Metric
- Convert thickness from centimetres to metres.
- Multiply length × width × thickness.
- Apply the selected waste allowance once.
Concrete calculator
Concrete slab calculator
Estimate the wet concrete volume for a rectangular slab and compare common premixed bag quantities. Enter the slab’s internal length, width, uniform thickness, and your chosen waste allowance in metric or US customary units.
Concrete estimator
Enter slab dimensions
Results update as soon as every value is valid.
Estimated requirement
Concrete needed
Approximately1.32cubic metres
Approximately 1.32 cubic metres of wet concrete, including 10% waste.
- Base slab volume
- 1.20 m³
- Waste added
- 0.12 m³
- Secondary equivalent
- 1.73 yd³
- Waste allowance
- 10%
Premixed concrete
Estimated bag quantities
Calculated counts show the mathematical quantity. “Buy” counts round up to whole bags and use nominal manufacturer-style yields.
Metric bags
Active units| Bag | Nominal yield | Calculated | Buy |
|---|---|---|---|
| 20 kg | 0.009 m³ | 146.7 bags | 147 whole bags |
| 25 kg | 0.011 m³ | 120.0 bags | 120 whole bags |
| 30 kg | 0.013 m³ | 101.5 bags | 102 whole bags |
| 40 kg | 0.018 m³ | 73.3 bags | 74 whole bags |
US customary bags
| Bag | Nominal yield | Calculated | Buy |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 155.4 bags | 156 whole bags |
| 60 lb | 0.45 ft³ | 103.6 bags | 104 whole bags |
| 80 lb | 0.60 ft³ | 77.7 bags | 78 whole bags |
Bag estimates include only the selected waste allowance. Check the yield printed on the actual product before buying.
Before using the estimate
Assumptions
- The slab is rectangular and has a uniform thickness.
- Entered dimensions are the internal dimensions used by the volume calculation.
- Waste is user-defined and is applied once to the base wet-concrete volume.
- Bag yields are nominal estimates; the yield printed by the manufacturer takes precedence.
- Actual requirements can vary with excavation, compaction, spillage, uneven subgrade, batching, and product-specific yield.
This calculator is a material estimate, not structural or project-specific professional advice. See the shared calculation methodology.
Transparent method
Formula and calculation method
The engine converts every dimension to metres and keeps the full precision in cubic metres internally. Display conversions and bag rounding happen only after the required volume is known.
US customary
- Convert feet and inches consistently to canonical units.
- Calculate cubic feet and convert the result to cubic yards.
- Apply the selected waste allowance once.
Base volume = length × width × thickness
Required volume = base volume × (1 + waste percentage ÷ 100)
Whole bags = required volume ÷ nominal bag yield, rounded up
Verified example
A 4 m × 3 m slab, 10 cm thick
For a 4 m long, 3 m wide slab with a 10 cm uniform thickness, the base volume is 1.20 m³. A 10% waste allowance adds 0.12 m³, giving approximately 1.32 m³ or 1.73 yd³ of wet concrete.
The values above are generated from the same tested calculation engine used by the calculator.
Common questions
Concrete slab calculator FAQ
How much waste allowance should I add?
Use an allowance appropriate for the job’s uncertainty. The calculator defaults to 10%, but you can enter 0–100%. Site conditions and ordering practices determine what is suitable.
Why are bag quantities rounded up?
Concrete is sold in whole bags. The calculated count remains visible, while the purchase count rounds upward so a fractional bag is not treated as purchasable.
Does the estimate include reinforcement or sub-base?
No. It estimates wet concrete volume and nominal premixed bag quantities only. Reinforcement, formwork, and sub-base materials are outside this calculator.
Can I use it for a non-rectangular slab?
This calculator covers one rectangular slab only. Divide a composite project into rectangles and assess whether that approach is appropriate before combining estimates.
Why might delivered concrete differ from this estimate?
Excavation tolerance, uneven subgrade, compaction, spillage, batching, and actual product yield can all change the amount used on site.
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