Blinding concrete is a thin concrete layer placed beneath foundations, footings or other structural work to provide a clean, level working surface. Its quantity is calculated from the area covered and the specified thickness.
For a rectangular foundation area, first calculate the plan area, convert the blinding thickness to metres, then multiply the two values. The resulting volume gives the theoretical concrete requirement before any project-specific allowance.
Table of Contents
- What Is Blinding Concrete?
- Measurements Needed
- Blinding Concrete Quantity Formula
- Worked Example
- How Thickness Changes Quantity
- Calculating Blinding Beneath Footings
- Calculating Blinding for Strip Foundations
- Multiple Foundation Sections
- Irregular Foundation Areas
- Adding a Concrete Allowance
- Estimating Cement, Sand and Aggregate
- Common Calculation Mistakes
- Practical Measurement Workflow
- Related BuildersMeasures Tools and Guides
- Frequently Asked Questions
What Is Blinding Concrete?
Blinding concrete is a relatively thin concrete layer used below structural foundation work or other construction elements. It provides a cleaner and more uniform surface for setting out, reinforcement placement and subsequent construction.
Unlike structural concrete, blinding is often specified primarily for its working-surface function. Its required thickness, mix and application depend on the project drawings and construction specification.
Measurements Needed
Before calculating the concrete quantity, collect:
- Length of the blinding area
- Width of the blinding area
- Specified blinding thickness
- Number of separate foundation sections
- Any irregular areas or changes in width
- Specified material allowance, if applicable
Keep all dimensions in consistent units. When the area is in square metres, convert the concrete thickness to metres.
Blinding Concrete Quantity Formula
For a rectangular section:
Then calculate the concrete volume:
For several sections:
| Thickness | Equivalent metres |
|---|---|
| 50 mm | 0.05 m |
| 75 mm | 0.075 m |
| 100 mm | 0.10 m |
| 125 mm | 0.125 m |
| 150 mm | 0.15 m |
Worked Example
Suppose blinding concrete is required beneath a rectangular foundation area measuring 14 m × 9 m. The specified blinding thickness is 75 mm.
Step 1: Calculate the area
Area = 126 m²
Step 2: Convert thickness
Step 3: Calculate concrete volume
The theoretical blinding concrete quantity is 9.45 m³.
If the project requires an ordering allowance, calculate the allowance separately rather than changing the specified blinding thickness.
How Thickness Changes Quantity
Thickness has a direct relationship with concrete volume. A thicker blinding layer requires more concrete over the same area.
For a 126 m² foundation area:
These are calculation examples only. The actual thickness should come from the project specification.
Calculating Blinding Beneath Footings
Blinding beneath isolated footings is usually measured from the specified blinding footprint rather than from the structural footing volume.
For one rectangular footing blinding area:
Suppose one footing requires a blinding layer measuring 2.4 m × 2.0 m at 75 mm thick.
If there are 10 identical footing bases:
Where the blinding extends beyond the footing footprint, use the actual specified blinding dimensions instead of the footing dimensions.
Calculating Blinding for Strip Foundations
Strip foundations are often measured by total strip length and average or specified width.
A continuous foundation has a total blinding length of 48 m. The specified blinding width is 0.80 m and thickness is 75 mm.
The theoretical blinding quantity is 2.88 m³.
For a foundation with several strip widths, calculate each width separately and add the resulting volumes.
Multiple Foundation Sections
A foundation layout can contain isolated footings, strip foundations, ground beams and other areas requiring separate blinding.
Do not combine sections prematurely when their dimensions differ. A section-by-section takeoff makes the calculation easier to check.
| Section | Area | Thickness | Volume |
|---|---|---|---|
| Footing group | 18 m² | 0.075 m | 1.35 m³ |
| Strip foundation | 32 m² | 0.075 m | 2.40 m³ |
| Additional foundation area | 10 m² | 0.075 m | 0.75 m³ |
| Total | 60 m² | 0.075 m | 4.50 m³ |
Irregular Foundation Areas
Not every blinding area forms a perfect rectangle. Where the foundation layout is irregular, divide the blinding footprint into simpler shapes.
