算BuildCalc

Blinding and Cushion Calculator

Plan size, concrete volume and side formwork for blinding.

Quick answer

With the default inputs: Footing base length 20 · Footing base width 10 · Projection per side 0.1 etc.Total blinding concrete 21.02 m³

Edit the inputs below to recalculate.
Length LDepth HWidth W
Length and width are the footing base; blinding projects 100 mm each side and its thickness is the pour depth.

Inputs

Usually 100 mm, 150-200 in poor ground
100 mm typical, 150 on soft ground or large footings
pcs
%

Result

Total blinding concrete
21.02m³
Blinding plan size20.20 × 10.20m
Plan area206.04m²
Concrete per pad20.604m³
Net concrete20.60m³
With waste21.02m³
Perimeter60.80m
Side formwork0.00m²
Formwork per m³0.00m²/m³

Blinding plan size is the footing base plus twice the projection; code projection is normally 100 mm, giving rebar cover and shutter room. Whether formwork is measured depends on method: cast against undisturbed soil (earth mould) and the sides are formed by the ground, needing no formwork — only concrete. Cast on fill, soft ground or against a brick template and all four sides must be shuttered. Most sites use the earth mould and omit formwork from the budget, though real formwork can be claimed as a variation. Blinding is plain concrete, C15 or C20, with no reinforcement.

Formula

Blinding lengthL = footing + 2 × projection
Blinding widthB = footing + 2 × projection
Volume per padV = L × B × thickness
Total concreteV = V × count × (1 + waste)
PerimeterP = 2 × (L + B)
Side formworkS = P × thickness × count
A 20 by 10 m footing with 100 mm projection and 100 mm blinding, one pad, earth mould: blinding plan 20.20 by 10.20 m, 206.04 m², 20.60 m³ net, 21.02 m³ with waste, and 6.08 m² of formwork if shuttered on all sides.

How to work it out

  1. Blinding plan = (footing length + 2 × projection) × (footing width + 2 × projection). Projection is normally 100 mm, or 150-200 mm on soft ground.
  2. Blinding concrete = plan area × thickness × count × (1 + waste).
  3. Perimeter = 2 × (blinding length + blinding width).
  4. Formwork area = perimeter × thickness × count, but only where forms are actually needed — casting against the excavation gives zero.
  5. Blinding is normally C15 at 100 mm, or 150 mm on soft ground.

Where it goes wrong

Material and cost

C15 blinding runs 350-420 per m³ and is cheaper placed by crane skip or chute than pumped. Brick earth forms 80-120 per m², concrete formwork 40-55 per m².

On site

Basis

FAQ

Why project the blinding 100 mm?
Two reasons. Cover: bottom bars need concrete around them and room to tie, so the wider blinding keeps the mesh off the soil and makes fixing practical. Formwork: the footing shutters stand on the blinding and need space outside for braces and tie bolts, which 100 mm provides. In poor ground or with groundwater, use 150-200 mm so sloughing cannot eat into the blinding.
Should blinding formwork be priced at all?
It depends on the method, not on a rule. Bills of quantities allow blinding formwork as a provisional item, but if the blinding is cast directly on the trimmed excavation (earth mould) there was no formwork, so claiming it is over-measurement that audit will cut. Judge from ground conditions and excavation: firm ground that trims cleanly suits an earth mould; fill, sand, or a base above existing level needs shuttering. Price it as none, state the method in the construction plan, and claim by variation if formwork was actually erected.
How thick and how strong?
Normally 100 mm of C15 plain concrete. Thicken it to 150 mm for large footings with high soil reaction, or on soft and muddy ground — where it also needs a φ6 at 200 mm mesh and stops being plain concrete. With groundwater the blinding doubles as a waterproofing substrate, so it must be flat and thick enough to survive site traffic. Never go below C15; weaker blinding crumbles underfoot.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Blinding and Cushion Calculator. 2026-10-25. https://jianzhujisuan.com/en/cushion-layer

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.