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Concrete Rebar Area

Solves the steel area from equilibrium rather than assuming a lever arm — it iterates the compression block depth a = A_sf_y/(0.85f′_cb) against the moment equation until they agree. Also reports ACI minimum steel, which governs shallow beams more often than strength does.

Design moment Mu (kN·m)

Beam width b (mm)

Effective depth d (mm)

Concrete f'c (MPa)

Steel fy (MPa)

Strength reduction φ

As required (strength)

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ACI minimum As

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Provide

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Governed by

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Compression block depth a

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ρ vs ρmax (ductility)

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Singly-reinforced rectangular sections in flexure only — no shear or torsion, no compression steel, no T-beam flange, no deflection or crack-width check, no development length or bar spacing. An over-reinforced section fails without warning and is not permitted.

PRELIMINARY SIZING ONLY — not a code-compliant design check. These are first-pass equations; they omit member stability, local buckling, shear and combined-stress interaction, connections, second-order effects and jurisdiction-specific factors. Codes vary by country and edition. Structural design must be performed and sealed by a qualified engineer.