Quantifying the relationship between crack tip constraint and fracture toughness is key to improving the structural integrity assessment of critical infrastructure. The role of in-plane and out-of-plane constraint on fracture toughness requires greater understanding, particularly for low strength materials where test data are limited. In this work a structural steel, grade S275, is assessed both experimentally and via computational methods to investigate the role of constraint on fracture toughness. Single edge notch bend (SENB) samples, tested under three point bend conditions, at temperatures within the ductile-to-brittle transition range (+20 °C to −70 °C) are presented. Both standard (B=W, B=0.5W) and non-standard (B=0.75W, B=0.25W) bend test geometries are assessed. Here it is shown that the in-plane and out-of-plane constraint parameters are interdependent. Out-of-plane (thickness) constraint trends which are generally accepted to occur under high in-plane conditions are not present when the in-plane constraint is reduced. The amount of ductile tearing incurred during fracture toughness testing is typically greater when both in-plane and out-of-plane constraint is reduced. Thin samples are shown to be more ductile than thicker samples for equivalent in-plane conditions. These findings are relevant to pressurised components operating at low temperatures. •DBT regions are affected differently by in-plane and out-of-plane constraint.•Out-of-plane constraint has significant impact on the dominant failure mechanism.•The failure mechanism for shallow notched bend tests is not thickness dependent.