Door Header Size Calculator

A door header is the horizontal structural member that carries loads across an opening and transfers them into the framing or supports on each side. Removing a section of a load-bearing wall interrupts the normal load path, so the header must resist bending and shear while keeping deflection within acceptable limits. This calculator provides a transparent preliminary estimate for a rectangular section. It is not a substitute for an engineered design or a prescriptive building-code table. The calculation treats the header as a simply supported beam under a uniformly distributed line load. The safety factor multiplies the entered service load to produce a design load. Maximum bending moment is the design load times span squared divided by eight, while maximum support shear is the design load times span divided by two. The estimated depth follows the rectangular section-modulus relationship using a representative allowable stress and width for the selected material. Depth is rounded upward to a practical twenty-five-millimeter increment. Real buildings are often more complicated than this model. Loads may come from roofs, floors, masonry, snow, wind, or concentrated posts rather than being uniformly distributed. The entered load must already be a line load in kilonewtons per meter; an area load cannot be copied into the field without first determining the tributary width. The header span should include the actual distance between bearings, which may differ from the nominal door width. Bearing length and support capacity also require checks. Wood, steel, and concrete behave differently. Wood design depends on species, grade, moisture, duration of load, built-up member fastening, and lateral restraint. Steel headers need an appropriate section shape plus checks for buckling, welding, corrosion, and fire protection. Reinforced concrete cannot be represented fully by an unreinforced rectangular stress model because reinforcement, cover, cracking, and detailing govern its behavior. The material choices here therefore provide only broad comparison values. Use the result to understand how span, load, and safety factor influence required depth or to prepare an early project conversation. Never purchase or install a structural header solely from this result. Confirm the load path, local code, member availability, deflection limit, bearing, connections, and construction sequence with a qualified engineer, architect, or building official. Temporary shoring may also be required while an existing load-bearing wall is altered.