Rectangular sections
Triangular sections
Trapezoidal sections
Add the individual areas before multiplying by the specified thickness.
An irregular foundation blinding footprint contains:
- Rectangle: 40 m²
- Rectangle: 18 m²
- Triangle: 6 m²
At 75 mm thickness:
Adding a Concrete Allowance
The theoretical volume is based on the specified dimensions. Actual ordering often includes an allowance for measurement tolerances, uneven surfaces, minor losses and delivery or batching considerations.
If an allowance is specified, use:
For 9.45 m³ of blinding concrete with a 5% ordering allowance:
The estimated order quantity is approximately 9.92 m³.
Estimating Cement, Sand and Aggregate
Once the blinding concrete volume is known, cement, sand and coarse aggregate can be estimated when the concrete mix specification provides the required proportions or material consumption per cubic metre.
A concrete calculator can help with the material conversion stage. Use the BuildersMeasures Concrete Calculator after determining the required concrete volume.
For a nominal mix expressed as a ratio, the calculation needs to account for the dry-volume factor and the specified proportions. Do not assume one concrete mix ratio applies to every blinding application.
Common Calculation Mistakes
Using millimetres without conversion
When the area is in square metres, convert thickness from millimetres to metres before calculating volume.
Using footing volume instead of blinding volume
Blinding is a separate layer. Its quantity should be calculated from its own footprint and thickness.
Ignoring blinding overhangs
If the specified blinding extends beyond the footing or foundation, include the full blinding footprint in the area calculation.
Using one width for different foundation sections
Different sections might have different blinding widths. Measure each section according to the drawings.
Adding an allowance by changing the layer thickness
Keep the specified thickness and ordering allowance as separate calculation stages.
Assuming the concrete mix
The quantity of cement, sand and aggregate depends on the specified concrete mix and batching method.
Forgetting small foundation sections
Steps, isolated footings, foundation projections and changes in layout can affect the total quantity. Review the complete foundation plan before ordering.
Practical Measurement Workflow
A clean blinding concrete takeoff can follow this sequence:
- Identify every area receiving blinding concrete.
- Measure the length and width of regular sections.
- Divide irregular sections into simple shapes.
- Confirm the specified blinding thickness.
- Convert thickness into metres.
- Calculate the volume of each section.
- Add all section volumes together.
- Apply a specified ordering allowance if required.
- Use the final volume for concrete material or supply calculations.
- Check the result against the foundation drawings before placing the order.
Related BuildersMeasures Tools and Guides
Frequently Asked Questions
How do I calculate blinding concrete quantity?
Multiply the blinding area by the specified concrete thickness after converting the thickness into metres. For a rectangular area, use length × width × thickness.
What is the formula for blinding concrete?
The basic formula is blinding concrete volume = area × thickness. For a rectangular area, the calculation is length × width × thickness.
How thick is blinding concrete?
Blinding thickness varies by project. Use the thickness shown on the construction drawings or specified by the project requirements rather than applying one universal thickness.
Is blinding concrete structural concrete?
Blinding is commonly used as a working layer beneath structural foundation elements. Its purpose and specification differ from the structural concrete forming the footing, slab, beam or column.
How do I calculate blinding for strip foundations?
Multiply the total blinding length by the specified blinding width and thickness. If the strip widths differ, calculate each section separately and add the volumes.
Should I add waste to blinding concrete?
An ordering allowance can be added when appropriate, but the allowance should be calculated separately from the specified concrete thickness and based on project conditions.
Can I use the footing dimensions to calculate blinding concrete?
Only when the blinding footprint has the same dimensions as the footing. If the blinding extends beyond the footing, use the actual specified blinding dimensions.