Concrete Slab Calculator: measuring notes and method
Measure the actual pour boundary
Sketch the slab as non-overlapping rectangles. Record length, width and specified thickness for each, and save separate project items where thickness changes. For a thickened edge, add only the depth below the slab already counted.
Before booking the delivery
Keep measured volume, allowance and supplier rounding separate. Ask the supplier about order increments and minimum delivery, and list reinforcement, formwork and pumping separately from the concrete quantity.
CHECK A PRACTICAL EXAMPLE
How much does a thickness change affect the order?
Compare a 6 × 4 m rectangle at two specified thicknesses, with 10% allowance in both estimates.
100 mm thick
- Material to order
- 2.64 m³
- Measured volume
- 2.4 m³
125 mm thick
- Material to order
- 3.3 m³
- Measured volume
- 3 m³
An extra 25 mm across this footprint adds 0.6 m³ before allowance. Confirm the specified thickness before booking a volume; the calculator does not select it.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Volume = length × width × depth × quantity. Order volume = volume × (1 + waste ÷ 100).
Product reference: QUIKRETE product yields
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Length
- 6 m
- Width
- 4 m
- Depth / thickness
- 100 mm
- Quantity
- 1
- Waste allowance
- 10 %
Material to order: 2.64 m³
Imperial worked example
- Length
- 19.685 ft
- Width
- 13.1234 ft
- Depth / thickness
- 3.937 in
- Quantity
- 1
- Waste allowance
- 10 %
Material to order: 3.453 yd³
Follow the calculation
Lengths in this working are in metres.
- Measured volume = 6 × 4 × 0.1 × 1 = 2.4 m³.
- Allowance multiplier = 1 + 10 ÷ 100 = 1.1.
- Volume with allowance = 2.4 × 1.1 m³.
Imperial concrete example
A 20 ft × 12 ft slab at 4 inches thick has volume 20 × 12 × (4 ÷ 12) = 80 ft³. Divide by 27 for about 2.963 yd³. With 10% extra, the estimate is about 3.259 yd³. Keep full precision until the final result.
Slab thickness, driveways and concrete strength
Use the thickness specified for your project. A 100 mm or 4 inch value is an arithmetic example, not a universal driveway specification. A 20 MPa or 30 MPa strength designation does not change length × width × thickness. Specify required strength, exposure requirements and other mix properties separately with your supplier.
Concrete bags versus ready-mix
At a mixed yield of 12 L per bag, 2.64 m³ requires 2,640 ÷ 12 = 220 bags. Use zero extra waste in the bag calculator when transferring this volume: the slab estimate already includes its allowance.
Unit conversions
1 m³ = 1,000 L ≈ 1.30795 yd³. One cubic yard is 27 ft³.
Reviewed 19 October 2026. How calculations are checked · Report a correction