Free calculator

Lintel load calculator

Work out the load on a lintel over a window or door. It takes the 45° load triangle when the wall above can arch, the full height when it cannot, and adds any floor or roof bearing on the wall above.

Example values: a 1.8 m opening in a cavity wall with a timber first floor 0.4 m above the lintel. Typical UK figures, so change them to suit your job.

Opening and wall

Clear opening

Wall weight

kN/m²

Wall above lintel

Lintel self-weight

kN/m

Floor bearing on the wall

Height is from the top of the lintel to the floor bearing.

Height above lintel

Loaded width

Dead, gk

kN/m²

Imposed, qk

kN/m²

Roof at the top of the wall

Set the loaded width to 0 if the roof does not bear on this wall.

Loaded width

Dead, gk

kN/m²

Imposed, qk

kN/m²

Result

Masonry on the lintel

3.43 kN

Total load, gk + qk

5.98 + 4.05 kN

Characteristic average UDL over 1.98 m

5.07 kN/m

ULS (1.35gk + 1.5qk) average UDL

7.15 kN/m

Characteristic reaction, each end

5.02 kN

ULS (1.35gk + 1.5qk) reaction, each end

7.08 kN

Arching used: there is at least the 60° triangle height of wall above. Confirm the arching conditions against BS 5977-1. Load triangle 0.99 m high, interaction zone up to 1.71 m, on a 1.98 m base.

The average UDL gives the right end reactions, but not the true moment or shear along the lintel, because the masonry part is triangular.

Elevation over the opening (to scale)

01Method

How it works

Bonded masonry over an opening arches, so a lintel only carries part of the wall above it. This calculator follows the method in BS 5977-1. Two triangles stand on a base of 1.1 × the clear opening, which allows for the bearings. The masonry inside the 45° load triangle sits on the lintel. The band between that and a 60° triangle is the interaction zone: its masonry is not carried, but loads applied in it count at half.

Base b = 1.1 × clear openingLoad triangle height = b / 2 (45°)Interaction zone top = (b / 2) × tan 60° = 0.866 bMasonry on lintel = wall weight × b × (b / 2) / 2 = wall weight × b² / 4Floor at height h in the load triangle: length carried = (b − 2h) + ½ × 2h × (1 − tan 30°)Floor at height h in the interaction zone: length carried = ½ × (b − 2h × tan 30°)Floor above the 60° triangle: nothingReaction at each end = total / 2; average UDL = total / b

The length carried is multiplied by the floor's line load (load per m² × loaded width). A floor in the load triangle is carried in full where it lies inside the triangle and at half where it runs on into the interaction zone at the same level.

Assumptions behind the numbers:

Arching is used only when you say the masonry can arch and the wall above the lintel is at least as tall as the 60° triangle. That is a minimum: confirm the height of masonry needed above the zone against BS 5977-1 or the lintel maker's guidance. Otherwise the full height of wall over the base, and every floor and roof load, go on the lintel.
Masonry can arch when it is in stretcher bond, has solid wall of enough length either side, and has no movement joint, corner or other opening close to the lintel.
Floor and roof loads are taken as uniform along the wall. A beam or post bearing on the wall above is a point load, which this calculator does not cover.
The roof bears at the top of the wall, at the height you enter for the wall above the lintel.
The average UDL is the total over the base. It gives the right end reactions, but not the true bending or shear along the lintel: the masonry part is triangular, and for the same total a triangular load gives a mid-span moment of WL/6 against WL/8 for a uniform one.
For a cavity wall, enter the weight of the leaves the lintel carries. To check the leaves apart, run it once per leaf and put the floor only on the leaf it bears on.
BS 5977-1 covers lintels up to 4.5 m in single-storey buildings and up to 3.6 m in two- and three-storey houses, with no opening inside the load triangle. The calculator warns when the opening is over 3.6 m.
In US mode the same geometry is used in feet and psf. The 1.1 × opening base and the 60° interaction zone are UK practice from BS 5977-1, not a US code method. Snow is not handled separately: enter the larger of Lr and S as the roof live load.

