Round, Square and Flat Bar Weight Calculator
Mass per metre and total for round, square and flat bar.
Quick answer
With the default inputs: Bar type Round bar · Size a 20 · Flat bar thickness 6 etc.Total mass 148.08 kg
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Result
Total mass
148.08kg
| Type | Round bar |
| Mass per metre | 2.468kg/m |
| Mass each | 14.808kg |
| Total length | 60.00m |
| Section area | 314.16mm² |
| In tonnes | 0.1481t |
| Length per tonne | 405.2m/t |
| Surface area | 3.77m² |
The 0.00617 factor for round bar is π/4 × 7.85 ÷ 1000, the same formula as rebar (Φ20 round bar is 2.468 kg/m). Square and flat use 0.00785. All three are the same steel; only the section differs, so a wrong answer usually means the wrong formula was used.
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Formula
| Round | W = 0.00617 × d² in kg/m |
| Square | W = 0.00785 × a² in kg/m |
| Flat | W = 0.00785 × a × b in kg/m |
| Total | per metre × length × quantity |
| Section area | πd²/4, a², a×b |
| Factor origin | 0.00617 = π/4 × 7.85 ÷ 1000 |
Φ20 round bar, 6 m each, ten lengths: 2.468 kg/m, 14.808 kg each, 148.08 kg total with a 314.16 mm² section. The same 6 m in 20 × 6 flat bar is only 0.942 kg/m — 2.6 times lighter.
How to work it out
- Pick the shape first, since the three sections use different formulas: round W = 0.00617 × d², square W = 0.00785 × a², flat W = 0.00785 × a × b.
- All dimensions in mm give kg/m directly. The 0.00617 for round bar is π/4 × 7.85 ÷ 1000.
- Multiply by length for one piece and by the count for the total.
- When ordering by piece, note the stock lengths (mostly 6 m, some 9 m) and the length tolerance of +50 mm.
- Surface treatment adds weight separately — hot-dip galvanising is about 3-5%.
Where it goes wrong
- One formula for all three shapes — at the same nominal size round and square differ by 21% (0.00617 against 0.00785), so the wrong formula is a fifth out.
- Giving flat bar only a width — it is a rectangle and needs both dimensions; treated as square it grossly overstates the weight.
- Using the outside diameter for deformed bar — the nominal diameter is an equivalent section diameter and the actual outside dimension is larger; nominal is what the weight is based on.
- Ignoring minus tolerance — hot-rolled bar is toleranced ±0.4 to ±1.0 mm, proportionally larger on small diameters.
Material and cost
Q235 round bar 4000-4800 CNY/t, 45# 4500-5500, square and flat 4100-4900, stainless 304 round bar 18000-26000.
On site
- Rebar uses d²/162, which is identical to 0.00617 d² — the two formulas cross-check each other.
- On flat bar, width is the long side and thickness the short side; drawings write -50×6 for 50 wide by 6 thick.
- Small diameters deviate proportionally more, so settle by weighbridge on accurate work.
Basis
- GB/T 702-2017 Hot-rolled steel bars — dimensions, shape, mass and tolerances
- GB/T 1499.1-2017 Steel for reinforced concrete, Part 1 (plain bars)
- GB/T 1499.2-2018 Steel for reinforced concrete, Part 2 (ribbed bars)
FAQ
Are round and ribbed bars interchangeable?
Not in structural members. Plain round bar has about 40% of the bond of ribbed bar, and codes require ribbed steel for load-bearing reinforcement. Round bar is for stirrups, secondary steel such as distribution and tie bars, and embedments. Substituting plain for ribbed in main steel multiplies anchorage length by over 1.5, and failing to do so leaves a latent defect.
Why is the formula the same for rebar and round bar?
Both are steel at 7.85 g/cm³, so mass per metre depends only on nominal area. Φ20 round and Φ20 ribbed both compute as 2.47 kg/m. Ribbed bar actually weighs 2-4% more because the ribs add steel not counted in the nominal section — which is exactly why rebar is settled on theoretical weight.
Does flat bar orientation matter?
The weight is identical but bending capacity differs by more than three times. A 20 × 6 flat stood on edge has a section modulus of 400 mm³ against 120 mm³ laid flat. For handrails, brackets and purlins the flat must stand on edge; laid flat for convenience it buckles underfoot.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Round, Square and Flat Bar Weight Calculator. 2026-10-25. https://jianzhujisuan.com/en/steel-bar-weight
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.