Concrete Calculator
Calculate concrete volume in cubic metres and the number of 40 kg bags needed for your slab, footing, or column. Includes waste factor.
Quick Calculator Get a fast estimate
m
m
m
e.g. 0.1m = 100mm slab
%
Concrete Volume (with waste)
2,2 m³
Net Volume
2 m³
40 kg Bags Required
147 bags
Area
20 m²
Depth
100 mm
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How to Use This Calculator
Enter the length, width, and depth of your concrete pour. Set your waste factor — 10% is standard, 15% for irregular shapes. The calculator shows volume in m³ and number of 40 kg premix bags required.
Concrete Formula
Volume (m³) = Length × Width × Depth
Volume with Waste = Volume × (1 + Waste% ÷ 100)
Bags (40 kg) = Volume ÷ 0.015 m³/bag
Each 40 kg premix bag yields approximately 0.015 m³. For pours over 1 m³, ready-mix concrete is more economical.
Need more detail?
Extended Calculator More options, charts, and scenario comparison
m
m
m
%
Concrete Volume (with waste)
2.64 m³
Base Volume
2.40 m³
40kg Bags Needed
177
Bagged Mix Cost
R 18 585
Slab Visualization
Material Cost Comparison
R
| Option | Quantity | Estimated Cost |
|---|---|---|
| 40 kg Premix Bags | 177 bags | R 18 585 |
| Ready-Mix Concrete | 2.64 m³ | R 6 336 |
| Saving with Ready-Mix | R 12 249 |
Ready-mix is generally more economical over 0.5 m³. Prices vary by region and supplier.
Example Calculation
Driveway Slab — 6m × 3m × 100mm
Base Volume = 6 × 3 × 0.1 = 1.8 m³
With 10% waste = 1.98 m³
40 kg Bags = 1.98 ÷ 0.015 = 132 bags
For this size, ordering 2 m³ ready-mix at ~R2,500/m³ is more economical.
Concrete Slab Thickness Guide
| Application | Minimum Thickness | Strength (MPa) |
|---|---|---|
| Pedestrian path | 75–100mm | 20 MPa |
| Driveway / carport | 100–150mm | 25 MPa |
| Residential floor slab | 150–200mm | 25 MPa |
| Structural floor slab | 200–250mm | 30 MPa |
| Heavy vehicle traffic | 250mm+ | 35–40 MPa |
Need full precision?
Professional Calculator Complete parameters, sensitivity analysis, and detailed breakdown
Concrete Volume & Mix
m³
Material Cost (Site Mix)
R 6 921
Cement (39 bags of 50kg)
R 4 485
Sand (2.56 m³)
R 897
Stone (3.85 m³)
R 1 538
Material Prices (Adjust for Your Area)
R
R
R
Reinforcement Estimator
m²
mm
mm
Rebar Required
213.08 kg (0.213 tons)
Estimated Cost
R 4 688
Delivery & Curing Schedule
m³
| Item | Value |
|---|---|
| Total Volume | 5.00 m³ |
| Truck Loads Required | 1 |
| Mix Strength | 25MPa |
| Minimum Curing Before Use | 7 days |
| Full 28-Day Strength | 28 days |
Curing: keep concrete moist for minimum 7 days. Do not apply loads before reaching minimum strength.
Frequently Asked Questions
For a pedestrian path: 75–100mm. Driveway or carport: 100–150mm. Structural floor slab: 150–200mm. Heavy vehicle traffic: 200–250mm. Always consult a structural engineer for load-bearing slabs.
Bagged premix is practical for small jobs under 0.5 m³. For anything larger, ready-mix concrete is more economical, faster, and gives a more consistent mix. SA ready-mix typically costs R2,000–R2,800/m³ depending on strength and location.
A standard house slab uses a 25 MPa mix (approximately 1:2:3 — cement:sand:stone). Structural engineers may specify 30–40 MPa for specific applications.
In 2025, ready-mix costs approximately R2,000–R3,000/m³ including delivery, depending on mix strength and location. Bagged premix (40 kg) costs R85–R120/bag. Bulk site-mix from cement, sand, and stone can be cheaper for large volumes.