Concrete Slab Calculator

How to Calculate Concrete for a Slab

Concrete slabs are the foundation of patios, garage floors, shed bases, paths, and driveways. Getting the right volume is essential because running short mid-pour creates a cold joint, and over-ordering wastes money. This calculator gives you precise volumes, bag counts, and costs.

Recommended Slab Depth by Project

The depth of your slab determines its load-bearing capacity. Using too little concrete risks cracking under load. Using too much wastes money unnecessarily.

Concrete Volume for Common Slab Sizes

Quick reference for typical residential slabs including 10% waste allowance.

For any slab over 1 m3 (about 84 bags), ready-mix delivery is strongly recommended over premix bags.

Reinforcement Guide

Reinforcement mesh prevents cracking from shrinkage, thermal expansion, and minor ground movement. The right mesh depends on the slab's load.

Placement: Mesh should sit in the lower third of the slab. Use plastic mesh chairs (spacers) to hold it at the correct height. If the mesh rests on the ground, it provides almost no reinforcement.

Overlap: Overlap mesh sheets by at least one full grid square (200 mm) and tie with wire.

Slab Preparation Steps

Proper preparation is what separates a slab that lasts 30 years from one that cracks in 2.

1. Mark out the area — Use string lines and corner stakes. Check for square by measuring diagonals (they should be equal).
2. Excavate — Dig to slab depth + sub-base depth + 5 cm. For a 10 cm slab with a 10 cm sub-base, dig 25 cm total.
3. Install formwork — Use straight timber boards (50 x 150 mm or similar) held in place with stakes every 60-90 cm. The top of the formwork marks the finished concrete level. Use a spirit level and add a 1-2% fall away from buildings for drainage.
4. Lay and compact sub-base — Spread gravel or hardcore in 5-10 cm layers, compacting each layer with a plate compactor. A well-compacted sub-base prevents settlement cracking.
5. Add a DPM if needed — For garage floors and indoor slabs, lay a damp-proof membrane (DPM) over the sub-base with 15 cm overlaps at joints.
6. Place reinforcement mesh — Set mesh on spacer chairs. Cut to fit with bolt cutters, leaving 5 cm clearance from formwork edges.
7. Pour, level, and finish — Pour concrete, spread with a rake, screed level with a straight edge, then finish with a float or trowel.
8. Cure — Cover with polythene or spray with curing compound. Keep moist for at least 7 days.

Expansion Joints: Where and Why

Concrete shrinks as it cures and expands and contracts with temperature changes. Without planned joints, it will crack randomly.

Cut joints with an angle grinder and diamond blade within 6-18 hours of pouring (before random cracking starts), or form them with a jointing tool while the concrete is still workable.

Cost Estimate by Slab Type

Professional install costs include labour, formwork, sub-base preparation, reinforcement, and finishing. Ready-mix delivery is the clear winner for cost and speed on anything over 1 m3.