Concrete Slab Calculator
Calculate concrete volume and cement, sand, and gravel quantities for any rectangular slab.
Estimates for planning only — verify quantities against local codes and a qualified structural engineer before ordering materials.
How it works
- 1Enter the slab length, width, and thickness in your preferred unit system.
- 2Set a waste allowance percentage and choose your cement:sand:gravel mix ratio.
- 3The calculator finds the wet concrete volume and breaks it into cement bags, sand, and gravel using a 1.54 factor.
Use cases
- Estimating materials for a residential patio or driveway slab.
- Sizing a garden shed or workshop floor pour.
- Verifying a contractor’s material quote before ordering concrete.
Frequently asked questions
How is the concrete volume calculated?
Volume = Length × Width × Thickness × (1 + waste ÷ 100). For a 5 m × 4 m slab at 100 mm thick with 5% waste: 5 × 4 × 0.1 × 1.05 = 2.1 m³.
What does the mix ratio mean?
A 1:2:4 ratio means 1 part cement, 2 parts sand, and 4 parts gravel by volume. A richer 1:1.5:3 mix gives higher strength and is used for structural elements.
Why is typical slab thickness 100 mm (4 in)?
A 100 mm (4 in) slab is the standard minimum for light foot traffic and residential floors. Driveways carrying vehicles typically use 150 mm (6 in), and structural slabs may need 200 mm or more.
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