Shear stress describes the internal force that develops when a load tries to slide one layer of a material past an adjacent layer, rather than pulling or pushing straight through it. It's the governing calculation for bolts, rivets, pins, welds, and any other fastener or joint that carries load by resisting sliding rather than tension or compression.
Shear versus normal stress: direction is everything
The formula τ = F/A looks identical to normal stress, σ = F/A, and the arithmetic really is the same — but the physical setup is different. Normal stress comes from a force acting perpendicular to a cross-section, pulling it apart (tension) or pushing it together (compression). Shear stress comes from a force acting parallel to the cross-section, trying to slide one face past the other, the way scissor blades cut paper by sliding past each other rather than crushing it. A single component can experience both kinds of stress simultaneously depending on how the load is applied — see the companion Stress Calculator for the normal-stress case.
Why bolts and pins are sized by shear
Most bolted and pinned connections transfer load primarily through shear, not tension. A bolt clamping two plates together resists the plates sliding relative to each other along the bolt's cross-section — that's a shear load. Engineers size the bolt diameter (and therefore its cross-sectional area) so the resulting shear stress stays comfortably below the bolt material's allowable shear strength, which is itself typically a fraction of the material's tensile strength.
Single shear versus double shear
A joint is in single shear when the fastener has one shear plane — one place where the two connected pieces can slide relative to each other, as in a simple lap joint. A joint is in double shear when the fastener passes through three overlapping pieces (or a clevis-and-pin arrangement), creating two shear planes that share the load. Because two planes resist the same total force, the shear stress on each plane in a double-shear joint is about half what it would be in an equivalent single-shear joint — divide this calculator's single-shear result by 2, or halve the force before entering it, to model the double-shear case.
Limits of this simple model
This calculator assumes the parallel force is distributed uniformly across the shear plane — a reasonable first approximation for a bolt, pin, or rivet in direct shear. Real fasteners can see uneven stress distribution near edges and holes, combined loading (shear plus bending), or fatigue effects from repeated loading that a single static τ = F/A calculation doesn't capture. For safety-critical joints, cross-check this result against the fastener manufacturer's rated shear strength and apply an appropriate safety factor before finalizing a design.