02Example

Worked example

A 1.8 m window in a cavity wall weighing 3.5 kN/m². There is 2.6 m of wall above the lintel, in stretcher bond with solid wall either side, so it can arch. A timber first floor bears 0.4 m above the lintel with a loaded width of 2.0 m, at 0.8 kN/m² dead and 1.5 kN/m² imposed. The roof bears at the top of the wall with a 3.0 m loaded width, at 0.9 kN/m² dead and 0.6 kN/m² imposed. Allow 0.2 kN/m for the lintel.

b = 1.1 × 1.8 = 1.98 m; load triangle 0.99 m high; 60° triangle 1.715 m high2.6 m of wall ≥ 1.715 m, so arching appliesMasonry = 3.5 × 1.98² / 4 = 3.430 kNFloor at 0.4 m: (1.98 − 0.8) + ½ × 0.8 × (1 − 0.577) = 1.180 + 0.169 = 1.349 mFloor Gk = 0.8 × 2.0 × 1.349 = 2.159 kN; Qk = 1.5 × 2.0 × 1.349 = 4.047 kNRoof at 2.6 m is above the 60° triangle: 0 kNLintel = 0.2 × 1.98 = 0.396 kNGk = 3.430 + 2.159 + 0.396 = 5.985 kN; Qk = 4.047 kNCharacteristic = 10.03 kN; ULS = 1.35 × 5.985 + 1.5 × 4.047 = 14.15 kNReactions: 5.02 kN characteristic, 7.08 kN ULS at each endAverage UDL over 1.98 m: 5.07 kN/m characteristic, 7.15 kN/m ULS

Switch the calculator to “No arching” and the same lintel carries the full 2.6 m of wall, the whole floor and the whole roof: 36.43 kN characteristic and 50.61 kN at ULS, or 25.30 kN on each bearing. Arching is worth having, but only where the masonry really can arch.

03Questions

FAQ

How do you calculate the load on a lintel?

Where the masonry above can arch, take the weight of masonry inside a 45° triangle standing on a base of 1.1 × the clear opening, and add the lintel's own weight. Add any floor or roof load applied inside that triangle in full, and half of any load in the interaction zone between the 45° and 60° triangles. Loads above the 60° triangle arch over the opening. Where the masonry cannot arch, the lintel carries the full height of wall and every load on it.

What is the 45 degree rule for lintels?

Bonded masonry over an opening arches, so the lintel only carries a triangle of it with 45° sides. Under BS 5977-1 the triangle's base is 1.1 × the clear opening, so it is 0.55 × the opening high. Over a 1.8 m opening the triangle is 0.99 m high and, in a 3.5 kN/m² wall, weighs 3.43 kN.

When can you not assume arching over a lintel?

When the masonry is in stack bond, when there is less wall above the lintel than the 60° triangle needs, when a movement joint, corner or other opening is close to the lintel, or when the piers either side are too short to resist the outward thrust. Then the lintel takes the full height of wall and every load above it.

Does a floor above a lintel count?

It depends where it bears. A floor bearing inside the 45° load triangle goes on the lintel in full. The part of it within the interaction zone, between the 45° and 60° triangles, counts at half. A floor bearing above the 60° triangle arches over and drops out. Without arching, all of it goes on the lintel.

How do I choose a lintel from the result?

Compare the load with the manufacturer's safe working load for the lintel type, length and wall build-up. Check whether their table is for a uniform load or for the triangular masonry load, which bends a lintel more than the same total spread evenly, and whether it covers the inner leaf, the outer leaf or both.

04Limits

Check the result

Check against the code and get a qualified engineer to sign off.

The load triangle is a simplified model with conditions attached. Check the opening against BS 5977-1 or the lintel maker's guidance, check the bearings at each end, and have a qualified engineer confirm the lintel choice.

For a steel beam in place of a lintel, use the steel beam load calculator, which takes the full wall height.

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