Slab Concrete Calculator
Estimates the concrete volume for a slab and splits it into cement bags, sand and coarse aggregate for your chosen mix ratio.
Recent
Formula
How the calculation works
Enter the slab length and width in feet and the thickness in inches, which for a residential roof is usually four to six inches.
Choose the mix ratio, where 1:2:4 is the common residential grade and 1:1.5:3 is stronger for a heavier load.
The tool multiplies length, width and thickness in feet for the volume and applies the cement, sand and aggregate factors for the chosen mix.
Common mistakes
- Using a wet volume for the dry materials, since the sand and aggregate factors already allow for the shrinkage that happens as concrete sets.
- Forgetting the wastage and uneven surface allowance, because a five percent margin covers spillage and a slightly thick patch.
When to use it
- Use it before ordering materials for a roof slab, so the cement, sand and stone arrive in the right proportion and the pour is not delayed.
- Use it to compare mix ratios, since a stronger mix needs more cement and less cheap aggregate for the same slab.
Worked example
A slab 20 feet by 15 feet and 5 inches thick has a volume of 125 cubic feet, which for a 1:2:4 mix needs about 69 bags of cement, 188 cubic feet of sand and 375 cubic feet of coarse aggregate.
Common questions
Does this include the steel reinforcement?
No, it covers only concrete, so calculate the steel separately from the bar bending schedule, since reinforcement is a distinct material and cost.
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