Practical guide
Slab Diagonal and Sub-Base Quantity Checks
For a rectangular slab, calculate the target diagonal from its side lengths. Estimate sub-base material separately from concrete, using its specified compacted depth and footprint.
Updated 16 October 2026
Reviewed by the 99Tools team. Editorial and corrections policy.
How to use the Concrete Slab Calculator
Add each distinct section and count only identical sections together. Select metres, millimetres, feet or inches as appropriate. For a triangle, the second plan dimension is the perpendicular height to the entered base, not a sloping side. Known area helps with irregular footprints already measured elsewhere. Depth must come from the project specification.
Worked slab volume example
A 3 m × 4 m solid section at an illustrative 100 mm thickness contains 1.2 m³. A 5% allowance gives 1.26 m³ before any supplier increment. At an example order increment of 0.1 m³, the suggested order is 1.3 m³. The rectangle target diagonal is 5 m. These dimensions and allowances demonstrate arithmetic, not a house slab design.
How to square a concrete slab
For a rectangle, target diagonal = √(length² + width²). Check both diagonals, intended side lengths and opposite sides together. Equal diagonals alone do not prove an arbitrary four-sided outline is the intended rectangle. Circular, triangular and known-area sections do not have a rectangular set-out diagonal in the result.
Separate sub-base quantities
Optional sub-base volume uses its own footprint deductions and specified compacted depth. The entered loose-to-compacted factor starts at 1, meaning no conversion. Concrete-volume deductions do not automatically remove sub-base area. Confirm supplier order units and material conditions rather than assuming one compaction factor or tonnes conversion applies everywhere.
Thickness, reinforced slabs and mesh
The thickness input is for quantity arithmetic, not a recommendation. A house floor, suspended slab, roof or heavily loaded surface can need a different design. Rebar, wire mesh, laps, cover, chairs, joints and support details must follow the specification. The calculator accepts a reinforcement quote but does not select steel size, spacing, sheet count or strength.
Count separate layers and openings once
Concrete thickness and sub-base depth describe different materials. Keep the measured footprint and each set of deductions separate. A concrete recess or service detail does not automatically remove material from every layer below it. Record the specified depth and supplier ordering units for each material, and do not turn compacted volume into loose delivery or tonnes without a confirmed conversion.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
Frequently asked questions
How do I calculate a concrete slab?
Multiply each section’s plan area by the specified thickness in metres, combine the measured volumes and adjust only for uncounted thickenings, deductions and one allowance.
How much does a concrete slab cost in South Africa?
Use current quotes for the actual design, site and scope. The budget option totals ordered concrete and entered whole-job allowances; it does not provide a live national price per m².
How thick should a slab be for a house?
The required thickness depends on the design, loading, ground or support system and other project conditions. Enter the specified value; this calculator does not determine a suitable thickness.
Can I use square feet or inches?
Yes. Select the plan and depth units separately. A size such as 10 × 12 needs a stated unit, and a 2-inch input is a measurement option rather than a thickness recommendation.
How do I calculate a triangular slab?
Use triangle base × perpendicular height ÷ 2 for plan area, then multiply by the specified thickness. Do not use a sloping side as the perpendicular height.
Does it calculate reinforced or hollow-core slab design?
No. It estimates entered solid-pour quantities and quoted costs. Reinforcement and precast systems require their own design and supplier information.
How long does a concrete slab take to dry?
It varies with the concrete, thickness, environment, curing and finish requirements. Curing, drying and strength gain are different; use the specified assessment and moisture limits rather than one universal waiting period.