Calculate your result
Quick answer
Calculate each section’s plan area and multiply by its specified thickness in metres. Add only concrete not already counted, subtract measured volume deductions, then apply one ordering allowance. Convert the result to ready-mix orders, bags using confirmed finished yield, or a budget using current quotes. The tool does not choose slab thickness or reinforcement.
How it works
Use this Concrete Slab Calculator for South Africa to estimate a specified solid pour, premix bags or an itemised budget from your quotes. Add rectangular, triangular, circular or known-area sections with their actual thickness. Measure extra thickenings separately and use the supplier’s finished yield when estimating bagged concrete.
Formula
Rectangle volume = length × width × thickness. Triangle volume = base × perpendicular height ÷ 2 × thickness. Circle volume = π × diameter² ÷ 4 × thickness. Sum sections, add only uncounted thickenings, subtract measured deductions and apply allowance once.
Assumptions and common mistakes
Check units, input values and the method before relying on an estimate. Review the existing tool notes and official sources below.
Formula
Rectangle volume = length × width × thickness. Triangle volume = base × perpendicular height ÷ 2 × thickness. Circle volume = π × diameter² ÷ 4 × thickness. Sum sections, add only uncounted thickenings, subtract measured deductions and apply allowance once.
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.
Ready-mix concrete slab calculator and bags
Ready-mix mode uses ordered m³ and your quoted supply rate. Minimum load count is a capacity division, not a delivery schedule. Bag mode requires the selected complete premix’s finished mixed litres per bag; bag mass is not yield. Site-mix mode scales a confirmed recipe and does not infer strength, water dosage or a universal mix for a slab.
Concrete slab cost calculator South Africa
Choose the slab budget option and enter concrete price per final ordered m³ plus separate whole-job quotes for reinforcement, formwork, sub-base/preparation, membranes, labour, delivery/pumping and other work. Use a consistent VAT basis; no extra VAT is added. Contingency applies to the entered subtotal. Zero fields are unpriced items. Avoid counting labour or supply charges already included in another quote.
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.
Precast, hollow-core and roof slabs
Do not price a precast or hollow-core system as a solid block of concrete using its gross depth. Obtain the specified system, layout and supplier quote. Only separately specified in-situ toppings, grout or other pours belong in this solid-volume estimate. Roof slabs also need falls, drainage, support and compatible waterproofing details beyond a cubic-metre count.
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.
Pouring, curing, drying and repairs
Follow the specified preparation, placement, compaction, finishing, curing and loading plan. Curing manages moisture and temperature for strength development; drying describes moisture loss. A fixed age is not proof that a slab is dry enough for every finish or ready for structural loading. Cracks, leaks and waterproofing need assessment and a suitable repair system, not a blanket sealant recipe.
Area, depth and concrete volume examples
| Illustrative area | Illustrative depth | Measured volume, no allowance |
|---|---|---|
| 20 m² | 75 mm | 1.5 m³ |
| 20 m² | 100 mm | 2 m³ |
| 20 m² | 150 mm | 3 m³ |
| 35 m² | 120 mm | 4.2 m³ |
These are arithmetic examples without allowance. The example depths are not design recommendations.
Last reviewed: 16 October 2026. Reviewed by the 99tools team. Editorial and corrections policy.
Quantity planning only. Confirm product suitability, concrete specification, reinforcement and site requirements for the actual project.
Related construction calculators
- Concrete Calculator South Africa
- Foundation Calculator South Africa
- Cubic Metre Calculator South Africa
- Premix Concrete Calculator South Africa
- Concrete Bags Calculator South Africa
Concrete guides
- How to Calculate Concrete Volume in South Africa
- How Many Concrete Premix Bags Do I Need?
- Concrete Mix Quantities: Cement, Sand and Stone
- Ready-Mix Concrete Order Guide South Africa
- How Much Concrete for Fence Posts?
- Slab Diagonal and Sub-Base Quantity Checks
- Compare Concrete Calculators: PPC, AfriSam and BUCO
- Foundation Quantities: Strip, Pad and Raft
- How to Calculate Foundation Cost in South Africa
- How to Square a Building Foundation
- Concrete Slab Cost in South Africa
- Solid, Precast and Reinforced Concrete Slabs
- Concrete Slab Curing, Drying and Repairs
- 40 kg Premix Concrete: Yield and Coverage
- Premix Concrete Prices and Product Checks
- Cubic Metres, Litres and Area Conversions
- Cubic Metres to Tonnes and Wheelbarrows
- Concrete Bags per m³ and Fence Post Volumes
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.
Related calculators
Concrete Calculator South Africa
Use this concrete calculator for South Africa to estimate cubic metres, litres or cubic yards for slabs, foundations and post holes. Add separate sections.
Foundation Calculator South Africa
Calculate foundation concrete in South Africa from measured strips, pads or raft sections. Add your quoted costs and check a rectangular set-out diagonal.
Premix Concrete Calculator South Africa
Calculate premix concrete bags from project dimensions and confirmed finished yield. Check packs, coverage at your depth and material costs from your quote.
Cubic Metre Calculator South Africa
Calculate m³ from dimensions or area and depth. Convert litres, cm³ and cubic feet, then check coverage, wheelbarrows, mass or cost using your own inputs.
Helpful guides
How to Calculate Concrete Volume in South Africa
Calculate concrete volume by converting every dimension to metres and multiplying length × width × depth. Keep measured volume separate from allowance and.
Ready-Mix Concrete Order Guide South Africa
Calculate the required finished volume, confirm the specified concrete and obtain a complete South African supplier quote. A volume result alone is not a.
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.