Blinding and Cushion Calculator
Plan size, concrete volume and side formwork for blinding.
Quick answer
With the default inputs: Footing base length 20 m · Footing base width 10 m · Projection per side 0.1 m etc.Total blinding concrete 21.02 m³
Edit the inputs below to recalculate.Inputs
Result
Total blinding concrete
21.02m³
| Blinding plan size | 20.20 × 10.20m |
| Plan area | 206.04m² |
| Concrete per pad | 20.604m³ |
| Net concrete | 20.60m³ |
| With waste | 21.02m³ |
| Perimeter | 60.80m |
| Side formwork | 0.00m² |
| Formwork per m³ | 0.00m²/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 length | L = footing + 2 × projection |
| Blinding width | B = footing + 2 × projection |
| Volume per pad | V = L × B × thickness |
| Total concrete | V = V × count × (1 + waste) |
| Perimeter | P = 2 × (L + B) |
| Side formwork | S = 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
- Blinding plan = (footing length + 2 × projection) × (footing width + 2 × projection). Projection is normally 100 mm, or 150-200 mm on soft ground.
- Blinding concrete = plan area × thickness × count × (1 + waste).
- Perimeter = 2 × (blinding length + blinding width).
- Formwork area = perimeter × thickness × count, but only where forms are actually needed — casting against the excavation gives zero.
- Blinding is normally C15 at 100 mm, or 150 mm on soft ground.
Where it goes wrong
- Measuring formwork regardless of method — where the excavation is neat and concrete is cast against it, there is no formwork, and claiming it weakens the estimate's credibility.
- Counting the projection on one side only — 100 mm each side adds 200 mm to the overall dimension.
- Specifying C20 or better — blinding carries no structural load, so C15 is enough and a higher grade is wasted money, though below C10 durability suffers.
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
- The blinding top is the footing soffit level, so set it with a level to ±10 mm or the footing depth varies.
- Let it take foot traffic before fixing steel — usually 24 h; if pressed, use walkboards but avoid dishing the surface.
- Around piles, leave out the pile positions rather than casting straight across.
Basis
- GB 50202-2018 Standard for acceptance of building foundation construction, Chapter 4 (ground and foundations)
- GB 50007-2011 Code for design of building foundations (blinding requirements)
- GB 50666-2011 Code for construction of concrete structures
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.
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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.