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Calculator & tools

Modulus of rupture calculator

Estimate the breaking load of a span from the species' published modulus of rupture.

The span

Modulus of rupture 12,400 psi

Net 1.5" × 5.5"

The single biggest factor in the answer — depth is squared.

in

4 ft

Result

Working load (÷ 2.5)

853lb

Breaking load reduced by the safety factor below

Breaking load

2,131lb

Centre-point load at failure, clear specimen

Bending moment

25,575in·lb

Section modulus

2.063in³

Depth in load

1.5in

The dimension that gets squared

Common practice for sawn lumber in bending is around 2.5.

A beam resting on two supports with a single load applied at the centre. P is the load, L the span between supports, b the breadth across the load and d the depth in the direction of the load.PLdb
Simply supported beam, load at mid-span.

Formula. P = (MOR × b × d²) ÷ (1.5 × L), where d is the depth in the direction of the load, b the breadth across it and L the clear span — the standard centre-point bending relation solved for load.

This is an estimate, not a design value. Modulus of rupture figures are published for small, clear, defect-free specimens at a stated moisture content. A real board has knots, slope of grain, checks and whatever moisture it picked up on the way to you, and it may be loaded repeatedly rather than once. Use this to compare species and sizes, not to decide what will hold a load. Anything structural needs a licensed engineer and the applicable building code.