Timber Beam Span & Bearer Sizing Calculator

A simply supported timber beam is checked for two independent limits. Strength is governed by the bending moment wL²/8 divided by the section modulus bd²/6, compared against the characteristic bending stress for the F-grade. Serviceability is governed by mid-span deflection, limited to span over 300 for floors. Because deflection scales with the cube of section depth, increasing depth is far more effective than increasing width. Worked example: a 3 kPa floor load over a 1.2 m tributary width is 3.6 kN/m. On a 4 m span the maximum moment is 3.6 × 4² ÷ 8 = 7.2 kN·m. A 90 × 240 mm section has a section modulus of 90 × 240² ÷ 6 = 864,000 mm³, so the bending stress is 7.2 × 10⁶ ÷ 864,000 = about 8.3 MPa, which is then compared with the design strength of the timber grade after the modification factors. Deflection against the span ÷ 300 limit often governs, so check both. Use AS 1684 span tables or an engineer for structural members.