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Lintel Sizing Calculator — What Size Lintel Do I Need?
Work out the minimum lintel length for your opening and get guidance on lintel type based on wall construction and load conditions.
Clear width of the door or window opening
150mm minimum end bearing for steel lintels (Catnic/IG); use 200mm for heavy loads or wide spans
Enter the price of the lintel for your records
Safety notice
Structural elements must be designed or checked by a structural engineer before construction. Treat these figures as initial guidance for planning and budgeting, not a structural design.
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How We Calculate This
This calculator determines the minimum lintel length by adding the required bearing length to each side of the opening width. It then provides guidance on suitable lintel types based on your wall construction and loading conditions.
The formula
Minimum lintel length = Opening width + (Bearing × 2)
For example, a 1200mm opening with 150mm bearing each side needs a lintel at least 1500mm long. We then recommend the nearest standard manufactured length.
Lintel type guidance
Our recommendations reference common UK lintel ranges from manufacturers such as Catnic and IG Lintels. The correct lintel depends on:
- Wall type (single leaf, cavity, or solid)
- Load condition (roof only, floor + roof, multi-storey)
- Opening width (wider = heavier duty lintel)
- Cavity width (for cavity wall lintels)
Important disclaimer
This calculator provides general guidance only. Lintel selection for structural openings must be confirmed by a structural engineer and approved by Building Control under the Building Regulations (Approved Document A). This is particularly important for load-bearing walls, wide openings, and multi-storey buildings.
Frequently Asked Questions
The standard minimum bearing for a steel lintel is 150mm each side of the opening. This means a 1200mm wide opening needs a lintel at least 1500mm long (1200 + 150 + 150). Some heavy-duty lintels or wider openings may require 200mm or more bearing. Always check the manufacturer's installation guide.
For a standard UK cavity wall, you need a combined cavity wall lintel that spans both the inner and outer leaf. For typical roof and light floor loads, a standard-duty lintel such as the Catnic CG90/100 or IG L1/S 100 is used. Where heavier floor loads bear above, step up to a heavy-duty Catnic CH90/100 or IG L1/HD 100. These come with an integral damp-proof tray. The lintel must suit your specific cavity width, typically 50mm, 75mm, or 100mm.
Yes, replacing or installing a lintel is structural work and requires Building Regulations approval. If you are creating a new opening in a load-bearing wall, you will also need a structural engineer's calculation to specify the correct lintel. Building Control must inspect before the opening is made permanent.
Standard domestic steel lintels (Catnic, IG Lintels) are available in lengths up to about 4800mm. However, the maximum clear span depends on the load and lintel duty. For openings wider than about 2.4m with significant loads, a structural engineer should specify the lintel size and type.
Yes, precast concrete lintels are still used, particularly for internal walls and smaller openings. They are cheaper but heavier and have lower load capacity than steel lintels of the same depth. For cavity walls, steel lintels with integral DPC trays are strongly preferred to prevent cold bridging and damp issues. If a steel beam is needed, use our Padstone Sizing Calculator to work out the padstone bearing area where the beam sits on the wall.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.