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Masonry · two leaves
How many bricks and blocks for a cavity wall?
Both leaves, the ties between them and the insulation in the cavity, from one elevation. It also checks you have left a clear cavity behind partial fill — the thing that fails inspection.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Each leaf = net area × units per m² × (1 + waste %)
Ties = net area × tie density, plus extras at every reveal
Residual cavity = cavity width − insulation thickness; under 50 mm it must be full fill
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
Rate
Default
Why
Bricks per m²
60 no.
Blocks per m²
10 no.
Wall ties
2.5 per m²
Plus 3 per metre of reveal at every opening.
Insulation board
1.08 m²
1200 × 900 cavity board.
DPC roll
30 m
Weep vents at
0.9 m
Over every cavity tray and at DPC level.
Minimum clear cavity
50 mm
Below this, partial fill is not permitted — go full fill.
A worked example
With the defaults the page opens on:
How long is the elevation? 10 metres
How tall is it? 5.2 metres
Doors and windows to take off? 6.5 square metres
What is the inner leaf? Block 100 mm
What is the outer leaf? Facing brick
How wide is the cavity? 100 millimetres
How thick is the insulation? 75 millimetres
Spare for breakages 5 per cent
Leaves — 45.5 m² each
Net area per leaf: 45.5 m² — L × H − openings
Inner leaf: 478 no.— block, +5% waste
Outer leaf: 2,867 no.— brick, +5% waste
Total units: 3,345 no.
Cavity
Residual cavity: 25 mm— less than the 50 mm minimum
Wall ties: 114 no. — 2.5 to the square metre, minimum length 215 mm
Insulation boards: 43 sheets— 75 mm, 1.08 m² a sheet
DPC / cavity tray: 1 rolls — at 30 m a roll
Weep vents: 12 no. — at 900 mm centres
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many wall ties per square metre?
2.5 per square metre at standard spacing, plus extras at every reveal. The calculator also gives the minimum tie length for your cavity width.
What is the minimum clear cavity behind partial-fill insulation?
50 mm. If the cavity minus the insulation is less than that, the page flags it - widen the cavity, thin the insulation or switch to full fill, because it will not pass inspection as drawn.
Do I count bricks for both leaves?
Yes: the inner leaf (usually block) and the outer leaf (usually facing brick) are counted separately from the same net area, each with its own waste, and the total is shown as the sum of the two rows.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
