What Size Steel Beam Do I Need?

No size follows from the width of an opening, which is why nobody can answer this over the phone or off a website. The section is an output of a structural calculation done by an engineer who has stood in your house, and Building Control will want to see that calculation before the wall comes down.

So this page is what goes into it, what the engineer needs from you, and why a table of indicative sizes on a builder’s site would be worse than no page at all.

Why there is no table

A span table holds everything constant except the span, and in a house already standing, the thing above the wall is the variable.

Span tables work for a floor joist because a domestic floor is a standardised object: known loads, known spacing, repeated in every house on the street. An opening cut into an existing wall is the opposite. Two openings of identical width can differ by an order of magnitude in what they carry. One picks up a ceiling and half a metre of blockwork. The other picks up joists from both sides, a full storey of solid brickwork, a purlin off the roof, and a chimney stack outweighing everything else combined.

Which is also why an online beam calculator does not help. It asks you to enter a uniformly distributed load in kilonewtons per metre, and that figure is the actual question. Checking a section against a known load is arithmetic. Establishing the load is the work, and it happens with a tape, a torch and a lifted floorboard. Approved Document A gives sizing guidance for simple repetitive elements in small buildings and stops well short of this, so past that point somebody has to design it, sign it and insure it.

What the calculation needs

Half of it is measured on site. The other half is history only you and the deeds hold.

• The opening you want, plus a bearing at each end. The structural span is longer than the hole.
• What sits directly above. Ceiling only, a floor, a storey of masonry, a roof, a stair trimmer, a chimney breast and the stack over it.
• How much floor the wall collects. Joists deliver roughly half their span to the wall at each end, so a beam under a wall with floor bearing both sides carries a strip of house far wider than the wall. This is the step that gets skipped.
• What the wall above is built of, and how far up it goes. Solid brick, blockwork or timber stud, stopping at the ceiling or continuing to the roof.
• What is under each end. Engineering brick in cement mortar behaves nothing like soft stock in lime mortar, or a partition standing on a suspended timber floor.
• The finish above and either side, because brittle plaster and masonry tighten the limit the beam is designed to.
• The headroom you can give it, and whether the steel is hidden, visible, or lifted inside the floor above.
• How a beam physically reaches the room. A long section will not turn in a terraced hallway, and that occasionally changes the design rather than the delivery.

Eight inputs, and the first is the only one most people ring up with.

The checks the size has to survive

Five on the beam, and one on what it stands on.

The section is checked for bending, for shear, for local crushing of the web where the load arrives, and for the stability of its top flange wherever nothing holds it straight. Then for deflection, meaning how far it sags rather than whether it breaks, and on a domestic opening that is usually the check that settles the size. Why sag wins, and why asking for a slimmer steel gets you nowhere, is on the structural openings page.

Separately, the bearings are designed: a bearing length at each end, a padstone to spread that concentrated load, and a check on the stress in the masonry underneath. The last is not part of the beam size at all, and it is the one that comes back asking for a pier or a new pad foundation. It moves your quotation far more than the section does, which is where the money on a beam job actually goes.

What changes the answer, and the one thing that cannot

Span is the strongest lever you hold. Bending rises with the square of the span and sag rises faster still, so leaving a nib of wall in the right place, or dropping a column into a very wide opening, shrinks the beam and eases both bearings at once. Ask before the layout is fixed. A room planned around a nib is often better than the same space with a very deep steel across the ceiling.

Shape moves as well as size. A squatter, wider section does the same work in less depth where headroom is short, and two lighter members bolted together, an engineered timber, or a steel plate sandwiched between timbers can each be right where the beam has to be carried in by hand.

Reading the answer when it comes back

A beam arrives as a designation rather than a description. The first two figures are the nominal depth and width of the section in millimetres, the third is its mass in kilogrammes per metre, and the letters name the family: tall narrow sections normally used as beams, squatter ones normally used as columns but pressed into service where depth is tight, and channels with one flat face, often used in pairs.

The size is the least of what that drawing carries, though. The schedule also gives the bearing length, the padstone specification, restraint straps replacing the stability the wall was providing, how the joists meet the beam, the fire protection, and any pier or pad needed under a bearing. A note that something is to be confirmed on site is normal rather than evasive. Nobody can see through plaster either.

The practical answer

Ask a different question. Rather than what size beam you need, ask what is standing on this wall and where that load goes once the wall is not there. That is what an engineer answers, and the size falls out of the end of it.

Have the design done before the price is fixed, give the engineer access to the room above and into the loft, and expect the answer to arrive with a bearing length and a padstone attached rather than as a single number. Where the line falls on needing an engineer at all is a separate and narrower question.

We are builders and we do not size beams ourselves. We survey the wall, commission the design from the structural engineer we work with, and build to exactly what comes back, with the calculations checked on a full plans application before anybody props anything and the bearings looked at on the visit before it all goes behind plasterboard. Whether the wall can come out at all is dealt with separately. The survey and the quotation cost nothing.

Beam sizing is specific to a building and is a matter for the structural engineer and building control on your own project. Treat this as an explanation of how the number is arrived at rather than as a specification for your house.

Last reviewed August 2026.