Floor Slab Calculator

What this calculates

Calculates slab area, concrete volume (with waste), gravel sub-base volume and estimated wire mesh area for a simple rectangular ground floor slab.

Formula used

Area = length × width. Concrete = area × thickness. Concrete with waste = concrete × (1 + waste %). Gravel = area × sub-base depth. Mesh = area × 1.08 (for laps and cuts).

Worked example

A slab of 5 m × 4 m × 10 cm: area = 20 m². Concrete = 2.0 m³. With 8% waste: 2.16 m³. Gravel (8 cm): 1.6 m³. Mesh: 21.6 m².

Typical slab thickness

Patios, sheds and light foot traffic: 10 cm (4 in). Garages: 12–15 cm (5–6 in). Driveways: 12–15 cm (5–6 in). Any vehicular or heavy load: consult a structural engineer.

When not to use this calculator

Elevated slabs, slabs on fill, slabs with post-tension, structural slabs and any slab carrying vehicles or heavy equipment require engineered design.

How to use this calculator

1. Enter the slab length and width in meters (or switch to feet with the imperial toggle).
2. Enter the slab thickness in centimeters — 10 cm (4 in) for patios and sheds; 12–15 cm (5–6 in) for garages and driveways.
3. Enter the gravel sub-base depth — 8–10 cm is standard for residential slabs on stable soil.
4. Choose whether to include wire mesh — the calculator adds 8% for laps and cutting waste.
5. Click "Calculate floor slab" to see concrete volume (in m³ and yd³), gravel volume and mesh area.

Sub-base and compaction — why they matter

The sub-base (crushed stone or gravel layer under the slab) serves three critical functions: it distributes loads, provides drainage and prevents differential settlement. Getting it wrong is the leading cause of cracked slabs. Key requirements:

• Material: Crushed stone (not rounded river gravel) compacts better. Use clean aggregate — clay-contaminated sub-base retains water and heaves.
• Compaction: Compact in layers of 10–15 cm maximum. Uncompacted sub-base settles after loading, cracking the slab above.
• Thickness: 8–10 cm for light residential. 15–20 cm for garages with vehicles. Over unstable soil, increase to 20–30 cm.
• Moisture barrier: A 200-micron polyethylene membrane over the sub-base reduces moisture migration into the slab and prevents fines from being absorbed into the stone.

Wire mesh vs. rebar — what controls cracks

Wire mesh (welded wire fabric / WWF) provides crack control, not structural capacity. It keeps cracks tight if they form due to shrinkage or minor ground movement. It does not prevent cracks or carry loads. For structural slabs — vehicle traffic, heavy point loads or uncertain soil — designed rebar is needed. This calculator estimates mesh area only; a structural engineer must design rebar layouts.

Frequently asked questions