Lintel Bearing Length Calculator

How the lintel bearing calculator works

A lintel is the beam over a door or window opening. Its ends rest on the masonry either side, and that overlap, the bearing, is what carries the load down into the wall. Too little and the masonry under the end is overstressed. So the first question for any lintel is how long it has to be: the clear width of the opening plus the minimum bearing at each end, L = W + 2b.

Enter the opening width. The calculator applies the minimum bearing for the unit system you pick, gives the minimum length, and then the length you would actually order, because lintels are made in fixed steps. It does not choose the lintel type or check its strength. That comes from the manufacturer's load tables or an engineer, and it matters more than the length.

Measuring the opening

Measure the clear structural opening from masonry to masonry at the level where the lintel will sit. It is not the size of the door or window frame, which fits inside the opening with a gap around it. If the reveals are not square, use the widest measurement.

Minimum bearing in England

Approved Document A, the structural guidance for England, says in paragraph 2C24 that lintel bearings should be 150 mm or more, and may be reduced to 100 mm where the clear span is 1200 mm or less. The metric side uses exactly that: 100 mm each end up to 1200 mm, 150 mm above. A 900 mm opening therefore needs at least 900 + 2 × 100 = 1100 mm of lintel.

In practice most lintels are ordered with 150 mm each end anyway. Catnic's lintel size guide works out lintel length by allowing 150 mm bearing at each end (its example is an 1800 mm lintel for a 1500 mm opening), and its product load tables are based on 150 mm end bearings. A lintel sat on 100 mm is shorter than the one the load figures describe.

Stock lengths

Steel lintels come in set lengths. On the product pages of that UK range, standard lengths rise in 150 mm steps up to 3000 mm and in 300 mm steps above, up to 4800 mm for many types (4575 mm is made, 4500 mm is not). The calculator adds 150 mm each end to the opening and rounds up to the next stock length: 1200 mm for a 900 mm opening, 2700 mm for 2400 mm, and 3600 mm for a 3200 mm opening, which leaves 200 mm at each end. Openings wider than 4500 mm fall outside that range and need a specialist or heavy-duty section.

Minimum bearing for US block walls

For concrete masonry in the US, the Concrete Masonry & Hardscapes Association's TEK 17-02A advises specifying a modular lintel length of at least the clear span plus 8 in, which gives at least 4 in of bearing at each end. The US side uses that 4 in minimum. Because block is laid in 8 in modules, it also rounds the length up to the next multiple of 8 in: a 36 in opening needs at least 44 in, which becomes a 48 in lintel with 6 in bearing each end. A 40 in opening needs exactly 48 in.

Steel angle lintels in brick veneer are sized by their own rules and span tables, and local codes may ask for more bearing than the minimum, so check both before ordering.

Worked examples

A 1.5 m patio door opening in England needs 150 mm each end, so at least 1800 mm of lintel, and 1800 mm is a stock length. A 1.2 m window can sit on 100 mm bearings, giving a 1400 mm minimum, but ordered for 150 mm bearings it becomes a 1500 mm lintel. In a US block wall, a 6 ft (72 in) opening needs at least 80 in, which is already a multiple of 8 in, while a 5 ft (60 in) opening needs 68 in and is ordered as 72 in, with 6 in bearing each end.

What the bearing length does not tell you

Length is the easy part. The lintel also has to suit the wall (single leaf, cavity or solid wall), carry the load above it (masonry, floors, roof and any point loads from beams) and deflect no more than the finishes allow. Point loads near the end of a lintel, long spans, and openings close to corners or other openings need an engineer, and sometimes a padstone under the bearing. The calculator deliberately stops at length.

Using this calculator with other BuildMetricLab tools

The brick and brick tie calculators cover the walling around the opening, and the brick arch calculator is the alternative for an arched head. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.