# Concrete Slab Calculator

> Calculate concrete for slabs, patios, garage floors, and paths. Get volume, bag count, reinforcement needs, and cost estimates for any slab size.

Canonical: https://renoquant.com/concrete-slab-calculator
Prices updated: 2026-03

## 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.

| Project Type | Recommended Depth | Sub-Base Depth | Reinforcement |
|-------------|------------------|----------------|---------------|
| Garden path | 5-8 cm | 5-10 cm gravel | Optional fibre mesh |
| Stepping stones | 5 cm | 5 cm sand | None |
| Patio (foot traffic only) | 10 cm | 10 cm gravel | Wire mesh (A142) |
| Shed base | 10 cm | 10-15 cm hardcore | Wire mesh (A142) |
| Garage floor (cars) | 12-15 cm | 15-20 cm hardcore | Wire mesh (A193) |
| Driveway | 12-15 cm | 15-20 cm hardcore | Wire mesh (A193) + edge rebar |
| Structural foundation | 20-30 cm | As per engineer spec | Rebar cage |

### Concrete Volume for Common Slab Sizes

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

| Slab Dimensions | 8 cm Depth | 10 cm Depth | 12 cm Depth | 15 cm Depth |
|----------------|-----------|------------|------------|------------|
| 2m x 2m (small path pad) | 0.35 m3 | 0.44 m3 | 0.53 m3 | 0.66 m3 |
| 3m x 3m (small patio) | 0.79 m3 | 0.99 m3 | 1.19 m3 | 1.49 m3 |
| 4m x 3m (standard patio) | 1.06 m3 | 1.32 m3 | 1.58 m3 | 1.98 m3 |
| 5m x 4m (large patio) | 1.76 m3 | 2.20 m3 | 2.64 m3 | 3.30 m3 |
| 6m x 3m (single garage) | 1.58 m3 | 1.98 m3 | 2.38 m3 | 2.97 m3 |
| 6m x 6m (double garage) | 3.17 m3 | 3.96 m3 | 4.75 m3 | 5.94 m3 |
| 10m x 3m (driveway) | 2.64 m3 | 3.30 m3 | 3.96 m3 | 4.95 m3 |

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.

| Mesh Type | Wire Diameter | Grid Size | Weight | Best For |
|-----------|--------------|-----------|--------|----------|
| A142 | 6 mm | 200 x 200 mm | 2.22 kg/m2 | Patios, paths, shed bases |
| A193 | 7 mm | 200 x 200 mm | 3.02 kg/m2 | Garage floors, driveways |
| A252 | 8 mm | 200 x 200 mm | 3.95 kg/m2 | Heavy-duty floors, foundations |
| Fibre mesh (polypropylene) | N/A | Mixed into concrete | 0.9 kg/m3 | Paths, light-duty slabs |

**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.

| Slab Width | Joint Spacing | Joint Depth |
|-----------|--------------|-------------|
| Up to 3m | No intermediate joints needed | N/A |
| 3-6m | One joint at centre | 1/4 of slab depth |
| 6-12m | Joints every 3m | 1/4 of slab depth |
| Against walls or structures | Isolation joint at junction | Full slab depth |

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

| Slab Type | Size | Volume (with waste) | Premix Cost (DIY) | Ready-Mix Cost | Professional Install |
|-----------|------|--------------------|--------------------|----------------|---------------------|
| Garden path | 5m x 1m x 8cm | 0.44 m3 | EUR 185-260 | EUR 120-170 | EUR 350-550 |
| Patio | 4m x 3m x 10cm | 1.32 m3 | EUR 555-770 | EUR 160-220 | EUR 600-950 |
| Shed base | 3m x 2.5m x 10cm | 0.83 m3 | EUR 350-485 | EUR 120-160 | EUR 450-700 |
| Single garage | 6m x 3m x 15cm | 2.97 m3 | EUR 1250-1735 | EUR 360-500 | EUR 1100-1800 |
| Driveway | 10m x 3m x 15cm | 4.95 m3 | EUR 2080-2890 | EUR 600-830 | EUR 1800-3000 |

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.

## Frequently Asked Questions

### How thick should a concrete patio slab be?

A patio slab should be 10 cm (100 mm) thick for foot traffic and garden furniture. If the patio will support heavy planters, a hot tub, or is on soft clay soil, increase to 12-15 cm. Always pour onto a compacted gravel sub-base of at least 10 cm.

### How thick does a garage floor slab need to be?

A residential garage floor should be 12-15 cm thick with wire mesh reinforcement. If the garage will hold heavy vehicles or machinery, increase to 15-20 cm and add rebar. The sub-base should be 15-20 cm of compacted hardcore.

### Do I need reinforcement mesh in a concrete slab?

Yes for any slab 10 cm or thicker and wider than 2 metres. Wire mesh (A142 or A193) prevents cracking from shrinkage and minor ground movement. Place it in the lower third of the slab on spacer chairs, not on the ground.

### What are expansion joints and where do I put them?

Expansion joints are gaps cut or formed in concrete to control where cracking occurs. Place them every 2.5-3 metres in each direction, at changes in slab thickness, and where the slab meets walls or other structures. Use a joint strip or cut with an angle grinder after initial set.

### How long before I can walk on a new concrete slab?

Light foot traffic is safe after 24-48 hours. Wait 7 days before placing heavy furniture or garden equipment. Wait 28 days before driving vehicles onto a driveway or garage slab. Keep the slab moist during the first week for proper curing.

### Can I pour a concrete slab in cold or wet weather?

Avoid pouring below 5 degrees C as concrete cures very slowly and can be damaged by frost. In rain, cover the pour with a tarp. The ideal temperature range is 10-25 degrees C. In hot weather (above 30 degrees C), pour early in the morning and keep the surface moist.

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Interactive version with calculator: https://renoquant.com/concrete-slab-calculator. Figures are estimates; final quotes vary by site conditions, materials, and region.