengths, the same idea with imperial units. Steel at 7,850 kg/m³ is about 490 lb per cubic foot. That gives two forms, both listed among the conversion factors in the RSIC manual:
- lb/ft ≈ 3.4 × area in square inches
- lb/ft ≈ 2.67 × d², with d in inches
Checks:
- A US #4 bar is 0.500 in in diameter: 2.67 × 0.25 = 0.668 lb/ft.
- A Canadian 15M bar has an area of 0.31 in²: 3.4 × 0.31 = 1.054 lb/ft, against the published 1.055 lb/ft.
If you have kg/m and want lb/ft, multiply by 0.672 (2.2046 lb per kg × 0.3048 m per ft). For example, 1.570 kg/m × 0.672 = 1.055 lb/ft. The full conversion chart between the two systems is in our Canadian vs US rebar sizes post.
Using the formula backwards to identify a bar
The formula also works in reverse, which helps when you find unmarked bar on site and need to know what it is. Cut a straight piece, measure its length, weigh it, and work out the mass per metre. Then:
nominal diameter ≈ √(kg/m × 162.2)
Example: a 1.50 m offcut weighs 2.36 kg. That is 2.36 ÷ 1.50 = 1.573 kg/m, and √(1.573 × 162.2) = √255 ≈ 16.0 mm. It is a 15M bar.
A few cautions:
- Heavy rust or loose scale adds and removes weight unpredictably. Brush the sample clean first.
- Bars carry rolling tolerances, so expect a reading slightly off the nominal value. You are looking for the nearest standard size, not a perfect match.
- Weight tells you size, not grade. Grade comes from the rolled markings or the mill test report.
For reading bar markings in the first place, see how to identify rebar size.
When the steel formula does not apply
The 0.00785 constant is for carbon steel. A few cases need a different number or a different approach:
- Bent bars. The formula gives weight per metre. The length you multiply by is the developed length of the bent piece along its centreline, including hooks, which the detailer calculates on the bar list.
- Coated bar. Epoxy and zinc add a little weight, and coated bar is quoted as its own item. Ask your supplier how coated weight is calculated on the quote.
- Stainless steel bar. Stainless alloys have a slightly different density from carbon steel, so use the producer's published mass rather than the carbon steel table.
- GFRP bar. Glass fibre bar weighs roughly a quarter to a third as much as steel of a similar size, and its mass varies by manufacturer and grade. Use the manufacturer's data sheet, as shown in our GFRP rebar sizes and weights chart.
- Other steel shapes. Plate, angle and beams use the same density but different area calculations. Our guide to calculating structural steel weight covers those.
From kg per metre to a real order
The formula is the easy part. The hard part of any rebar weight is getting the lengths right: every bar counted, laps where the runs exceed stock length, hooks and bends measured along the bar, dowels and corner bars included. That comes from detailing the drawings, not from a formula.
We prepare free take-offs and quotes from structural drawings, and our detailed bar lists give you the weight of every mark by size. Send us your drawings or see our rebar products for straight bar, fabricated shapes and GFRP.
Frequently asked questions
What is the formula for rebar weight per metre?
Weight in kg/m equals the bar's cross-sectional area in mm² × 0.00785, because steel weighs about 7,850 kg per cubic metre. Using diameter instead, kg/m is about d² ÷ 162, with d the nominal diameter in millimetres.
Why is it d squared divided by 162?
The area of a round bar is π × d² ÷ 4. Multiplying by the density of steel (7,850 kg/m³) and converting mm² to m² gives d² × 0.006165, which is the same as d² ÷ 162.2.
What diameter should I use for 10M rebar in the formula?
Use the nominal diameter, 11.3 mm, not 10. Using 10 gives about 0.62 kg/m, which understates 10M by roughly a fifth. The correct nominal mass is 0.785 kg/m.
What is the imperial rebar weight formula?
Weight in lb/ft is about 2.67 × d², with d in inches, or 3.4 × the area in square inches. Both come from a steel density of about 490 lb per cubic foot.
