12 × 10 ft slab, 5 inch thick, M20, 5% extra
1.49 m³ of concrete. About 12 bags of cement, 0.62 m³ of sand and 1.25 m³ of aggregate.
Concrete calculator
The dry volume is 1.54 times the wet volume. Cement is taken as 1,440 kg per m³ in 50 kg bags. Mix ratios are by volume: M15 is 1:2:4, M20 is 1:1.5:3, M25 is 1:1:2. Always check the structural drawing for the grade.
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Mix
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Concrete needed
1.49 m³
52.5 cubic feet. You need about 12 bags of cement, 0.62 m³ of sand and 1.25 m³ of aggregate.
- Cement
- 12 bags
- Sand
- 0.62 m³
- Aggregate
- 1.25 m³
Concrete quantity for a slab: the working
Concrete is sold by volume, but jobs are measured in feet and inches. The sum is short. Multiply length, width and thickness, change to cubic metres, and then split the volume by the mix ratio.
A 12 × 10 ft slab 5 inches thick holds 12 × 10 × (5 ÷ 12) = 50 cubic feet, which is 1.416 m³. Add 5% for spillage and you need 1.49 m³ of wet concrete, or 52.5 cubic feet.
To find the materials, multiply by 1.54 for the dry volume. For M20, 1 : 1.5 : 3, the parts total 5.5. Cement is 1 ÷ 5.5 of the dry volume. With cement at 1,440 kg per m³, that comes to about 12 bags of 50 kg. Sand is about 0.62 m³ and aggregate about 1.25 m³.
These are estimates for small jobs. Larger or structural work should follow the drawing and a tested mix design, often with ready-mix concrete. And do not forget the steel, formwork and curing.
Sources
What this calculator does not account for
- Reinforcement steel and formwork.
- Water content and admixtures.
- Structural design, which a qualified engineer must set.
How this is calculated
Formula: Volume and mix split
wet =L × W × thickness; dry =wet × 1.54; cement =dry × a ÷ (a + b + c)What the symbols mean
- 1.54Dry volume factor, since the dry materials shrink when mixed and set.
- a : b : cCement : sand : aggregate, for example 1 : 1.5 : 3 for M20.
Good to know
- Cement is taken at 1,440 kg per m³ and sold in 50 kg bags.
- Thickness is entered in inches and converted to feet.
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<iframe src="/embed/concrete-calculator" title="Concrete calculator" width="100%" height="720" style="border:0;max-width:42rem" loading="lazy"></iframe>Questions
What is M20 concrete?
A mix with a 28-day strength of 20 N/mm². The nominal ratio is 1 : 1.5 : 3 of cement, sand and aggregate.
Why is the dry volume 1.54 times the wet volume?
Dry cement, sand and aggregate have air gaps, and the volume shrinks when water is added and the mix compacts.
Which grade is used for a house slab?
M20 is the usual minimum for reinforced work. Your structural engineer's drawing decides.
Should I add extra for spillage?
Yes. The tool adds 5% by default.