Steel Lintel Calculator — Catnic/IG Selection by Opening Width

This calculator gives a first-pass steel-lintel selection for a masonry opening from the clear opening width, cavity size, leaf support and load type. It is a guide — confirm the final part against the manufacturer’s load/span table.

Selection process

• Opening width: the clear (structural) span the lintel must bridge. Lintel length = span + 2 × 150mm bearing (Catnic minimum).
• Cavity width: for cavity walls the cavity is encoded in the Catnic code — CG50/100 suits a 50–65mm cavity, CG90/100 a 90–105mm cavity, and so on (the “/100” is the 100–115mm inner leaf).
• Leaf support: both leaves uses the Catnic Cougar cavity range (CG standard, CH heavy, CX extra-heavy duty); a single 100mm internal leaf uses the Catnic CCS single-leaf channel lintel (the blockwork sits inside the channel). Thin internal walls have their own codes — CCS for a 100mm leaf and CN100 for 100/140mm internal walls. A true solid one-brick (200/215mm) external wall instead needs a solid-wall lintel from the Catnic CN range (e.g. CN71A or CN50C standard duty, both for external 200/215mm solid walls); internal solid/block walls use the lighter Catnic CN102 internal range — check the merchant for the exact wall make-up and duty.
• Load type: standard masonry, heavy (floor + roof) or point load — heavier loads escalate the duty class.

How the load is estimated

The applied masonry load uses the BS 5977-1:1981 load triangle — a 45° triangle with its base equal to 1.1 × the clear span (apex height ≈ 0.55 × span), giving a loaded area of about 0.3025 × span². The full triangle is carried only where the masonry above reaches the apex height; with less masonry above, the loaded area is capped to what is present.

Where a floor or roof also bears on the opening the masonry estimate is increased by a rough first-pass allowance — about ×1.5 for a heavy (floor + roof) load and ×1.8 for a concentrated point load (a beam or joist landing on the lintel). These multipliers are a conservative ready-reckoner, notthe BS 5977 load-ratio method (1:1 masonry / 3:1 timber floor / 5:1 concrete floor) and not a manufacturer figure. For any opening carrying a floor, roof or point load you must take the safe working load from the manufacturer’s load/span table, which is the authority for the safe load and the final lintel choice.