Round, Square and Flat Bar Weight Calculator
Mass per metre and total for round, square and flat bar.
Inputs
Result
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.
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.
Related calculators
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.